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THE INTERNATIONAL SCIENTIFIC SERIES.
           volume xxi. 
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 THE FIVE SENSES OF MAN. 
J3?T<- *•*
        DEDICATED  TO






          HIS FATHER






    A.  BERNSTEIN






IN AFFECTIONATE LOVE AND GRATITUDK




                BY




         THE AUTHOR 
L 
THE INTERNATIONAL SCIENTIFIC SERIES.
THE
FIVE   SENSES   OF   MAN
BY
      JULIUS  BERNSTEIN

O. O. PROFESSOR OF HIYMiM.ucjv IN THE UNIVERSITY OF HALLE
  ' Man must persist in believing that the inconceivable is conceivable,
or he will never make a discoverer'—Goethe
WITH NINETY-ONE WOODCUTS
^'
NEW YORK:
D.  APPLETON AND  COMPANY,
           549 and 551 Broadway.
                 1876.
/ 
bS3ft 
PREFACE.
THIS BOOK lays no claim  either to  systematic  com-
pleteness  or to popularity understood  in  trie widest
sense of the word.   It contains, of course, a general
system, which  has been elaborated in  Treatises  and
Text-books in  the course  of time ;  but this has  been
interrupted in many ways by physical and physiological
explanations, which a  proper treatment  of  the subject
required, and it has therefore been  found necessary to
subject it to many changes in order  to give  it a popular
appearance.  So far the author has aimed at  popularity :
but he has  endeavoured at times to take the reader a
step beyond the domain of ordinary popular treatises ;
and, if he  has not succeeded in smoothing  his  path
sufficiently, he entreats the  reader's forbearance.
    As an  aid  to  the  understanding of the  subject,
figures are  given with the text, some of which were pre- 
Vlll
PREFACE.
pared especially, the majority borrowed from scientific
treatises, and considerably altered for  the work.   His
acknowledgments  are due  to J. Miiller, 'Atlas  der
Physik' (Leipzig 1871), from  which most of  the  phy-
sical illustrations are  taken ; Kolliker, 'Handbuch der
Gewebelehre des Menschen ' (Leipzig 1867) ; Helmholtz,
' Handbuch der Lehre von den Geweben' (Leipzig 1871);
Helmholtz,  ' Handbuch  der  Physiologischen  Optik'
(Leipzig  1867);   Helmholtz,   'Die  Lehre  von   den
Tonempfindungen ' (Braunschweig 1872).  The  reader,
who wishes to penetrate deeper into the part  of science
treated of, is referred to the above works.

                                  The Author.
Halle a S. : April, 1875. 
CONTENTS.
PREFACE.

INTRODUCTION
     FIRST  PART.

SENSE   OF  TOUCH.
                         CHAPTER  I.

The Qualities of the  Sense of Touch—The Sense of Touch and the
    Organs of Touch—The Sensitive Parts of the Skin    ,      .10


                        CHAPTER II.

Sensibility of the Sense of Touch—The Sensory Circles of the Skin—
    Relation of the  Sense of Touch to the Activity of the Brain—
    Delusions of the  Sense of Touch


                        CHAPTER  III.

The Skin's Sense of Pressure—The Sense of Temperature  .
PAGE
  vii


   I 
X
CONTENTS.
   SECOND  PART.
SENSE   OF SIGHT.
                         CHAPTER  I.
                                                              PAGB
Formation of the Eye—Refraction of the Light-Rays in a Lens—Path
    of the Light-Rays in the Eye   .      .       .       •       .48
                         CHAPTER  II.
The Adjustment of the Eye—Short Sight and Long Sight  .       .   62

                         CHAPTER  III.
Recognition of the Interior Portions of the Eye—The Ophthalmo-
    scope   .       .      .      .      .       .       .          78

                         CHAPTER IV.
Formation of the Retina—The Blind Spot and the Yellow  Spot—
    Position of the Perception of Vision in the Retina     .       .   89

                         CHAPTER V.
The Colours, of^the Spectrum—Combination of Colours—The Three
    Primary Colours—Colour-Blindness    .                        100

                         CHAPTER  VI.
Incidental Images—The Phantascope—Positive and Negative Inci-
    dental Images—Incidental Colours—Harmony of Colours         116 
CONTENTS.
xi
                        CHAPTER  VII.
The Movements of the Eyes—Binocular Vision—Simple and Double
    Vision —The Identical Spots of the Retina,—The Horopter    .  123


                        CHAPTER VIII.
The perception of Solidity—The Stereoscope—The Telestereoscope—
    The Pseudoscope—The perception of Luminosity—The struggle
    between the Fields of Vision   .       .       .       .       .137
                        CHAPTER  IX.
Optical Illusions—Apparent  Size of the Moon—Intuition—Illusions
    of the Sense of Colour—Mental and Corporeal Vision  .       .150
       THIRD  PART.

SENSE  OF  HEARING.
                          CHAPTER  I.
General construction of the Organ of Hearing—Sound as a Tone, a
    Note, and a Noise—Musical Instruments      .       .      .  164
                         CHAPTER  II.

Hearing through the Air and through the  Skull—The External Ear
    and the Auditory Canal —Recognition of the Direction of Sound
    —Arrangements for the Protection of the Ear  .      .      .177 
Xll                       CONTENTS.
CHAPTER III.
PAGE
The Tympanic Membrane—The  Tympanic Cavity, the Ear-Bones,
    and the  Eustachian Tube—The Vibrations of the Tympanic
    Membrane and of the Ear-Bones       .       .       •       .187
                         CHAPTER  IV.

The Function of the Tympanic Membrane—The Telephon—Flame
    Manometer—The Importance of the Funnel-shaped Form of the
    Ear—The  Muscles in the Tympanic Cavity
                         CHAPTER  V.

The Labyrinth—The Organs of Corti—The Transmission of Sound
    in the Labyrinth—Presence  of Sympathetic Vibratory  Appa-
    ratus   .      .      .       .       .       .       .      .210
                        CHAPTER VI.

The  Sensation of Music—The Monochord—The  Octave and  its
    Divisions—Perception of Grave and Acute Tones—The  Irrita-
    tion of the Auditory Nerve    .....  229
                        CHAPTER  VII.

Notes or Compound Tones—The production of Harmonics—The per-
    ception of the latter by means of Resonators—Graphical repre-
    sentation of Quality or Colour  .       .       .      #         2. -
                        CHAPTER  VIII.

Analysis of Notes after the Law of Fourier—Helmholtz's Theory of
    the Perception  of Notes—Formation of  Notes by  Electr
    Magnetic Tuning-Forks .                               " 
CONTENTS.                      xiH
                       CHAPTER IX.
                                                           PAGE
Harmony of Notes—Consonance and  Dissonance—Production of
    Beats—Dissonance to be attributed to the  existence of Beats—
    Influence  of  Harmonics upon  Harmony—Resultant  Tones—
    Chords—Noises.       .      .     .      .      .      .269
                    FOURTH  PART.

                SMELL  AND   TASTE.


                        CHAPTER I.
The Sense of Smell .......   285


                        CHAPTER  II.

The Sense of Taste ....••«  29S 
 
ILLUSTRATIONS.
 FIG.                                                           PAGE
  i. Section of Skin of the Finger.......17
  2. Tactile Corpuscules.........18
  3. Theory of Stationary Sensory Circles.....3°
  4. Theory of Moveable Sensory Circles......32
  5. The number of unexcited Sensitive Fields necessary for producing
       a Double Sensation........33
  6. Sensory Illusion........        3^
  7. Horizontal Section  of the Eye.......5°
  8. The Formation of an Image by a Convex Lens      ...   54
  9. Camera Obscura    .........   55
 10. Reversal of the Retinal Image.......57
 II. Cause of Indistinct  Retinal Images......63
 12. Alteration in the Curvature of the Lens during Adjustment    .   65
 13. Mode of observing  Reflected Images in the Eye  .    .    .   .   68
 14. Reflected Images observed in the Eye.....69
 15. Inversion of the Image upon the Retina.....71
 16. Action of Concave  Lens on Convergent Rays ....   74
 17. Irradiation..........75
 18. Irradiation   .     .......•    -7°
 19. Irradiation      .    .    .    •     •     ...    •   •   77
20. Production of a Shadow  by the Blood-vessels of the Eye .    .   82
21. Mode of examining  the Interior of the Eye.....84
22. The Ophthalmoscope........86
23. The Retina as seen  with the Ophthalmoscope    .    .    .   .   87
24. Section of the Retina    .         .         .     •    •    .9°
25. Mode of proving the existence of the Blind  Spot .    .    .   .   92
26. The connection between the Nerve  Fibres and the Rods and
      Cones  ...........95 
xvi
LIST  OF ILLUSTRATIONS.
FIG.                                                              rAGE
27. Formation of a Spectrum.......io1
28. Colours of the Spectrum   ......••   IQI
29. Colour Top..........Io6
30. Method for Mixing Colours.......io7
31. Illustration of the action of the Spectral Colours upon the Retina   113
32. Phantascope    ..    .    .     .     .     .     .     .   .   no
33. Muscles of the Eye.........I24
34. Identical points of the Retina.......I31
35. Illustration of Theory of Single Vision  .    .     .     .     .   132
36. Illustration of Theory of Single Vision     .     .     .     .  .   133
37. Illustration of Theory of Single Vision  .....   134
38. The Horopter..........135
39. Stereoscopic Pictures........139
40. The Principle of the Stereoscope      .     •          .     .   .   139
41. The Principle of Brewster's Stereoscope  .    .     .     .     .   14Q
42. Stereoscopic Pictures.........144
43. Stereoscopic Pictures........144
44. Principle of the Telestereoscope .......   146
45. Illustration of Luminosity     .......   147
46. Optical Illusion..........151
47. Optical Illusion.........153
48. Optical Illusion .     .     .    .     .     .     .     .     .   .   154
49. Optical Illusion.........   155
50. Optical Illusion .     ........159
51. Optical Illusion.........160
52. Optical Illusion......     .     .     .   .   160
53. The essential parts of the Ear .     -     .     .     .     .     .165
54. Self-registering Tuning-fork     .     .     .     .     .     .   .   168
55. The Phonautograph.........T69
56. Simple Transverse Vibration......   .   170
57. Organ-pipes........' .     .171
58. The Siren..........   .   172
59. Dove's Siren  ...           .                            j~-
60. Principle of Reed-pipes   .......
61. Reed-pipe.........        j„-
62. Artificial Larynx.......             g
63. Sound-waves .......     #               ~
64. Connection of Tympanic Membrane with Hammer-bone          188
65. Chain of Ear-bones.....
66. Mode of Vibration of Ear-bones .
                                             .....196 
                  LIST OF  ILLUSTRATIONS.              xvii

FIG.
                                                               PAGE
67. Telephon    ..........200
68. Flame Manometer......               202
69. Vibration of a Funnel-shaped Membrane.....203
70. The Labyrinth..........2II
71. Section of the Cochlea......     .     .  212
72. The Crista Acustica.........214
73. Section of Coil of the Cochlea......215
74. The Organ of Corti.........215
75. Position of Fibres upon the Basilar Membrane .     .     .     .216
76. Cell connected with  the Fibres of Corti.....217
77. Change of angle in the arch of the Fibres of Corti   .     .     .218
78. Gradual increase in width of the arch of the Fibres of Corti .   .  222
79. Monochord   ..........  23-5
80. Nodal Points..........235
81. Siren    ...........  237
82. Savart's experiment for finding the Lowest Perceptible Tone   .  239
83. Savart's experiment  for finding the Highest Perceptible Tone  .  241
84. Resonator.     .     .     .     .    .    .    .     .          2<o
85. Production of a Pure Tone by means  of Tuning-fork and
       Resonator ..........  252
 86. Analysis of Vibration into Fundamental Tone and  1st Harmonic  253
 87. Complicated Form of Sound-wave  ......  259
 88. Displacement of the Phases of the Harmonic   .     .     .  .  264
 89. Electro-magnetic Tuning-fork .......  266
 90. Representation of the degree of Roughness between each Interval
      of the Octave   .........  279
91. Olfactory Cells........        .287 
 
THE
FIVE   SENSES   OF  MAN.
                INTRODUCTION.

In the animal kingdom a number of organs have been
developed which possess the  property of making each
organism  acquainted with occurrences in the  outer
world,  and  which are therefore  called  the  'sensory
organs.'  They are found in  their highest  perfection
in man,  whose  mental power surpasses in the same
degree that  of the rest of organised beings.
   Every sensory  organ may be  shown to be anato-
mically connected with  the nervous system  by means
of nerve-trunks and nerve-fibres.  Touch, sight, hearing,
smell, and taste are inconceivable without the presence
of a nervous system, even if the sensory organs were in
their present full development.  An eye,  of which  the
optic  nerve  has  been destroyed, can present  to us no
picture of the outer world ; an ear, of which the auditory
nerve has been severed, conducts no  sound to us; an arm,
of which the  nerve is injured, can feel nothing.  Such an 
2
THE FIVE SENSES OF MAN.
eye may have all the appearance of a sound one, it may
receive the rays of light and form an  image of objects
on its background, and nevertheless no  sensation can be
produced, for the connection with the brain, the centre
of the nervous system, is wanting.  The case is the same
with the deaf ear, and with the arm devoid of feeling.
The sensory organs are, therefore, only instruments of
the mind, which has its seat in the brain, and by means
of nerves makes use  of these  instruments  to obtain
information  of external objects.   The  forces which
operate in the outer world—namely, light, heat, sound,
motion, and  chemical affinity—produce  in the sensory
organs  an irritation of the sensory nerves  connected
with them,  and  these convey  the  irritation which is
there received throughout their entire  length to the
brain.    Each organ  of  sense has  its  own  specific
irritation  by which it  is excited. The terminations of
the  optic  nerve in the  eye can only be  excited  by
light-waves,  not  by sound-waves, and the  latter can
only excite the terminations of the  auditory  nerve in
the ear.   For the tactile nerves of the skin mechanical
pressure  and heat  are  specific excitements ;   for the
nerves of taste and smell  some chemical substance is
necessary.
  t The sensation itself evidently first takes place in the
brain.   The sensation of light does not  take place in the
eye, where there is only an impression of light upon the
expanded surface of the optic  nerve ;  the sensation of
light cannot, however, take place in the optic nerve itself,
for it merely conveys  the fact  of the  existence of the
irritation to the brain.   The sensation of lio-kt, a process
to us most obscure, begins rather in  the brain,  which is 
INTRODUCTION.
3
irritated by the excited nerve;  and since we can follow
the optic nerve up to its origin in the brain, we there-
fore conclude  that this  process occurs- in  the  central
organ of the optic nerve.
    The eye, therefore, is nothing  more than an optical
instrument which receives the light, and the optic nerve
nothing  more  than  an  apparatus for conveying  the
intelligence of  an irritation to the brain.   It has been
observed in  operations that if the optic nerve is  either
torn, crushed, or even severed, at the moment when it is
broken a strong flash of light is observed by the patient.
This light  is  not real,  for it is only perceived by the
person under  operation.  The sensation of light arises
merely from the mechanical irritation of the optic  nerve,
and from  the extension of the irritation to the nerve-
centre, where it awakens the process of the sensation of
light, just as if the excitement had proceeded from the
eye.   In such cases the sensation of light occurs without
any external  objective  light, and always takes place if
the optic  nerve is  irritated  in  any  way  whatever  by
those  influences which have an irritating effect on other
nerves, such as electricity, heat,  and chemical action.
Objective light, that is to say, the light-waves of the ether,
takes  no part in this action ; it may therefore be accepted
as a fact that  in ordinary vision  no  trace of the light
which enters the eye finds  its way to  the brain, but only
a process  of irritation peculiar to the nerve, and which
can be produced in  the nerve-trunk  by  pressure, elec-
tricity, heat, and chemical action, just as well as in  the
eye by light.   In whatever way the irritation may have
been caused, the process in the optic nerve  is always  the
same, and the action on the nerve-centre always produces 
4
THE FIVE SENSES OF  MAN.
the sensation of light.  It must  be exactly the same
with  the other sensory  organs and their  nerves.  A
sound does not extend beyond the end of the auditory
nerve, and none of it is  conveyed to the brain by the
auditory  nerve.  The nerve, which  is  excited at  its
termination,  communicates its  condition to the brain,
and  causes  in  the  centre of the auditory nerve the
sensation of sound.
   The  sensation of sound, therefore, can  take place
without  a sound-wave reaching  the ear, if only  the
auditory nerve  is in  any way excited, whether it  be
by pressure, rupture, electricity, etc.  Thus the irritation,
which in the centre of the auditory  nerve  causes the
sensation of  sound, always takes place in the nerve.
   It  is clear that these  ideas must be extended to the
other  sensory organs  of  taste,  smell, and touch.   All
sensory nerves are only  intended to communicate the
fact of an excitement of the nerve from the terminations
of the nerves to their centre in the brain—the sensorium.
This irritation  of the  nerve  is by no means similar to
the first irritation.   It is neither light  nor sound, nor is it
pressure  nor  warmth, nor a current of liquid which can
be tasted, nor of a gas which can be smelt.  It  is rather
a  process  of a peculiar  kind, about  which  we  may
conclude that in all the nerves of the  body the irritation
is  one and the same, since, in the muscular as well as
in the  sensory nerves, it  exhibits the same phenomena
and obeys the same laws.
   The nerve, again, can no more appropriate a trace of
the excitement  than  possess a trace of sensation.  If
we have at  any place divided a sensitive nerve we
can excite the divided part  as  strongly as we please  ■ 
INTRODUCTION.
5
but  all sensation is gone.   The central  nerve-trunk,
however, is sensitive throughout.
   Sensation  can only take  place  in the sensorium.
The  excited condition of the sensorium is the material
fact  which  corresponds  to a sensation ;  and  it is  un-
necessary that the sensory nerve concerned should have
caused the  irritation; for  in  dreams  we have distinct
sensations  which are not caused by the specific excite-
ment of the nerves, but  only by the action of some
internal excitement  within the sensorium.  Abnormal
excitements also, which occur in the case of lunatics,
or abnormal  states  of  the  blood in febrile  diseases,
cause subjective perceptions, which are called phantoms
and hallucinations.
   From  these  remarks it is clear that we really have
no sensations of objects of the  external world them-
selves, but only  of the  changes which  occur in  the
sensorium.
   How is it then  that we, nevertheless, transfer our
inward sensations to the outer world, that we consider
as external to ourselves all that we  see, hear, or feel ?
This fact, which to  the healthy human mind  seems so
simple and natural,  requires consideration.
   The above  question  can  be  answered  shortly, as
follows:—From  our  very  birth we learn  by experience
how  to explain the sensations of our senses;  and by a
thousand  experiments, which we make with  eye,  ear,
and  limbs  in every-day life,  arrive  at the conclusion
that  the  object  of  sensations, that  is, their  ultimate
cause,  is external to ourselves.  The  newly-born child,
of course,  experiences  sensations.    The  light which
enters  its  eye acts  indisputably  on  its brain, for  the 
6            THE FIVE  SENSES OF MAN.

pupil contracts  under the influence  of  the light,  and
this  cannot  occur without  the  co-operation of  the
centre of the optic nerve in. the brain.   The  sensation
is, however, only  an  internal  one,  like  the  feeling of
satiety or of hunger ;  it is, of course, not yet recognised
as proceeding   from   external objects.   We  observe,
indeed, that a child  gradually begins by  moving  eye
and  head to fix the  object, namely, to bring the eye
into  such  a position that the image of the object may
fall upon the point of most distinct vision  in the  centre
of the retina.   Yet  in  this stage of development the
transference of what is  seen  to  the outer world  has
not  yet  taken  place.   The  fixing of  the  eye upon
objects does not take place suddenly, but is  gradually
developed from a spasmodic  attempt to move the eye,
which, perhaps,  is caused by the irritative  effect  of the
light; and if during this motion the  point of distinct
vision chances  to fall  upon an object  which attracts
attention by its  brightness, colour, or motion,  then by a
repetition of this process, the  child will gradually learn
by experiment to repeat the  required motion at will.
   The use of  the sense of touch  is contemporaneous
with the  use of vision.   The sensation of touch, also,
is not recognised at first as proceeding from external
objects, but is, perhaps, only perceived as an inward
sensation,  and  as a  check to movement.   Now the
hand—the  most important  organ of touch__is  one  of
the  objects which are seen at once ; and  since  it has
the  property of great  mobility in space,  the eye will
very soon  see  the  hand moving  and  touching,  and
many sensations caused by it will  be  simultaneously
perceived.   When the hand  touches  an object  the 
INTRODUCTION.
7
eye sees  not only the  object, but the hand itself also;
and when the hand is in motion we  perceive simulta-
neously  the inward  sensation   of  movement of  the
muscles,—that is,  the existence of  the  sensation of
touch, and,  by means of our sense  of sight, the visible
motion of the hand and the object.
   The  simultaneous  occurrence of  the  sensations of
touch  and sight  gradually  leads to  the impression that
an object perceived by both senses is external to our-
selves.   To this step of knowledge there belongs, of
course, a logical  conclusion, the existence  of which re-
mains a  problem of mental life, but which is carried out
unconsciously,  and certainly only  formed by degrees.
It consists  in this, that if the two  sensations of touch
and  sight always  take place simultaneously, they must
have one and the same  cause, and, therefore, that the
object which is  seen  and touched must be one and the
same.
    Still  one such logical conclusion is not sufficient to
enable us to recognise objects as external to  ourselves.
A second  follows, which  certainly may  appear  very
scientific, but is not so, since it is formed unconsciously.
    The  two simultaneous  sensations of touch  and
sight are two  sensations of dissimilar quality, which
have their nervous centres in different parts of the brain.
If the cause of the sensations  be found within  the or-
gans which perceive  them, then it  must  be present at
the  same time  in those  of  touch and sight, which are
both different in constitution.  That is to say, it must
be a double one.  However, according to the first logical
conclusion,  the  cause is  single, not double; therefore it
is not an internal, but an external cause.
            2 
 8            THE FIVE SENSES OF  MAN.

    The simultaneous action of the sensations of touch and
 sight is, in fact, for the human mind an important source
 of knowledge in the external world.  Yet we must not
 on this account conclude that  touch alone, without the
 assistance of sight, as in the case of  persons born blind,
 cannot lead  to knowledge.  It is probable that the sense
 of touch alone might enable us to distinguish  our own
 body and external objects sooner than vision.  For the
 act of touching  our body with our  hand calls forth a
 double sensation of touch, one through the hand and the
 other  through the part of the skin touched  ;  touch-
 ing an external object causes only a single sensation of
 touch through the tactile organ.  Upon this physiological
 basis, an  idea of  the external world might be formed,
 although it would be difficult to analyse such a mental
 act so as to arrive at  a simple logical conclusion.
    If  once  the idea  is formed that  the object touched
 belongs to the external world, the education of the sense
 of sight will take place rapidly.  The simultaneous and
 coincident sensations of sight  and touch—the  simulta-
 neous  perception  of  the  motion by the  eye,  and  the
 sensation of  it by  the hand—the commencement of the
 sensation of touch as soon as the eye sees  that  the ob-
 ject is  moved by the hand, all lead to the conviction
 that the cause of both sensations must be one  and the
 same, and that if the  sensation of touch is recognised as
 external to ourselves, this must also be the  case with the
 sensation of sight.
    The sensations of  the senses  must be distinguished
 from other kinds of sensations to which the body is sub-
ject, which are termed common  sensations.   With them
 may be classed especially the sensation of pain, which is 
INTRODUCTION.
9
spread almost over the entire body.   The characteristic
distinction between these common sensations and the sen-
sations of the senses is, tKat by the latter we gain know-
ledge of the occurrences and objects which belong to the
external world, and that we refer the sensations which
they produce to external objects, whilst by the former
we only feel  conditions of our own body.
    The limit between the sensations of  touch and  pain
may be illustrated by the following example given by
Ernst Heinrich Weber.  If we place the edge of a sharp
knife on  the skin, we feel  the edge by means of our
sense of touch ; we perceive a sensation, and refer it to
the object which has caused it.  But as soon as we cut
the skin  with the knife, we feel pain,  a feeling which
we no  longer refer  to  the cutting  knife, but which
we feel within ourselves, and which communicates to us
the fact of a change of condition in our own body.  By
the sensation of  pain we are  neither able  to recognise
the object which caused it, nor its nature.
    The  sensations of the senses, therefore,  appear to be
of a higher kind than the common sensations.  The
general  sensibility of the body is,  indeed, the general
ground  from which the sensations  of  the senses also
spring;  but they  are  distinguished  from it by a  more
complete perfection, since  they are  produced by the
action of the forces of the external world upon deli-
cately constructed  organs  on  the surface of the body,
and the mind is thereby brought into immediate commu-
nication with the  external world. 
         PART  I.

THE  SENSE  OF  TOUCH
                    CHAPTER I.

The Qualities of the Sense of Touch—The Sense of Touch and the Organs
           of Touch—The Sensitive Parts of the Skin. •

We know from experience that every part of our skin
possesses a  certain  sensibility, and that this  sensibility
varies in different parts.   This  property is'  given  to the
skin by a great number of nerves which originate in the
brain and spinal cord, and  extend in  a tree-like form
over the body.  The sensibility of any  part of the body
is due to these nerves alone, for as soon as such a nerve
is lost or diseased, the part of the body supplied by it
becomes devoid of sensation.
    We must,  therefore, consider the skin as a sensory
organ which encloses our entire body, and is adapted  to
render every part of the surface of our  body sensible  of
external impressions, and, indeed, of impressions of mani-
fold kinds, which arouse in us peculiar sensations, and are
inseparably connected with mental processes.  It is re-
markable how various the perceptions are which we can 
THE SENSE OF TOUCH.             II
form by the aid of the skin alone, and there can be no
doubt that if we had neither eyes nor ears, this organ of
sense alone would be sufficient to enable us to construct
an external world, which, however, would of course be
limited in compass.  For we could only include in our
knowledge the objects which we could immediately reach,
which we could bring into direct contact with  our body,
and which would betray their properties and  nature by
some kind of impression upon our skin, whilst a great
part of the objects and occurrences in  nature  would re-
main a secret to us.
    By handling  a  body we could learn its shape and
form  just  as thoroughly as by the eye, especially when
we remember that the attention  of the  mind would be
concentrated in a greater degree upon the sensation of
touch, as is really the case  with the  blind.  We learn
the size and dimensions of a body which we can touch,
as soon as we can compare it with the size of our hands
and our  body;  and our judgment is fully capable of
determining the nature of its surface, whether it is smooth
or rough,  even or uneven.   In  short, the idea of the
nature of bodies, in so  far as they differ in form, which
we could obtain by the use of the sense of touch alone,
would be  sufficient  to  give our  life  an incitement  to
activity.   Our  intelligence could even form matter for
abstract  thought, from these perceptions  alone, and by
this  means arrive at the conception of  a line, an angle,
a triangle, etc., from which it  would follow that  mathe-
matics is a science  independent  of the observations of
sight, although in  reality its origin has  been derived
from this source.
    With the aid of the sensibility of the skin, moreover, 
12
THE FIVE SENSES OF  MAN.
we could  learn much more about the nature of objects
than their mere form.  With  the aid of our organ  of
motion we can judge of the weight of a body, by estimat-
ing the amount of force which we have  to use  in order
to lift it.   At the same  time every object exercises a
pressure on the skin  by its weight, which we perceive  as
such,  and from  which  we can  obtain  an  idea  of its
weight.   Every other force, equally with weight, can act
upon  the  skin by pressure, and  thereby we are  able  to
estimate the strength of  the pressure  which  weighs
upon the skin.  A weight placed upon  the hand, and a
friendly pressure  of  the hand, differ but little from one
another in their  physical  properties, however different
their physiological action  may be, and in both cases we
measure by the pressure experienced  the force which
caused it.
   These properties, however, by no means exhaust the
power   of discernment  possessed by the skin ;  it  pos-
sesses a property which  is confined to  it alone,  and
which cannot  be replaced  by any other organ of the
body. Whilst we can estimate the form  of a body much
better by  the eye than  by the touch, and by the latter
can only complete the knowledge obtained by the eye;
further,  whilst  by the aid  of our muscles we estimate a
force which produces pressure,  the perception of  heat
and cold remains as a peculiar property  of the skin, and
is shared by no other organ of the body.   The eye, in-
deed, by the aid of certain signs, which it has gained by
experience, is often able to tell whether a body is hot—
when, for  instance, it is glowing or steaming—but a per-
ception  of warmth is not possessed by the eye.   This  is
possessed  by  the  skin alone, and it is of great import- 
THE SENSE OF TOUCH.
13
ance to the maintenance of the organism that this pro-
perty is spread over its entire surface ; for it surrounds it
like  a protecting wall  against its worst  enemy,  cold,
which, if not prevented, would destroy its existence.  We
are warned, however, of the approach of this  enemy by
a common sensation of the skin, an inward chill, which
is only caused by a cooling of the skin, and which warns
us against its approach. This peculiarity of the skin, how-
ever, also gives us warning of the approach of an excess
of heat, which can do  just as much harm to the body as
the other extreme.  The skin,  therefore, possesses diffe-
rent qualities of sensations.  Just as the eye, by the sen-
sation of sight, can distinguish between the  form of a
body and its colour,  so the sensitiveness of  the  skin
by contact with  an object  can  distinguish  many of  its
properties, its form, its firmness, whether it  is  hard, soft,
or liquid, and its temperature.
    The peculiarity  of the  skin by which it  recognises
the form of an  object, is called the sense of touch ; its
peculiarity of estimating the force with which  the object
which it  touches presses upon it,  is called the sense of
pressure ;  the peculiarity of recognising heat  or  cold,
the sense of temperature.  From the combination of these
three  sensations is formed our  faculty of discovering
the  properties  of an  object,  to  a certain  extent,  by
touch  alone.   The  tactile sense  of the  skin  is divided
into these three  qualities, which are generally united in
a simultaneous  sensation.  For a scientific investigation
it  is  necessary  to  separate them  artificially, just  as
white light  is separated  by the  prism  into its compo-
nents, in order that they  may be observed singly.
    Besides these specific sensations, the skin is endowed 
14           THE FIVE SENSES OF  MAN.

with common sensation, which is present also  in  the
inner organs of the body, namely, the sensation of pain.
It arises as soon as the excitement which produces sen-
sation  exceeds a certain strength,  whether this be  by
means of  pressure,  heat or cold, or chemically by means
of caustic substances.  We cannot, however,  consider
pain as a sensation of the senses, since it gives us no
knowledge of the nature of external objects.   At most
we can distinguish  whether the pain felt by the skin is
produced  by a burn, or by the corrosion of acids, or by a
sharp instrument;  but this takes place  only with the aid
of the  accompanying sensations of the senses—those,
namely, of pressure, touch, and temperature.   If we ab-
stract it from the latter, then the sensation of pain has
in all cases the same character, which  makes  itself ap-
parent  as soon as the cause of the irritation is gone,
since the pain still remains.   Excessive cold, when
caused, for instance, by  contact with  frozen  carbonic
acid, produces the same pain as a burn.
    By touch is understood in ordinary 1nnguage a some-
what complicated process.   In touching a  oody we em-
ploy the  organs best adapted  for the  purpose, namely,
the hands, the finger-tips of which are furnished with the
most delicate sense of touch.  This  action, however,
does not  consist in merely feeling  an  object, but  the
motion  of the hands plays a great  part in it, since  we
allow the  tactile organs to wander over  the object which
we are touching.  This action, which we may call active
touch, consists  of a combination of motion and sensa-
tion, which brings different points of an  object gradually
into  contact with the sensitive  parts of the skin.  Our
imagination then  collects  together the  points found 
THE SENSE OF  TOUCH.
IS
forms from them lines  and surfaces, and, in short, the
entire figure of a body, the single parts of which are thus
united to form a general picture.
    In ordinary life the tactile senses only serve as an aid
to the eye, which is the cause of our using it but little,
and in the dark we often find of how little  use  it  is in
recognising objects.   By practice, however, it may  be
brought to considerable perfection, as may be  seen in
the case of the  blind, who are compelled to  use it  by
necessity;  and,  further,  in  the  delicate tactile sense of
the tongue, which is accustomed to grope in  the dark,
and so extremely correct are the decisions it can  make
in the  mouth, that it  can  recognise every little corner,
angle, or tooth with the greatest exactness.
    Much simpler than ttiis action  is the  sensation and
perception of contact with the skin.   If, without  making
a  movement ourselves, a second person touches  with
sufficient force any point of the skin with a  pencil  or a
pin, we can recognise with closed eyes the part touched,
with more or ! "^certainty.  In the hands this power is
very perfect, and the more perfect the nearer we approach
to the tips of the fingers.  We can  distinguish between
touches on the three finger joints most correctly, and can
determine  the distance  of the point touched  from  the
finger-tip with tolerable certainty.  Similarly, the sensi-
bility of the face and of the forehead is strongly marked,
while that of the lips and the tongue  is more perfect
still.  Experience teaches us that there is  no point of
the skin's  surface which is insensible to  contact, when
made with greater or less force.  But the  determination
of the point touched is much less correct on the arms,
legs, and the other parts of the skin, than on the hands
and face. 
 l6          THE FIVE SENSES OF  MAN.

    The  property of determining  the  point touched  is
 called the skin's sense of locality, which is a factor of the
 tactile sense.  The perception of contact is not sufficient
 to explain this phenomenon ; the process consists much
 more in  the localisation of the contact perceived.  This
 power is an important psychical action, which evidently
 takes  place in the brain, and can only be explained by
 the supposition, that in our imagination a picture, as  it
 were, of our skin's surface exists, in which we seek for
 the position of the spot where contact has  taken place,
 and which we find with greater or  less certainty.  The
 question, what is the relation between  the skin's  surface
 and the  imaginative  faculty,  i.e. the  brain, in  which
 the  faculty is situated,  is sufficiently  answered  by the
 physiology of the nerves.
    A great number of sensory nerves  radiate from the
 brain and the spinal cord to the skin ; they all consist of
 a very great number of fibres which separate from each
 other  near the skin and here terminate in a peculiar
 manner.
    The skin itself consists of three layers (fig. i).1   Upon
 the cellular tissue d f under the skin,  which sometimes
 is very rich in fat, lies the first skin,  the dermis (from  c
 to b), which is  of  a  tolerably compact texture,  and by
 tanning is converted  into leather.  Its surface consists
 of a  greater  or  less number  of  cylindrical or  conical
 protuberances,  e, which are called papillce.  Upon-the
 dermis lies the mucous layer,  b, which consists of a great
 number of small microscopic cells completely filling the
depressions between the papillae of the dermis.   Lastly,
the outer layer is the cuticle or epidermis, a, which forms
1 Kolliker, ' Gewebelehrer.' 
THE SENSE OF  TOUCH,
17
a compact firm skin, but consists of an  intergrowth of
cells which are filled with a solid horny substance.   The
blood-vessels and nerves extend only as far as the surface
of the dermis, and to its papillae;  the mucous layer and the
epidermis are completely free from blood and nerves.  In
the figure are  seen also the sweat-glands, g, in the skin,
e-
                        Fig. 1.

whose ducts, h, penetrate to the surface of the skin, i.  The
nerves of the skin which terminate in single fibres only
extend to the dermis, and here they are observed  to end
in a peculiar  manner in the  papillae.  Many of them
contain  for instance, an  egg-shaped particle, b (fig. 2),
which a nerve fibre, c, enters and in which it is lost after 
18          THE FIVE SENSES OF MAN.
several convolutions, dd, round it. They are called tactile
corpuscles, and there can be  no doubt  that they act as
                     the instrument  of  the  sensation
                     of touch.   They are not found in
                     the same  numbers  in  all parts
                     of  the   skin,  occurring  in  the
                     greatest  number  in those parts
                     where  the  sensibility  is more
                     acute, and more sparingly where
                     it is weaker.
                         They are extraordinarily nu-
                     merous at the tips of the  fingers,
                     where, in the space of a square
                     line, about  ioo may be counted,
 and they are tolerably numerous over the whole surface
 of the hand, but occur  in much smaller numbers on the
 backs of the hands.  On the palm of the hand also the
 papillae, which, however, do not  all contain a tactile
 corpuscle, occur  in great numbers and  are arranged in
 regular  rows.  This  gives the  peculiar  striped  appear-
 ance of the skin which is  perceived  on  the surface of
 the hand.   The nerves  of the skin are observed  to pos-
 sess another terminal apparatus, similar to that of the
 tactile corpuscles, namely, long globules (pacinian  bodies),
 in the hollows of which the nerve fibres terminate.   In
 short, in the entire surface  of the  skin there  exist  ter-
 minal apparatus of  a   peculiar kind for the  sensory
 nerves, and  if we wish to follow the action of sensation
 further physiologically, we must  start with-the excitement
 of a nerve fibre which ends in a  definite part of the skin,
 and follow the course of the excitement to the brain.
    The  course of the nerve between brain and skinalon^
Fig. 2. 
THE SENSE OF  TOUCH.
»9
which the excitement passes can  be followed anatomi-
cally with a certain degree of  exactness.   A nervous
fibre which ends in the skin forms as far as its union «
with the spinal cord or brain,  a long, fine,  continuous
thread.   The fibres which terminate in the skin very soon
unite in small branches, and finally in thick nerve-trunks,
before they enter  the central  organ of the nervous
system, but in  no case do two nervous fibres coalesce in
these nerve-branches.  We may, therefore, assume that
every part of the skin is provided with isolated connec-
tions with the  centre of the nervous system, which  are
united there just  as  telegraph lines unite at a terminus.
    The physiology of the nerves, in which great advance
has been  made by  the study of the action of motory
nerves, throws  light upon the course of nerve irritation,
and leads to  the result  that  the irritation  of a fibre
passes through its entire length without being communi-
cated to the adjoining fibres.   In  a telegraph wire, of
course,  the electric  current must by complete isolation
be prevented from  passing along any other wire,  if the
intelligence is to reach the station intended.   The nerve-
fibres on the contrary, in which we -have  to do with an
action entirely different from, and much  less rapid than
electricity, do not require such  isolation, and  although
they are closely packed together  to  the  number of
several  thousand, yet they allow of no  transfer of the
irritation from  one to the other.
    When, by touching a part of the skin, we irritate the
nerve which ends there, the irritation, insulated in  the
nerve, passes along it to its origin in the  nervous  centre.
As soon as the irritation has arrived there, the action of
sensation takes place ; we know nothing of what  this 
20
THE FIVE  SENSES  OF MAN.
 consists, except that it can only take place in the nervous
 centre.   This sensation  is immediately connected with
? the consciousness that the irritation has been experienced
 by a certain definite part of the skin ; our  imagination
 even places the entire action of sensation in the skin itself,
 although it cannot take place in the skin, but only in the
 brain.   It  is absolutely impossible for us to  separate the
 sensation itself from the part irritated.  In our imagina-
 tion the locality of the irritation and that of the perception
 coincide exactly, and we are first taught by physiological
 experiments and the  diseases of the nerves  that this is,
 in reality, not the fact.   The fact  that the division of a
 nerve deprives a part of the body of sensation, shows us
 that the sensation itself  cannot take place in the  limbs,
 nor in the nerve itself, and that a nerve can only produce
 it when its connection with the brain and the spinal cord
 is intact.
     The law is equally applicable  to all sensory nerves,
 that we refer the sensation  zvhich  they produce  to the
 organs  at  the  terminations of the nerves, to  the point
 zvhere the cause of the irritation acts.  This is the case
 not only with  the sensory nerves  of the skin,  but to a
 much greater  extent with the optic nerve, the irritation of
 which takes place in the retina of the eye.   The  sensa-
 tion of  sight can only take place,  however, in the  brain,
 for it is lost as soon  as the optic  nerve is divided, and
 yet we  transfer the  object seen  to the external  world
 surrounding us.  This  fact, which  is called the law of
 eccentric sensation,  can  only  be  explained  by the as-
 sumption that our imaginative faculty is entirely formed
 by  experience.  We  know  from   experience  what  our
 sensation is as soon  as  the  contact with the  skin has 
THE SENSE OF  TOUCH.
21
taken place, and since we  have convinced  ourselves a
thousand  times, that  this sensation  is only caused by
contact with the skin, since, further,  we have no token
that the excitement passes through a long nerve  to  the
brain, therefore  our imagination, in consequence  of the
experience gained, refers the entire action of the sensation
to the spot where the recognizable  cause of the irritation
is present.
    With the aid of these observations we  can explain
the sense of locality possessed by the skin, i.e. the power
of determining  correctly the  locality  of the irritation.
From all parts of the skin run nerve-fibres, isolated like
telegraph wires, to the  nervous centre  of the brain in
which the consciousness of sensation takes  place.   The
brain is, therefore, the terminus of these nerve-branches,
and, as it  were, receives and  explains   the messages
transmitted  to  it.   The brain, however, distinguishes
very clearly by what branch such  a message  has come,
and just as a clerk in a telegraph office, where a great
number of  wires meet from all  sides, knows by experi-
ence from what direction each wire brings its message, so
the brain  also  knows, by the  experience it has gained,
where an irritation  has occurred, when it reaches it by a
certain nerve-fibre,  and therefore refers the whole sensa-
tion to that part of the skin where the irritation  took
place.   It is probable that the brain, by  its  imaginative
faculty, has formed a  complete  picture of  the surface of
the body,  which  has been  obtained gradually and  by
experience, and is  always  being more highly perfected.
The nerve-fibres are in a definite relative position to this
picture, or  rather with the part of the  brain where  this
picture is formed, and we can  imagine  that every spot 
22
THE FIVE SENSES OF MAN.
upon the surface of our body is connected with its corre-
sponding  spot in  the picture  of the  brain.   When,
therefore,  an irritation is conveyed by  a certain  nerve-
fibre, there arises in the brain a picture of the point of
the skin where the irritation has taken place.
    It is evident that if a true localisation  of the  sensa-
tion is to take place, all the connections of the skin with
the brain must be in good order.   If  the irritation were
to pass from one nerve-fibre to another the brain would
no  longer be able  to  distinguish the place  from which
it came, and could not localise the irritation.
    A very interesting observation is often  made in men,
in whom, for the  purpose of an  operation, a piece of
skin is removed from its original position  and  made
to grow in  another  place.  In  order,  for instance,  to
replace a nose, which has been  destroyed  by disease,
or  has been wanting from  birth, surgeons  detach a
triangular piece of skin from  the forehead, so that it
is  only attached by its apex to the  root of the nose
between  the eyebrows ; they  then fold the piece over
downwards, and sew  it  on the skin  of the  face  in  the
position of the nose.   The new nose after a short time
grows  on completely; if, however,  it  is  pricked with
a needle,  the person  does not  feel the  prick  on  the
nose,  but on  the  forehead—the original  position  of
the skin.  This sensation  does not continue long after
the operation,  and the person gradually  becomes con-
scious  that  the  locality  of the sensation  is now a dif-
ferent  one, and thus, by experience, learns to determine
the locality  correctly.   A similar and no less interestinc
observation  is made in  the case  of  persons who have
undergone amputation.  They very frequently make the 
THE SENSE OF TOUCH.
23
remark that they have sensations in the amputated legs
or arms, just as  if the limbs were  still existing.  They
feel in them the effects of frost and warmth ;  they say
they  feel  pain  in  this or that toe  of the lost  leg ; that
they feel tickling,  itching, etc., and  distinguish  the exact
point where these  feelings take place.  The explanation
of these facts  is  quite  simple.   In the stump of the
amputated leg lie the divided nerve-trunks, which have
provided the entire limb with sensory  nerves.   In the
healed scar the nerve-stems are often irritated ;  and since
the irritation of the nerves is conveyed to  the brain, it
causes sensation, and simultaneously produces—we might
almost say from habit—the picture of the same part of
the body in which they naturally end.  The brain, there-
fore, refers all these sensations, from the  experience it
has  gained,  to  the  same  limbs  in which  the irritated
nerves originate, even when the limb itself is wanting.
    This very remarkable phenomenon is a manifest proof
that  the  action of  sensation takes place  only in the
nervous centre  of the brain.  Since even  when a part
of the body is  wanting, the sensation of  its  existence
and irritation does  not  disappear. 
24
THE FIVE  SENSES OF  MAN.
                    CHAPTER  II.
Sensibility of the Sense of Touch—The Sensory Circles of the Skin—Re-
   lation of the Sense of Touch to the Activity of the Brain—Delusions of
   the Sense of Touch.

The power  of  touch is not equally  developed in all
parts of the  body.   We know from experience that it is
fully developed in  the hands, since we use these organs
for  the  purpose  of touching  objects.   The  hand, by
means  of its mobility and  its  articulated structure, is
more especially  adapted for this  action than  all  the
other  parts  of the body;   and  it is  a  general  fact
in the animal  kingdom, that all organs adapted for
touching are endowed with  the  greatest mobility,  such
as the feelers of an  insect, the trunk of an elephant,
and  the tongue  of  all animals.    Mobility alone,  how-
ever, is not  sufficient to give such organs  the function
of a tactile  organ, but  its surface must be especially
provided  with a fine sense of touch, in order to render
it  capable of perceiving  fine distinctions  of space.
   Other parts of  the body than  the hand, especially
the  feet, are  very little  adapted  for touching, not  only
on account of'their inconvenient  position and defective
form, but because  their skin possesses a far less culti-
vated sense of locality.   It  would, for  instance, be  very
difficult  to   recognise with  closed  eyes  the  form  of a 
THE SENSE OF TOUCH.
25
body, however simple, by the aid of the arm alone ; with
the surface of  the feet it could be done more  easily,
while with the hand we should very quickly  make our
decision.  It seems extremely natural that the members
most  capable of motion should possess the finest sense
of touch ;  and it may with justice be said, that it is most
probably  developed by use, and, further, is  transferred
by inheritance.   Apes, who use the feet for grasping
just as much as  the  hands, have  the sense of touch
equally cultivated in both members, since both are used
to the same extent for touching.  In mankind,  whose
foot is  transformed into  an instrument  of progression,
the power of touch in this member  returns to the hand ;
but it is interesting to observe that occasionally in men,
who are without arms, the  sense of touch can be highly
developed in the feet by  practice, and as the mobility
increases  in  an equal degree, they are  able to write
and execute many kinds of work.
    The delicacy of the sense of touch, or, more exactly,
the sense of locality in different parte of the skin, has
been  measured  by  Ernst  Heinrich Weber, by a very
ingenious method.  It consists  of  the following  highly
interesting  experiment,  which  everyone  can perform
with  the  greatest ease.  Two persons are required  for
this experiment, one of whom tests the sense of touch
of the other.  For  this  purpose a pair  of compasses
are taken, whose  points, somewhat blunted, are  placed
at a certain distance from one another  on a part  of the
skin of the  other person.   The latter must then say,
with  closed eyes, whether he feels the contact of two
separate  points,  or  whether both points seem  to  be
merged into one. 
26           THE FIVE  SENSES  OF MAN.
   The result of this experiment upon the less sensitive
parts of the skin is very surprising.   If, for instance, the
points are placed on the forearm  in the direction of the
length of the arm, at a distance of about four centimetres
(1-58 inch) apart, the sensation is then evidently a double
one ; but  as  soon as the  distance between the points is
less than three centimetres (ri8 inch), the contact is then
felt as that of a single point—that is to say, both contacts
are united into a  single  sensation,  and the  person  on
whom the experiment is made feels considerable surprise
when, on opening his eyes, he sees that, contrary to  his
sensation, two points of the skin have been touched at
such a considerable distance from each other.
    We  may now discover by experiment for each part
of the skin,  how near  the points  may be brought  to-
gether without  the impression produced being  that of
a  single point,  and the  smaller the distance the finer
is  the  sense  of locality for that part of the skin.   By
this test the  tip of the tongue is found to be the most
sensitive,  for the two points are distinguished when at
a distance of only a millimetre apart  ('0394 inch). Then
come the tips of the fingers, which can distinguish a dis-
tance of two millimetres  ('079 inch).   In the hand the
sense of locality varies with the joints of the hand ; it is
considerably finer on the palm of the hand than on the
back, which  cannot distinguish a distance of four or five
millimetres (*i 57-196 inch) between the points of the
compasses.
    In the face, the lips possess a tolerably fine sense of
locality.   If  the  points of the compasses are placed on
the  cheek near  the ear,  so that both  can  be  clearly
distinguished, and  then brought slowly over the skin 
THE SENSE OF TOUCH.
27
to the lips, a  sensation  is  experienced as though the
points were being  separated  from  one  another.  The
sense of  locality increases,  therefore,  as we  approach
the  mouth ;  and  while   the  distinction between the
two  points becomes more clear, it seems  as if  they
were  being separated also.  The sensation  is  precisely
similar  if the  points  are placed across the forearm, and
slowly  moved  towards the hands as far  as the tips of
the fingers.  Here also we imagine  that the lines drawn
by the points,  although  really parallel,  are  always
diverging, since  the  sense of locality increases as  we
approach the fingers.
    The skin of the back  has the dullest sense of touch,
since when the  points are at a distance of five to  six
centimetres apart  (i*97-2-36 inches), they are still per-
ceived  as a single touch.   It is quite astonishing  how
greatly the distance  between the two points must be
increased on the back before w.e are clearly conscious of
a double impression.  In the arms and legs the tactile
sense increases with the distance from the trunk, agreeing
with the corresponding mobility of  these  parts;  and the
sensibility, again, is greater on the inner or bent side than
on the outer or extended side.  If, now, for any  part of
the  skin  we measure exactly the distance between  the
two points,  at which the sensation of the two points
passes into that of a single one, and make this measure
in many directions,  we obtain for  the part of the skin
under  examination  a figure of a  circular form, within
which  the  simultaneous contact of  any two points is
felt as  that of a single one.   In many parts of the skin,
 on the arm, for  instance, this figure  has  an oval shape,
since the power  of distinguishing two points is more per- 
28           THE FIVE  SENSES OF MAN.
feet  in a transverse  than in a  longitudinal  direction.
Such a  figure, which has more or  less of a  circular
shape, is called the sensory circle of the skin ;  and by
exact measurement the skin "of the whole body has been
divided  into a number of sensory circles,  which  differ
extremely in size, and considerably  in  form.
   An   interesting  observation   is  explained  by  the
existence of such sensory circles.   If a metallic tube, with
triangular, circular,  or rectangular sides,  is pressed on
the skin, it  is not  easy to recognise the form of  the
tube. ' The smaller the sensory circle, the easier this
is, and the smaller the diameter of the  tube may be.
Upon  the insensible  parts  of  the  skin, however,  the
sensation is always the same, whatever  the form of  the
tube may be, as long as  its size does  not greatly surpass
the  diameter of a  sensory  circle.   For when  all  the
points of the sides lie  within a single sensory circle,
then they all unite  to a single  point.  Upon the arm,
therefore, we cannot  distinguish between  a  triangular,
circular,  or rectangular tube of two centimetres (79 inch)
diameter, while  this can easily be done  upon  the  palm
of the hand.
   Figures  more complicated than  the above  are, of
course, still  more difficult to recognise.  This is well seen
from the ease with which  letters, or  even words of mo-
derate size, are understood when written with a pencil
on the palm of the hand, while  it becomes difficult to
do so, if they are written on  the arm, and the letters
must be of a very large size to be understood if  written
on the back.   Moreover, in this case the recognition is
easier than when a figure already made  is placed upon
the skin,  since there is  time to devote the attention to 
THE SENSE OF  TOUCH.             2g
each single point of the figure.  In  any case, a  letter
must extend over several sensory circles in order  to be
understood.
    In  order to  obtain  an insight into the facts  men-
tioned above, we must return to the anatomical relation
between the skin and the nervous centre  in  the  brain.
We know that all parts of the skin are provided with
separate nerve-fibres,  in which  the  irritation is isolated,
and by which  it is communicated ; and we  have sup-
posed  that the brain has learnt  from experience, to what
part  of the skin  the  nerve-fibre  belongs,  which  has
brought intelligence  from  without.  We have said, in
order to render the representation more exact, that every
nerve-fibre in the brain is provided with means of recog-
nising  the  part  of the  skin  in which  the  nerve ends.
Still  these  words explain  nothing further,  and  it is
better to rest with this conclusion, that the property of
the brain to recognise the locality of the irritation  of the
nerve-fibre is a power which has been gained by practice.
    However, the observation has been made, that this
property of the  brain has its limits, since in every part
of the skin  two  points  at a  certain distance  apart pro-
duce a single, not a double  impression, and in  certain
parts of the skin this  distance may be very considerable.
All the points which  lie within a sensory circle cannot
 be distinguished  from each  other by the brain, and all
the irritations, which are communicated  in the brain from
these points, are united by the  imagination into a single
irritation.
    The explanation of these phenomena, which  would
 most  naturally occur to us,  consists  in the  assumption
 that an entire sensory circle is always  provided  with a 
3<J           THE FIVE SENSES OF  MAN

single nerve-fibre.  We can imagine that the terminal ex-
pansion of a fibre is extended over the space of each sin-
gle sensory circle, and that the terminal apparatus of the
fibres varies in size according to the size of these circles.
    There are, however, important  reasons against  such
an  assumption.   It  has, in fact, been observed that in
the skin the nerve-fibres are considerably divided ;  but
since in the parts of the skin of low sensibility the sen-
sory circle has a diameter of 6 centimetres (2-36 inches),
it would, therefore, follow that a single nerve-fibre would
                 extend over a similar  space.   This is,
                 however,  improbable,   since  although
                 in  a  large sensory circle, there is  no
                 power of distinguishing points situated
                 at a considerable  distance, yet there is
                 no single point in this circle  which is
                 not sensitive ;  and  if  a large sensory
                 circle  only receives  a single fibre  from
                 the brain,  then this must be  split  up
                 into an inconceivable number of fibres,
                 in order to supply the  whole circle.
                    Moreover,  the following phenomena
                 are against   this  supposition.    Let
                 (fig.' 3) the sensitive regions  1 and 2,
                 which touch each other, be measured
                 on the arm round the centres c and d.
                 Now,  if the circle  1 is  provided  with
                 a fibre 1, and  the circle 2  with a fibre
                 2, it  is easily  seen that at the points
                 a  and b, which lie within the  circle 1,
the  sensation would be a single  one,  since the same
nerve-fibre would be excited.   Contact,  however,  with
Fig. 3- 
               THE SENSE OF TOUCH.             31

the centres c and d, which are situated  at  the same
distance apart as  a  and b, according to this assumption,
will not produce a single, but  a double impression, since
they belong to different nerve-fibres.  There are  two
points even still nearer, m  and n, where the ^impression
will  appear double, since  they also lie within the  ter-
minal limits of two fibres.   It  is, however,  quite the
contrary.   Contact with the points c and d produces in
this  case  a single impression, and much  more so with
the points m and n.  We see, therefore, that the terminal
limit of a single nerve-fibre  cannot possibly have the same
size as an entire sensory circle.  If it were so, the circle
must have distinct limits,  within which the phenomena
must obtain, as shown above.  If we move  the points
of the  compasses forward from the sensory circle 1 to
the circle  2, then  the perception of a  single impression
must suddenly pass into the perception of  a double one,
as soon as the points of the compass overstep the limit ;
and since  this is  not the case, it therefore follows  that,
in this sense,  such  definite limits do  not exist in the
skin.
   The best explanation of all the facts  hitherto men-
tioned  is  given by  Ernst  Heinrich Weber, an investi-
gator who has done good service  in  this, as well as in
other branches of  physiology.  He assumes that the
terminal  limits of a nerve-fibre are much smaller than
the sensory circles as found by measurement, so that the
latter always contain a great number of isolated  nerve-
fibres.  If, now, two terminal limits are excited, and if a
certain number of isolated  fibres which are not excited
lie between them, then the impression  is  only a single
one.  These unaffected terminal limits which lie between
             3 
32           THE  FIVE SENSES OF  MAN.
the irritated  points, and whose  fibres  are  not irritated
simultaneously, make the brain  conscious  that the skin
is touched in two separate points, and at the same time
the number of unexcited fibres gives the brain a measure-
                      ment, by means of  which it can
                      estimate  the distance  between
                      the points touched.
                        In fig. 4 the  small hexagonal
                      figures  represent  the  terminal
                      boundaries of the  nerve-fibres.
                      Let us assume that  twelve inter-
                      mediate  figures  are necessary  to
                      enable us to perceive  a double
                      impression.  Therefore a and b
                      will lie  upon  the  limits  of  a
                      single sensation.   Similarly with
                      c  and d, which  have  the same
number of intermediate  terminal boundaries.   We now
see why the sensation of a single contact is not  sud-
denly changed  into that of a double one, on  moving
the points  of the compasses; for as long as less than
twelve terminal  limits lie between  the points,  we still
perceive a single contact.   This idea makes  it clear
that a sensory circle can  have no fixed boundary on the
skin, but that we may imagine  it removed at will, as is
represented  by   the dotted  line fig. 4, as  long as  it
includes the  definite  number of terminal boundaries of
the nerve-fibres.
   The division, therefore, of the skin  by  the termi-
nations of the nerve-fibres is like a mosaic, each termina-
tion having its separate little piece  of mosaic, which w.e
may call the sensitive field of this nerve.   Contact with
Fig. 4. 
THE SENSE OF  TOUCH.
33
a sensitive field excites in our brain the idea of a certain
definite part of the skin; but if two neighbouring sensi-
tive  fields are touched,  we  do  not feel two  separate
points, since we have no sign  to separate them by.  We
shall see clearly that  this must be  so, if we place on
the skin some straight edge, the  sharp edge of a plate,
for instance, and  consider the sensation it  excites more
closely.   Our immediate sensation tells us, that, com-
mensurate Avith  our  experience,  a  straight
line  has touched us.   This line divides (fig. 5)
a  certain number of  sensitive fields  in each
of which a single  nerve-fibre is excited.  If,
now, in the approximate  sensory circles 1 and
2, 2  and 3, etc., our  sensation is that of separate
points ;  if we can  separate the locality of  the    Fis« 5-
point 1  from  that  of  point 2, then the entire  line must
appear to consist  of  points.   Since,  however, the sen-
sation is not that of broken  contact,  it follows  that we
cannot separate  locally  two sensitive fields from  each
other.  A separation  of locality is much more probable,
if between the two excited sensitive  fields, one at  least
is not excited, since it is only this  unexcited sensitive
field which  makes  us conscious  of  two  excitements
whose  localities  are different.
    A single  unexcited sensitive field between two ex-
cited ones  is not sufficient to enable us  to perceive a
double  impression.   The  phenomena  of  the sense of
touch are explained  in  a much less forced manner, as
soon as we assume with Ernst  Heinrich Weber that  a
greater  number  of sensitive  fields must  intervene be-
tween the excited fields, before we  can perceive a double
impression. 
34           THE FIVE SENSES OF  MAN.
   All the sensations of the senses then, of which we are
capable, pass into perceptions of the  senses, as  soon as
certain  mental  operations have been aroused by  the
sensory excitement.
   The observation may very frequently be made that,
under certain conditions, we are not conscious of impres-
sions, although  they  may have undoubtedly acted  on
our sensory nerves in no  inconsiderable degree.  If, for
instance, one person is engaged in an  animated conver-
sation  with another, and we meanwhile lay  our hand
upon the shoulder of the former, it frequently happens
that this contact, though readily noticeable, is not felt, or
rather that  it does not have  the desired effect.   We are
then accustomed to say that  the conversation claims so
much of his attention that he is unable to receive other
impressions; or, more  simply,  ' that he is too much
absorbed in conversation.'
   This expression is  grounded on views which  are
quite correct, and in accordance  with  physiology.  The
impression made on the  shoulder had doubtless in  this
case excited the nerves of the skin,  and they  had  un-
doubtedly conveyed the irritation to the spinal cord  and
the brain,  and  aroused there the  action of sensation.
But at this moment, the brain  was  unable to  become
conscious of the irritation  which reached it and the  sen-
sations which they cause,  and thereby to produce a per-
ception ; because, although the brain was at this moment
in full activity,  the organs of consciousness and  the  will
were entirely occupied in awakening and forming ideas,
and transforming them into intelligible language by the
aid of the motory and muscular nerves.  It was evidently
this latter process in the  brain, which prevented it from 
THE SENSE OF  TOUCH.
35
becoming conscious of the sensations which acted on it,
so that  it annihilated  the  irritation which reached the
brain and prevented its producing perception.
   These examples are only introduced here in order to
show how closely sensory impressions are connected with
mental  action, both of- which  take place in the brain.
The conclusions which  we formed concerning  the sense
of touch must be extended to the other senses also ; but
it  is  the many interesting  experiments made  on the
sense of touch which force us to conclude that no sensa-
tion is ever produced  by the sensory organs alone, but
always  in the brain, where all sensory nerves originate.
    Some very remarkable observations which have been
made in experiments upon the sensory circles are satis-
factorily explained  by  these  conclusions.   Upon the
same spot of  skin the size of  a  sensory  circle not only
differs in different people, but varies considerably in the
same person at different times.   The most interesting
fact,  however, is,  that  constant  practice considerably
diminishes the limit within which a single impression is
produced in certain parts of the skin, in those parts, for
instance, which are not  naturally very  sensitive  and
where the sensory circles are large. If, however, this prac-
tice ceases,  the delicacy of  the sense of touch will de-
crease also.   It  is  also  interesting to observe that the
blind, who are compelled to replace the power of vision
by the sense of touch, have much smaller sensory circles
than other people, and we can hardly doubt  that this is
due to practice.
   Such facts give an evident proof that the property of
the skin of measuring the distance between two points is
really a property of the  brain.   By practice the brain 
36           THE FIVE SENSES OF  MAN.
 learns to distinguish  two approximate  points  on the
 skin, since  it  has become conscious of the fact that,
 between these two excited points, a number of points are
 situated which are not excited.  The greater this number,
 the more distinctly can  we recognise the existence of
 the points ; but there is a number which is not sufficiently
 great to enable us to recognise the points.  By practice,
 however, this number can  be diminished ; that is to say,
 fewer non-excited points between those which are excited
 will  then be  necessary  to create the impression of a
 double sensation.   Since, during this process in the skin
 the arrangement of the nervous system has remained the
 same, practice being nothing more than the act of learn-
 ing,  it must therefore clearly follow, that we have here to
                  do with a process of the brain, and
                  that the sensory circle of the skin is
                  not  really situated in the skin but in
                  the brain.
                    A  very interesting  illusion of the
                  sense  of touch, which  is  probably
                  already known  to the  reader  as  a
                  trick, is  no  less a proof of the views
                 just  given.  It is an  experiment which
                  even Aristotle  regarded psychologi-
                 cally.   If we cross the first and middle
                 fingers  and then pick  up a pea from
                 the table, we have  a distinct impres-
sion  that we are holding two peas, and even when we look
closely and convince ourselves that there is only one pea,
we can hardly get rid of  the impression.   The illusion
is, moreover, particularly strong if we roll the pea back-
wards and forwards between the fingers.
 
THE SENSE OF TOUCH.             27
   The reason of this curious illusion is evidently that
the tactile surfaces  of the skin are placed in an unaccus-
tomed position.  If we  were to hold the  pea with the
first and second fingers in their usual position we should
know by experience that there was only one pea.   If,
however, we  cross  the fingers our experience  not only
leaves us in the lurch, but deceives us in the impression
produced by the sensation.
   The cause of the illusion proceeds in reality from the
experience of the brain which  has  been inculcated  by
practice, and in such  a case has  led us into error. • Thus
when we cross the fingers we bring the outer side of each
finger into contact with the  pea simultaneously.   But
these two sides of the fingers  are, in the ordinary position
of the  hand, invariably turned away from  each other,
and, if when  in this position, the two  sides are  touched
simultaneously, the brain knows by experience that it
can only be caused by two different  objects.
    Now this experience of the brain is not affected by
a change of circumstances,  but  remains  unaltered in
whatever position  we   place the fingers.   If we  cross
them, and hold between  them a  small ball, their position
will still be the natural one, as far as concerns the sense
of touch which the brain has  gained by experience, and
therefore, our imagination changes  the  crossed position
into  the ordinary one.    When  this  takes place our
imagination  must also convert one ball into two.
    This shows us how firmly the representation of the sur-
face of our body is  imprinted upon our brain.  The brain
knows perfectly well the natural position of every  part
of the body, and the situation of every  point of skin, a
knowledge which  it has gained  by  many years of prac- 
38           THE FIVE SENSES  OF MAN.
tice and experience.  It is also able to recognise a great
number of the movements of different parts of our body.
When the hand is moved over any object, it is placed in
many different positions with regard to the body, and
by  the  touch we obtain  a true representation of  the
object, whilst, by experience, we can tell the position of
the hand at the same time.  We can however only recog-
nise all  these positions  and movements of our tactual
limbs correctly  after sufficient practice and experience.
But as soon as we bring our limbs into an unaccustomed
position, our  power  of  recognising correctly tactual im-
pression ceases, and we  are no longer able to estimate
rightly the position  of the objects touched. 
CHAPTER III.
      The Skin's Sense of Pressure—The Sense of Temperature.

The skin not only possesses the faculty  of perceiving
contact with a body,  but it is also able to calculate the
pressure with which the contact takes place.
  If we lay an object upon  the outstretched  hand,  we
are able to determine the weight of it  by  raising and
lowering the hand, till we are sufficiently satisfied  as to
the amount of the weight.
    Our decision is founded principally upon the amount
of exertion, which the muscles of the arm are obliged to
use in lifting the weight; and as we have a distinct sen-
sation of the activity of these muscles, and as this sensa-
tion increases with an increase of action in the muscles,
we are thus able to. determine  the  difference between
different weights.    But in  such  an  experiment  the
weight laid  upon the hand acts not only by the tension
of the muscles, which are trying to support it, but it also
makes an impression upon  the skin, which we feel as
well as the tension of the muscles.  We have to do, there-
fore, with two influences which  almost invariably work
together when we lift an object, but which in a scienti-
fic  examination  of  the  question  must  be  considered 
40           THE FIVE SENSES OF  MAN.
independently.  Let us lift, for example, a weight with a
ring attached to it,  from  the ground  with  the arms
straightened at the side, then the weight acts by pressure
upon that part of the skin of the hand which grasps the
ring, and, by tension, upon the muscles  used in lifting.
The tension upon the muscles  remains the  same  with
the same  weight, but the influence of the pressure upon
the hand  may  differ  with  the form of  the  handle by
which the weight is held.   If the handle consists  of a
broad  ring, then  the pressure  of the weight  will  be
spread over a large  portion of the surface of the hand,
and each  point of the skin will  only experience a  very
moderate amount of pressure ; if, however, we take a very
small ring, then the whole pressure is concentrated upon
a small surface of skin, and we then experience much
more pressure, which  may even become painful.
    Our decision upon  the weight of  a body  is  then
guided by these two sensations  which work  simultane-
ously, and, therefore, in science we distinguish two sepa-
rate properties which correspond to these  sensations.
We designate the property of realizing  the  amount of
muscular  action the muscular sense,  and  the property
which estimates  the amount of pressure upon the skin,
the skin's sense of pressure.
    In  ordinary life the muscular sense and the sense of
pressure almost always act  simultaneously whenever we
lift an object, and, therefore, through the influence of the
sense of pressure we  are frequently subject  to illusions
when estimating  the weight of objects.   For if we lift a
heavy body when a very small portion of it presses upon
our skin,  it will  generally appear  heavier  than  if it
pressed upon a  larger surface, because  the pressure on 
THE SENSE OF TOUCH.
41
the skin is greater.  In such a case we are at least very
liable to make an incorrect  estimation of the weight of
an object.  But if, by  other means, we are  able to esti-
mate its weight correctly, then we should say  that the
body was  not nearly so heavy as it appeared to be from
the pressure it exercised, but that it was only very incon-
venient to carry.
    But we calculate  the weight of a body chiefly with
the help of the muscular sense, and the pressure is only
an accompanying sensation, which exercises a smaller
influence  upon our decision.  This is proved by some
very interesting experiments made by Ernst  Heinrich
Weber, which  we will follow.  He wished  to show the
smallest difference  of weight which  could  be felt when
the weight was lifted  by the  hand, and  he found that
19^ oz. could generally be distinguished from 20, but not
19! from  20.  This ratio of 19^ to 20 remained nearly
constant for  all weights,  i.e. 9f could be distinguished
from 10, or 39 from 40.
    In these  experiments muscular sense and the sense
of pressure are combined, and both influence the decision.
But other experiments may be mentioned in which the
sense of  pressure  alone operates,  the  muscular sense
being quite excluded.
    If we lay  the open hand upon the table, and put a
weight upon it, we are even then able to determine the
weight by its pressure, without  making any  muscular
exertion at all.   We can also distinguish in this manner
between different weights, but  it has  been  proved that
our judgment is then not nearly so accurate as when we
lift an object.  We are not then able to distinguish 19^
oz. from 20,  but at the most 14^ from 15. 
\2
THE FIVE SENSES OF MAN.
    Nature has, then, provided us with two powers which
we can use together as balances to determine the weight
of a body.   The one which we call the muscular sense is
the more sensitive balance, whilst the other, the sense of
pressure, is  much less so.   In reality, we use both these
means simultaneously in  determining the weight  of an
object.
    Many interesting discoveries  have been made by
these experiments upon the sense of pressure.   If we
wish to discover  what two weights  we can  distinguish
from each other by pressure, we should best effect it by
placing both weights quickly one after the other upon
the same  part of the skin, for  example, upon a finger
stretched  out on the table.  It  would be much more
difficult for us to  decide if we laid both weights simul-
taneously upon two  different fingers, since  we could not
turn our attention at the  same time upon  two different
parts of our skin.   But  even when we choose the same
place on the  skin, our decision depends  on the time
which elapses between the two trials.   Weber remarked
that, with  the  greatest attention, he  could  distinguish
14^ oz. from 15, if more than 10 seconds had not elapsed
between the deposition of  the two weights.    If the time
is longer, the answers begin to be uncertain, and we can
then only  distinguish those  weights  from each  other
whose  ratio  is greater.  After the  lapse  of half a
minute  for example,  Weber could  only distinguish 2
from 2,\ or 12 from 15.
   These experiments have a particular interest, because
in them an intellectual force, which we generally designate
under  the  name  of  memory, accompanies  the sensory
perception.  For the sense of pressure  ceases as soon as 
THE SENSE OF TOUCH.
43
the pressure is removed;  our brain, however, possesses
the faculty of  retaining  for  a time  the  impression
received, and if afresh pressure follows quickly upon the
first, we are able to compare the strength of both  im-
pressions.  But the remembrance of the sensory impres-
sion rapidly diminishes in strength, as may be proved by
the above  experiments,  so that we  cannot accurately
distinguish a second impression 'from the first if the time
between  them is too great.
    Weber has also examined the delicacy of the sense
of  pressure in different parts of the skin, and  made
experiments to discover whether the sense of pressure
stands in any  relation  to  the  sense  of touch—in  other
words, whether the skin's sense of pressure is developed
similarly with its sense of  locality.  This is by no means
the case.   For the sense  of pressure in the tips  of  the
fingers is not  much more  delicate than that of the fore-
arm, whilst the sense  of locality in the tips of the fingers
is nine times more delicate.  Several parts of the skin—
for example, the forehead and the skin of the stomach—
have  a tolerably  delicate sense of pressure, whilst  the
sense of  locality is not at  all remarkable, the skin of  the
stomach  in particular only possessing a  very dull  sense
of locality.
    When touching a body we not only feel its form and
the pressure produced by the contact, but at the  same
time the  degree of its heat.   In ordinary language we
say a body is cold, warm,  or hot.  This term is, however,
purely subjective,  for it is  regulated by  the impression
which the temperature of  that body has made upon  our
skin, and often, therefore, does  not agree with the indi-
cations of the thermometer.  We  call  that body cold 
4-4
THE FIVE SENSES OF MAN.
which draws warmth from our skin, that one warm which
gives it warmth, and as our skin is at a  temperature of
30-360  Cent.  (88°-98° Fahr.) the zero  of our percep-
tion of  temperature lies rather high.
    We  know that in physics the idea of cold is unknown ;
it only treats  of the single  force  of  heat, which is re-
garded  as a vibration of molecules.  Cold  is something
purely subjective, and only depends upon the  tempera-
ture of  our bodies.   Cold-blooded animals must there-
fore have a perfectly different standard of  perception to
warm-blooded animals.   The feeling  of heat only  lasts
so long as a body imparts heat to a particular part of the
skin ; if the temperature of the skin  has become equal
to that of the object touched, then the feeling of  heat
ceases.  If we place our hand,  the  skin  of  which is
generally rather cool, into water at 360 Cent.  (98° Fahr.)
then we shall experience a feeling of heat so long as  heat
is imparted to the hand.   The hotter, however, the hand
becomes, the less will be the feeling of heat.  If we  then
plunge the hand into water  at  300 Cent. (88°  Fahr.) it
will seem cold for the first moment) although  it really
possesses a moderate degree of  heat,  since  now  heat
leaves the hand and is imparted to the water.  If, how-
ever, the hand has been  cooled beforehand  in  the  air,
then the water at 300 Cent. (88° Fahr.) will  appear to us
to be pleasantly warm.  Our skin is, therefore, only a
relative  measurer of heat, and only feels absolute tempera-
tures in an imperfect degree.   If  we grasp  a  piece  of
metal and a piece of wood, which have the same tempera-
ture, the metal feels  considerably  colder than the wood,
although the thermometer would  give the  same indica-
tion for both.  Metal is  a good conductor  of heat, and 
THE SENSE OF  TOUCH.
45
extracts heat rapidly from the skin, whilst wood will do
so but slowly.  The rapidity with which  loss of heat
takes place in our skin has thus a great influence upon
the degree of our perception of  cold.
   The power of  the skin of perceiving the difference
of temperature has, by Weber, been submitted to exact
calculation.  Place, for instance, the hand, or a finger, in
water of different temperatures, and see to what extent
it is  able  to mark the difference.  Weber found  that
with  the finger he could perceive  a difference of about
£°  Cent.  (-|° Fahr.),  a  tolerably delicate  sensibility,
greater than we should have expected, since it is greater
than  that of our  ordinary thermometer.   And yet our
indications for absolute temperature are very uncertain ;
it would be impossible for us to determine accurately the
true temperature of water at 190 Cent. (6o° Fahr.), it only
being in our power to say that the temperature  lay be-
tween i6°-20° Cent. (51°-68° Fahr.) The delicacy of our
perception of relative temperature, when the trial is made
quickly, is, however, nearly equal for all temperatures up
to blood heat ;  but it is not  the same in all parts of the
body.  It depends principally upon the thickness of the
skin, for heat penetrates the thinner  skin most easily.
Thus the back  of the hand  is more sensitive than the
palm, and the eyelids, lips and tongue are most sensitive
of all.  The  elbow also possesses great sensibility, the
reason  of the well-known  practice of mothers, who, in
washing their children, will plunge their elbows  into the
bath  to try its temperature.   This may also be explained
by the thinness of the  skin, and  the  absence of fat in
that part.
    Moreover, we must  also assume that there  may be 
46           THE FIVE SENSES OF  MAN.
in the  skin  certain organs of temperature, which have
their own nerves  and are more strongly developed  in
some parts of the skin than in others.  For it has been
proved that the nerve-trunks themselves have not the
faculty of producing a feeling of heat, if heat is directly
applied to them.  For instance, a sensitive  nerve  runs
directly under the skin of the  elbow on the bone, which
causes great pain if struck.  Now if we place the elbow
in hot water we experience  a sensation of heat  in the
part immersed, not in the whole arm, although the nerve
extends throughout the arm and hand.   We experience,
however, a dull feeling of pain in the whole  arm if the
water is too hot.  Thus the nerve-trunk is irritated by
heat, which irritation, however, does not  create a sensa-
tion of heat, but pain.   If the elbow is placed in ice-cold
water this  sensation of pain is just  the same, proving
that the  nerve-trunk  can neither feel warmth nor cold.
The sensation of pain which occurs in this case is another
reason why the elbow should be so  sensitive to a great
and injurious degree of heat.
    We must, therefore, assume that-the nerves of the skin
possess certain organs of temperature which are adapted
to produce an  excitement of the nerve by heat.   Such
organs, however, have not yet been discovered, at least
none are recognised as such.  It may be that the tactile
corpuscles as well as producing tactile impressions  may
assist  our sense of temperature ;  but nothing certain can
be said on this point.  Weber has discovered the interest-
ing fact that warm bodies appear lighter than cold bodies.
If a cold  coin is placed upon the forehead of some person
whose eyes are shut and then upon the same spot two 
'  THE SENSE OF  TOUCH.
47
warm coins, the weight would  seem to him  the same,
whilst he  could distinguish cold weights  correctly.  A
connection seems, then, to exist between the sense of tem-
perature and the sense of touch, but it has not yet been
scientifically examined. 
        PART   II.
THE  SENSE  OF SIGHT.
                   CHAPTER I.
Formation of the Eye—Refraction of the Light-rays in a Lens—Path of
                 the Light-rays in the Eye.

Whilst we can only become acquainted with the pre-
sence of bodies through  the  organ of touch, when we
bring the surface of our skin in direct contact with them,
we can perceive through  the eye,  bodies at a greater
distance, by means  of what  we call light.  It almost
seems as if no limit could be set to the passage of light
through space.   We receive rays of light from stars in
the remotest spheres of the universe at an incalculable
distance, and are thus  informed of their presence.  In-
deed we are now enabled, by means of spectrum analy-
sis,  to determine their chemical composition.   Moreover,
to whatever distance a ray of light  penetrates through
space, as long as it possesses a certain intensity, it is able
to produce an impression  upon our organs of sight, and
thus opens out to us a region of knowledge, the limits of
which are far wider than those which enclose the domain
of the other senses.
    Physics  teach  us  that  light  is  transmitted  by the 
THE SENSE OF  SIGHT.
49
Ether, a substance of extraordinary tenuity, which ex-
tends throughout the universe, penetrates all substances,
exists also in empty space, and that it is produced by
vibrations  of  the ether of extraordinary rapidity.   As
these vibrations reach the interior of the eye through its
transparent organs,  they produce in us  a sensation  of
light, and  by  means of the wonderful formation  of the
eye, we are not only able to receive the  impressions  of
light emitted by bodies, merely as such, but also to per-
ceive their form, size and nature.
    Let  us now proceed to study  the  most important
parts of the eye.
    Fig. 7  represents a horizontal section of the right eye,
after Helmholtz, which will explain to us all  the  impor-
tant parts of  this organ, which nature has endued with
such wonderful delicacy.  We here  see the eye-ball sur-
rounded by a hard membrane, m n, called  the sclerotic
coat, which in living persons appears as the so-called white
of the eye, in  the anterior segment  between  the eyelids.
The membrane is rather thick and  strong, and provides
the eye with a sufficient protection from external dangers;
it is not transparent but perfectly translucent in a strong
light.   At the outer edge  of the eye it passes into the
cornea, h h, which is almost  equally thick, but  is distin-
guished by its glass-like transparency.   This membrane
rises  and thickens in the middle like a watch glass, and
also  forms the transparent covering to the eye, through
which the light passes into the interior.
    Now in the interior there follows upon the sclerotic coat
a far thinner and more delicate membrane, coloured deep
black, g, called the  choroid, because it contains  a  great
number of blood-veins.  Its black  colour, however, pro- 
50
THE FIVE  SENSES  OF MAN.
ceeds from a considerable number of black pigment-cells,
which are arranged like a mosaic upon  its inner  surface.
This membrane  joins the  iris, pp, which is  visible  in
every living eye,  and which contains a dark opening, the
pupil.  Through  this opening the light passes into the
                         Fig. 7.

interior of the  eyeball  and reaches its  posterior wall.
The black colouring of the choroid is a great assistance
to the eye's vision.   Were it absent, the light, which falls
upon the background of the eye would be  reflected as
from any other bright surface ; a number of rays of light
would thus be  distributed irregularly in  the eye, and 
THE SENSE OF SIGHT.
51
so dazzle it, besides considerably injuring the distinctness
of the image in the eye.  The iris is also provided with
black pigment similar to that of the choroid, and assumes
a lighter or darker colour, according to  the number  of
these pigment-cells. In a blue eye the iris has only a thin
layer of pigment-cells on its  posterior surface, and has,
therefore, a bluish appearance from the outside.  The
greater the number of pigment-cells the darker is the
colour of the eye ; this variation in the amount of pig-
ment is the cause of the different shades which we see in
eyes, from the darkest brown to the lightest blue or grey.
In all races of men individuals may be found  in whom
this pigment is wanting, not only in the eye, but through-
out the body,  in the hair and  skin ; such people are
called Albinos.  The want of this pigment in the eye in-
jures the sight in a remarkable manner.  The iris has a
whitish-red appearance,  the pupil is generally bright red,
and the dazzling  effect of the daylight causes  a winking
of the eyelids, which is an endeavour to replace the pro-
tection of the pigment which  is wanting.  We also find
albinos among  animals : for  example,  the white  rabbit,
upon whose eyes  many interesting observations  may be
made.  Thus, for instance, a delicate reversed  picture of
the surrounding objects  may  be seen upon the posterior
surface of an eye freshly extracted from such an animal,
shining through the transparent coat of the eye, whilst
this is not the case with the dark-coloured eye of another
animal.
   Now within the choroid is situated a delicate mem-
brane called the retina (fig. 7, i).   It forms the  continua-
tion  and extension of the optic nerve, which, like the
stalk of an  apple, penetrates  the  posterior side of the 
52           THE FIVE  SENSES  OF MAN.

eye in a somewhat slanting direction with the side of the
nose,  pierces the sclerotic coat  and choroid,  and  then
spreads out on all sides, so as to  form,  by means of the
peculiar terminations with which  it is provided, a kind of
nerve-carpet, which is  the most  delicate  sensory organ
created by nature.  The retina, towards the front of the
eye, touches with its edge the outer circumference of the
iris, and lies quite open to the transparent interior of the
eye, so that the rays of light  fall  directly upon  it, and
create in it an impression of light.  Before, however, the
rays of light reach the retina, they pass through a num-
ber of transparent  organisations which are situated in
the circular hollow of the eye, and are  here ingeniously
fitted together like the parts of a microscope or telescope,
with  this difference  only, that they are packed tightly
together, so as to allow  no trace of air between them.
The outer covering of this transparent body is the cor-
nea, mentioned above ; then, towards the interior, follows
first the aqueous humour, w, secondly the crystalline lens,
L ;  and thirdly, the vitreous humour, G.  The aqueous
humour fills, as we  have seen, the space  between  the
cornea, the iris and the lens.   Directly behind the  iris
lies the crystalline lens, L, so well known, and resembling
a very thick burning-glass.  It is more convex upon its
posterior than upon its anterior surface; in a living eye
it is as clear as crystal, and consists of  a somewhat soft
substance, which becomes harder towards the back of the
lens.  It has been discovered that this substance is  not
the same throughout, but that it  consists of small vessels
which are arranged in intricate lines, thus  giving  a sex-
radiated structure to the lens.  The lens, however, does
not lie unconfined behind the iris, but  is enclosed  in a 
THE SENSE OF SIGHT.
53
transparent capsule, which, again, is held in position by a
peculiar  elastic  membrane, k k and e e, of which more
hereafter.  It is sufficient to say here that this membrane
joins the circular hyaloid membrane, which  lies  directly
upon the retina.  The spheroidal space between  the lens
and the retina is filled by the vitreous humour,  a clear
gelatinous  mass, which  is directly  surrounded  by  the
hyaloid membrane.
   The rays of light, therefore, which fall upon  the eye
penetrate the cornea, the aqueous humour,  the crys-
talline lens, and the vitreous humour, before they reach
the retina, and on  their way are  refracted in such  a
manner  that they  unite into  a  distinct  picture  upon
the background of the eye.
    It is  well known that with the  aid  of a glass lens
we can throw  a representation of  any object  upon  a
screen.   If, for example, we  take the front convex lens
out of a pair of  opera-glasses, and, holding it opposite a
window,  place  a piece of paper behind it to  act as a
screen, a small reversed image of the window will appear
upon it, which, upon holding the paper at a certain dis-
tance, will become clear and distinct.  Again, the camera
obscura is well known, which in its simplest form  consists
of a box, in one side of which a convex lens is  fixed in
an  opening, while the opposite side consists of trans-
lucent paper, or a  piece  of  ground glass  on which the
picture is formed.  In principle the  action of  the  eye
resembles that of a camera obscura, which is commonly
used  in  the  production of photographic  pictures;  but
the eye, as we shall see, is in many respects much more
perfect than the camera obscura.
   The  manner in which a  convex  lens is  able to 
54           THE FIVE SENSES OF MAN.
produce an  image of an  object is seen in fig. 8.  Let
the arrow A B represent an  object at a certain distance
from the lens, F and F' represent the foci of the lens, the
distance of which is easily found by allowing a  sunbeam
to fall  upon  the lens, and finding  the  point of con-
vergence upon a screen, which with a strong glass is
very bright and  hot.  In the  focus  all the rays unite
which are parallel to each other, and fall perpendicularly
upon  the  lens; and, on  the other hand, when rays of
light  fall  upon  the lens from the focus they take a
parallel direction  on the other side.  The  upper point B
                        Fig. 8.

of the object, amongst others,  sends a ray upon the lens
which  passes  through the centre O.   This  ray  suffers
no  refraction, because  the lens  stands  in the  same
relation to it  as  if it were  a  pane of glass bounded by
parallel surfaces.  A second ray passes  through the
focus F, strikes the lens, and is continued  on the other
side in a  line parallel to the line f'of, which is called
the optical axis.   Both  rays meet in  the  point b, and
here, therefore, an image of the point of the arrow is
formed.  The image of  the  head  of the  arrow a is
formed in exactly the  same manner, as may be seen
from the figure, and thus a reversed image  of the object
is formed, which decreases in size  the further the object 
THE SENSE OF SIGHT.
55
is  removed, and which, when  the  object  stands at an
immense distance, as, for instance, the sun, contracts to
a single point—the focus.
   Fig. 9 shows a camera obscura as it is used by pho-
tographers.  In the brass tube h i is the lens; g is  the
ground glass  plate  upon which  the image is  to be
received, and the case a  b consists of two parts, which
slide  in and out, and  enable us to find  the  position of
the image for objects  at different  distances.  Besides
this, there  is a  screw on the  brass tube  which  allows
the lens to be moved backwards and  forwards, so that
the picture  may  be sharply defined  upon  the  glass
plate.  We  shall see that the  eye also possesses  an
apparatus which serves  the purpose of forming  distinct
images, but which acts in a very different manner.
   The  refraction  of the rays  of light in the eye  is
much more  complicated than in a simple convex lens;
for, in the eye,  the ray  of light passes through several
substances,  and is  refracted  at the  surface of  each
substance.   The common result, however,  produced by
all  these refracting media of  the  eye is exactly the
             4 
56           THE  FIVE SENSES OF MAN.
same as in a simple convex lens, and  a small inverted
image of the external object is formed upon the retina,
which,  as  we have  already seen, may  be  plainly per-
ceived shining through  the extracted  eye  of  a white
rabbit.   There are three principal surfaces by which the
rays  of light are refracted :  the outer  surface of the
cornea, the anterior and  posterior surfaces of the lens.
The  most powerful refraction takes place  at the outer
surface of the eye, because here the rays of light pass
from the  rarified  medium  of the air to  the  dense
medium of the  cornea and  aqueous  humour,  both of
which possess about the same refractive power.   This
power  is  greater  in  the lens than   in  the aqueous
humour, or in the vitreous humour, so  that in passing
through the eye the rays of light are still more strongly
deflected inwards.  The refracting  power of the lens is
increased by the fact of its consisting of concentric layers,
the  inner  of  which refract more powerfully than  the
outer.  It  has been  calculated that a  greater effect is
thus  attained than if  every  part  of the lens had the
same refracting power  as the centre of the lens.
   We now see  clearly that  the  image  formed  in the
eye is due to causes somewhat different to those which
produce the  image in the  camera obscura.  In  the
latter we have  only to deal with  a  single  refracting
body surrounded by air.   In  the eye, on the contrary,
the ray of light  passes from the air into a number of
refracting  substances  without  passing  again into  the
air, the picture here being formed behind  the vitreous
humour, upon the  retina.  The path of the refracted
rays   is, therefore, much more complicated in the eye
than in a glass lens ; but still  the  analogy  is sufficient 
THE SENSE OF  SIGHT.
57
for  a general examination of the  path of the rays of
light in the  eye.
    The  distinctive  feature  of the  central  point  of a
convex lens  (fig. 8) is the fact that rays pass through
it  without suffering refraction.    In optical  language
this point has, therefore, been called the optical centre,
and in a  double  convex lens of equal  curvature  this
point  coincides with  the  geometrical  centre.   It  is
different,  however, in  a double  lens  of unequal curva-
ture,  and  still  more  complicated  in a  system of  re-
fracting media, like those which  we meet with in the
eye.   From  the  mathematical calculation  of Listing,
it appears that even  in a system of this kind a point
exists  through which  the  rays  pass  almost without
suffering  refraction;  this  point,  which  is  called  the
optical centre of the eye, lies in the crystalline  lens, not,
however,  exactly  in its centre, but  between that point
and the posterior surface of the lens.   A luminous ray
passing through  this  point reaches  the  retina without
refraction, at least, with such a slight refraction, as to be of
no real consequence.   In fig.  io k  is the optical centre
                        Fig. IO.

of the eye  let A B be an object, then  the  rays A k a
and B k b will reach the retina without refraction.  Now
the size  of the eye has been so fixed  with regard to
the refracting  media,  that within  the limits of vision 
58           THE FIVE SENSES OF MAN.
the image of an object is formed nearly at the distance
of the  retina; and,  moreover, the  eye  possesses the
property  of  adjustment  for  different  distances.   If,
therefore, the eye  is  adjusted  to  receive  an impression
of the object A B, then all  the rays emanating from A
will unite in a point  a upon the retina, and those from
B  in  the point b,  and form upon the  retina a small
reversed  picture a b.   It is evident from the  figure
that all rays which  do  not pass through the optical
centre are refracted so as to unite into  an image upon
the retina.1
   When we look at a  very distant object, a star  for
instance, the rays fall parallel  upon  the  eye, and since
they  are  united to a single point upon the  retina,  it
follows  in this  case that  the focus of the media  of the
eye falls exactly upon the retina.   So perfect is, there-
fore, the  formation of the  eye, that  the form of the
refracting surfaces and the  refracting power of the eye
are exactly  proportioned to the  distance between the
retina and  the  outer  surface of the  cornea.   This
is, of course,  only the  case with a  normal  eye, the
relative  proportions  being  different in a short-sighted
or long-sighted eye.
   In this  manner images of all kinds  of  objects are
formed  upon the  retina.   The iris  here plays an im-
portant  part, since it increases the  distinctness  of the
image.    It  serves as a  kind of  screen to  the eye,

   1 In order to trace exactly the path of a ray in the eye, two optical
centres must be found by calculation, a line must then be drawn from the
luminous object to the first optical centre, and one parallel to it from the
second centre to the retina.  These two centres are situated very near to
each other, and can practically be taken as a single centre. 
THE SENSE OF SIGHT.
59
covering  the  edge  of the  lens and excluding  the
marginal rays, which, in every lens, render the picture
indistinct, because their focus  does not exactly coincide
with that of  the  rays  which  pass through the central
part of  the  lens.   A similar  screen  is used  in all
optical  instruments,  in telescopes, miscroscopes, and
in  the   camera  obscura;   otherwise  the  pictures  are
seen with indistinct  edges.  The iris, however,  is  a
screen of a much more perfect description than  those
which  are used  in our instruments.  It not only  ex-
cludes the injurious marginal rays, but it also regulates
the  amount of  light which is sufficient and  necessary
for  the  eye.   When  we  direct  the  eye towards  a
luminous object,  the  sky  or a  flame, the pupil con-
tracts,  and  enlarges   considerably when we look  in
the  dark.  This contraction may be easily observed
 if a person is placed  before  a bright window with his
 eyes shaded  by  his hand, and the hand  is suddenly
 removed.  The  pupils are considerably larger  in  the
 twilight than in  broad daylight.  In  this  manner the
 quantity of light which penetrates the eye  is regulated,
 for  the  smaller the pupil the smaller is the quantity of
 light which penetrates it from  any one point, and the
 larger the pupil the  greater also  will be the quantity of
 light.   Night-birds, such as  owls,  are remarkable for
 their wide  pupils, which enable them to see well  in the
 dark, whilst  they shun the daylight because they are
 blinded by it.
     If we return once more  to our comparison  of the
 eye  with  the   camera obscura,  we  are  struck with
 another advantage which  it  possesses  over  the  latter
 instrument.   This advantage  lies in the fact that the 
6o
THE FIVE SENSES OF MAN.
background of the eye is a concave surface, whilst the
surface  of  the ground  glass  plate which receives the
image in the camera obscura  is plane.  The advantage
to the  eye is here twofold,  which will immediately
strike anyone who  has  examined a  picture  in  the
camera.
   The image which a convex lens produces of a large
body with  perpendicular lines, as, for instance,  the
arrow  in   fig. 8, is not  perfectly perpendicular,  but
slightly curved, and this curvature may  be  exactly
calculated  by drawing a circle round the centre of the
lens  at the  distance of the  picture.   Let us suppose
the  whole  field of  vision  to be  depicted  by a  lens,
then the  image  formed  by  the lens  is not  thrown
upon an  even  surface,  but  upon  a concave  surface,
whose centre  lies in the lens.   This  accounts  for the
lines towards the  edges of  the picture  in  a  camera
obscura never being  perfectly  perpendicular, but always
more or less  curved.  This may be seen very plainly
in photographs  where the  vertical side  of a house is
represented near the margin.  This defect is not very
great if the image lies near the optical  axis (F  F'), but
as soon as  the  images  are  situated  at too great a
distance from it, they appear distorted and indistinct.
This  is the  reason why  the  size  of photographic
pictures  is  limited,  and  that  it  is only  possible  to
represent a small portion of  the field  of vision.   It is
different with the  eye,  since it possesses  a concave
background upon which the field of vision is depicted,
and with which the curved form  of the image coincides
exactly.  Thus  the  defect  of the  camera  obscura is
entirely avoided;  for  the eye  is  able to embrace a 
THE SENSE OF SIGHT.
61
larger field of vision, the margins of which  are  depicted
distinctly and  without distortion.  If the retina had  a
plane surface, like the ground glass plate in a camera,
it  must  necessarily be  much larger than  is really  the
case if we were to see as much ; moreover, the central
portion of the  field of vision alone would  give a good
clear picture. 
62
THE FIVE SENSES OF MAN.
                   CHAPTER  II.

      The Adjustment of the Eye—Short Sight and Long Sight.

WE  know from experience  that  we  are able to see
distinctly objects at  different distances  from our eye.
But on careful observation we shall find that we cannot
form  simultaneously  a  perfect  picture  of  objects  at
different distances  from  us.   Suppose that  we are in
a room at a little, distance from the window, and that
we then hold up a finger a few  inches before one eye,
and  close  the other; if we  now  fix the eye  steadily
upon  the finger so as to see it distinctly, the window
in the background will be seen  indistinctly.   If we fix
our eyes  upon  the window  frame so as  to  see it  dis-
tinctly, then the outline of the finger becomes indistinct,
so that we can  see distinctly at  will either the window
frame or the  finger.  Thus  the  eye is adjusted for
objects at different distances, and  this property of the
eye is called its  adjustment.   When our  eyes are wan-
dering over objects at different  distances from us,  this
adjustment  is   constantly at  work, although  we  are
quite  unconscious of it ourselves.
   In fig. 8 the image of the arrow A B  is depicted at
a b.   If, however, the object A B is  at a greater distance, 
THE SENSE OF SIGHT.
63
then the image a b will  be formed nearer the focus F',
at the same time becoming smaller.  The position of
the image  changes with  that  of the object.   When,
therefore,  a picture is  about  to  be  produced  in  the
camera obscura, the inner case b containing the ground
glass  plate g  must be moved forward into  the outer
case a for distant  objects, and  drawn  out  again for
near objects.   By means of the screw r the lens may
be  moved a little  backwards and forwards, which  will
still further ensure the  clear delineation of the picture.
This  also  applies to the  eye.   The  object A B in  fig.
10  may be depicted quite  clearly at a b in the eye.
If  the  object  approaches  or  retreats whilst the  eye
remains in the same position, the picture  of the object
will become indistinct, because in  the- first case it would
be  formed before, and in the second behind the retina.
                        Fig. n.
Let  us  suppose a  luminous point a,  fig. 11, to be  at
a certain  distance from  the eye, then a distinct image
of this  point  will be  formed  if the retina  lies at the
point of convergence c; if, however, the  retina is at a
greater  distance at gt g,„  so  that  the  focus  is formed
in front of it, then, instead of a point, a blurred circle
is produced.  A similar blurred image would be formed 
64          THE FIVE  SENSES OF MAN.

upon  the  retina if it lay at/,/,, in front of the image,
and if all  the  points of an object were not  depicted
upon  the  retina at their foci, the whole object would
appear indistinct  and blurred.  The  experiment, which
we  mentioned  at  the commencement of the  chapter,
may be easily explained in this manner.  When the
eyes are fixed upon the finger the image of the window
frame behind  it does not  fall  upon  the retina at the
focus of the rays it emits, and conversely, if the window
is  fixed by the eye, the finger appears blurred for  a
similar reason.  The image of the body which we see
distinctly  always falls upon the retina, whilst the image
of the body which  we see indistinctly  would  fall in
the first  case  before, and  in  the second behind the
retina.  From  this  the  important inference  is to be
drawn, that the eye possesses  a power of adjustment,
by  means of which objects at  various distances are
depicted distinctly upon the retina.
   We  might very well be inclined  to think that the
method employed for this adjustment of the eye should
be similar to that employed in  the  camera  obscura.
It was formerly considered as possible that in the process
of adjustment the retina moved  backwards and forwards
by the  elongation of the eye-ball, through the action
of muscles in the socket,  so  that the background of
the eye could with equal ease be moved in  either direc-
tion.  This supposition  has, however, been  refuted, and
it has been shown that  the method  employed-is quite
different to that of the  camera obscura, and so perfect
that  it cannot be imitated  artificially. Let us suppose
the eye  to be adjusted for a great distance, such as
some point upon  the horizon, or  in the sky.   The rays 
THE SENSE OF  SIGHT.             65
from this point will penetrate the eye in almost parallel
lines, and will, therefore, unite  at the focus of the eye,
which,  in a normal eye, lies exactly in the central point
of the  retina.  If, however, the object we are looking
at is nearer the eye, then, according to the laws of optics,
its  image will fall beyond the focus of  the eye, and,
therefore, behind the  retina.   The  eye, however,  is
enabled  by  adjustment to bring  the  point at  which
the image  is  formed back to the  retina,  which  can
easily  be explained  by a greater convergence of the
rays which penetrate the  eye, or in other words, they
must  suffer  greater  refraction in order that they may
meet  sooner  than  they would if  they suffered less  re-
fraction, and  thus produce a distinct image upon the
retina.
    The   reason why  the light-rays suffer  a  stronger
refraction  when the eye  is fixed upon nearer objects
is, that it  has been  observed, that when the eye, after
viewing a distant  object,  views a  nearer one, the crys-
Fig. 12.
talline  lens  becomes  thicker  by an  increase  in   the
curvature of its  anterior  surface.   Fig.  12 shows  the
alteration in  the curvature of  the  lens  when  a near
object is looked  at,  after a  drawing  by  Helmholtz. 
66           THE FIVE  SENSES  OF MAN.
The  left portion  of  the  figure  represents  the  eye
adjusted  for  distant  objects,  whilst  the  right  half
represents it adjusted for near  objects.   We here see
that the anterior  surface has  increased  in convexity,
and is  pushed forward  nearer the cornea, taking the
iris with it.  This advance  of the iris may  be easily
observed, by watching from the side the eye of a person
whilst the distance  of objects he looks at increases.
    The stronger  refraction of the  rays, which takes
place when we look at near objects, can undoubtedly be
explained by the  motion  of  the lens which has  just
been  described.  According to the laws  of optics the
refraction of the light-rays caused by a convex  lens
increases with its increase of curvature.  The distinctive
feature between  two lenses of equal size, one of which
is flat and thin, the other thick and of greater curvature,
is that  the focal  distance is greater in the former than
in  the  latter, "or  in other words, the latter produces a
greater  convergence of the  light-rays.   The  case  is
similar in the crystalline lens during adjustment.  When
the same object of our vision is at a small distance
the  lens is  more  convex  than  when the distance is
greater, and  its  refracting power is so much increased,
that  the image is again formed upon the retina  in-
stead  of behind  it.  The  curvature of the lens,  of
course, differs  with every variation  in the  distance  of
the objects seen, increasing  with their approach.   In
ordinary vision these alterations of  the  lens are  per-
formed as rapidly as the  changes in the direction  of
our vision,  and   through constant  exercise  they  are
performed with  such exactness, that we are able  to
form  a distinct image of every object upon the retina. 
THE SENSE OF  SIGHT.              67
    We .must now enquire further into the mechanism
by which this change  in  the  form of the lens is pro-
duced.  It is produced by means  of a muscle, which
is shown  in  fig. 7.  It lies at  t where the choroid runs
into the  iris.   Its fibres  are not directly  connected
with  the  lens,  but its formation  is somewhat  com-
plicated, and, therefore, its action in  producing adjust-
ment has long been a matter of discussion.   Helmholtz
has explained it in the following manner.  The lens is
attached at its edge to a radiating zonule (zonula  zinii),
which radiates outwards and maintains the lens in tension
(as represented  in the figure  by the line  e).   Now,  at
the point  where the fibres  of this ligament are  attached
to  the  outer membrane  of the eye,  are also  attached
the fibres of the muscle1  which produces adjustment.
In viewing distant  objects this  muscle is  relaxed, and
the zonula,  by means of its  elastic  tension,  extends
the lens in a radial direction towards the edge, thereby
diminishing  its  thickness  and  flattening its curvature.
In adjusting the eye  for  near  objects  the zonula  is
drawn  inwards  towards the edge of the  lens by the
muscle; its  tension, therefore, decreases, and  the lens
contracts  by means of its elasticity,  and consequently
its thickness  and  exterior curvature  are increased.
We experience  thus no  muscular exertion in viewing
distant objects,  the elastic tension  of the lens and  of
the zonula  alone  preserving  equilibrium.  In looking
at near objects, on the contrary, a muscular effort  takes
place  in the  eye,  and, therefore, in viewing near objects

   1 Ciliary muscle, or, Musculus tensor chotoidece—so called from its
drawing* forward  the edge of the choroid. 
68           THE FIVE SENSES OF  MA.tf.
the eye is undergoing muscular exertion, but in viewing
distant objects it is in a condition of rest.
                                                   1



                        Fig. 13-

   The question as to the means by which the thicken-
ing of the crystalline lens, when viewing near objects, can
be recognised, is still unanswered.   The contraction  of
the iris with which it is connected is, indeed, easily
observed, and at the same time the pupil  may be  seen
to decrease  in size.  Still this does  not perfectly prove
the increase  in the  curvature of the lens.  The action
may be detected with certainty by an observation  of
the image reflected  by the eye.
   The small  reflected  image of surrounding objects
which may be seen  in the eye is well known to everyone.
These images, which are very  distinct and perceptible
are produced upon  the anterior surface of the cornea,
and present the appearance of small, upright and some-
what distorted pictures, exactly similar to  those which
are produced upon reflecting balls often seen in German
gardens.  We have here  to deal with one of the proper-
ties of convex  lenses.   Upon a closer examination  of
the eye reflected pictures will also be observed upon the 
THE SENSE OF SIGHT.
69
anterior and posterior surfaces of the lens, which are not,
however, so distinct as that upon the cornea.  In order
to see them the following experiment may be made in a
dark room.  Let a candle C (Fig. 13) be  placed near to
the eye  under observation A.   The  eye  of the  person
making the observation will be at B, whilst the  eye A
looks in  the direction of/  B will then see  three reflected
pictures  in A, which  will have  the ap-
pearance represented in fig. 14.  A dis-
tinct image {a)  formed upon the cornea
will be  first seen against the dark back-
ground of  the pupil; then  an  image (b)
on the anterior surface  of  the  lens, also
upright and somewhat larger, but much
fainter ;  and  thirdly a smaller image  (c),
reversed but rather more distinct, on the     Fig. 14.
posterior surface of the lens.  The last
image is seen reversed because it has been reflected by a
concave mirror, whilst  the first  two were  reflected  by
convex mirrors.
    Now a marked  difference  will be  observed  in  the
image b on the anterior surface of the lens as soon as the
distance of the  object viewed  changes  from /to n.   In
the adjustment of the eye for the near object the picture
becomes smaller and more distinct,  but  in adjustment
for the distant object the picture appears larger and more
indistinct.  The inference  to be drawn  from this is, that
according  to the laws of optics the curvature of the sur-
face of the lens must have increased when the eye was
fixed upon the near object ; for the greater the curvature
of a convex lens, the smaller will be the picture which is
reflected.  The  two  other  pictures, a and c, on the con-
 
70
THE FIVE SENSES OF  MAN.
 trary,  remain the same during the  process  of adjust-
 ment,  and it follows that the curvature of the  cornea
 and of the posterior surface of the lens must remain the
 same.
    If we bring an object very near to the eye  and try to
 see it distinctly, for instance the print  of a book, we shall
 find that there is a limit beyond which  we cannot see
 anything distinctly, however great an effort  we make.
 This point, which is at a distance from the eye of about
 4 to 5 inches, is called  the  near-point of the eye.   By
 means  of a  very interesting  experiment,  known  as
 Scheiner's, the distance of this point for every eye may
 be exactly determined.
    Take a piece of cardboard and  make two small holes
 in it with a needle at a distance of about ^ of an  inch
 apart.   Now, if the holes are held close to the eye, and
 the needle behind the holes, two  needles will be  seen
 side by side,  if the needle is held very near to the piece
 of cardboard.   But  if the needle is gradually moved
 further off, a point  will be found where the two images
 combine into one, and that  point is the  near-point of
 the eye.
   The  accompanying  figure (fig. 15) will explain the
 phenomenon.   The eye is represented by the lens b, and
 before  it is placed a screen in which are two holes e and
/.  The  luminous point a sends  two pencils of  light
 through these holes to the lens, which combine ate, form-
 ing an image just  at the  point where the screen  n n
 represents the retina.  If, however, the retina were placed
 at the point m m, then two pictures of the point a would
 be formed at / and q, which would both be less distinct
 than the  image formed at c.   Now this is the case  in the 
THE SENSE OF SIGHT.              Jl
experiment mentioned above when the needle is held so
near the holes that the two  pencils have  not united
upon the  retina.   The eye then, even  with the greatest
effort, cannot increase the  refracting power of  the  lens
sufficiently to  adjust it for  the needle.   If, however, the
point is found at which it  is  possible  to see one image
only, this point will clearly be the near-point.
    Moreover,  it is interesting to observe  that when we
perceive two  images p and  q,  the image  q,  which is
                        Fig. 15-

formed by  the lower hole / appears  above somewhere
near Q,  whilst the  picture p appears below somewhere
near P.   For if the hole / is covered Q disappears, and
if e is covered  P disappears.  The reason is simple.  The
picture formed upon the retina is, we know, an inverted
one.   Therefore what is depicted upon the lower part of
the retina  is found  in  the upper  part  of the field of
vision, and that which  is depicted on the right, in the
left, and vice  versa.   Therefore in  our imagination the
retinal picture q is transposed to Q, and the picture/ to
P.
    For the success of the experiment it is important to
take care that the distance between the two holes should
not be  greater than the diameter of  the pupil, else  the 
 72           THE FIVE SENSES OF MAN.

two pencils of light will not  both be able to penetrate the
eye.   By an instrument  founded on the principle of this
experiment,  the  Optometer,  in which one  cylinder is
moved within another, as in a telescope, the near-point
may be determined with perfect accuracy,
   Now, in  most cases, there is also a limit to distinct
vision when the  eye is adjusted for distant objects.  It
is indeed  possible  for  persons with  perfectly healthy
clear-sighted eyes to see objects with clearly defined out-
lines at  any distance within the horizon, and they are
therefore able to  adjust their eyes  for infinite distances.
Many people, however, cannot do this, that is to say, they
are more or  less short-sighted, and  there is a  certain dis-
tance where their distinct vision ceases.   This  point is
calied the far-point of the eye.
    The  far-point of the normal eye is  at an infinite dis-
tance.   When the parallel rays of an object at an infi-
nite distance, a star for instance, fall upon the eye, they
unite in the focus  of the  eye, which in a  normal eye
would be exactly at the retina.  It is  evident, however,
that this ideal formation is seldom met with, from the
fact that people imagine that a star radiates light, whilst
in reality it is  only  a  luminous  point.  In the  short-
sighted  eye, the focus lies  more or less  in front of the
 retina within the vitreous humour;  this is caused by the
diameter of the eye, and particularly the lens, along the
line  of  its axis being too  great in comparison to the
refractive power of its media.   Now if a luminous point
is moved towards the eye, then according to the laws of
 optics, the image will be removed from the focus towards
 the  retina, which, however, it will  not reach  till  it has
 been moved to  within  a certain distance from the eye. 
THE SENSE OF  SIGHT.
73
For very short-sighted persons it is well known that this
distance is exceedingly small, so that  they are not even
able to form distinct images of objects in a room.  The
far-point for the eye of a short-sighted person will there-
fore  be the  point  where the  image of objects is im-
printed exactly upon  the retina, whilst every point at a
greater distance will only produce a hazy image.
   Short-sighted persons make use of spectacles consist-
ing of concave glasses  to enable them to see distinctly at
a distance.   These glasses have the peculiarity of increas-
ing the divergence  of the rays of  light, so  that the con-
vergent rays  which fall upon them will intersect each
other so as to form an image at a greater distance.   Fig.
 16 shows that rays which converge at the point B will,
by a concave glass, be made to converge at the point A.
Let us suppose that in a short-sighted eye the image of
a  luminous point  has been  formed at B,  and that  the
 retina lies behind  it at A; now by making use of a pair
of concave spectacles, the image will be formed upon the
 retina.  The more short-sighted  a  person is, the more
 concave must his  spectacles be,  to increase  the diver-
 gence of the incident rays  to such an  extent  that the
 image may be formed upon the retina.
    Another very  frequent  defect  in the  eye  is long-
 sightedness.   The structure  here  is  the opposite of that
just described, and consists in the focus of the eye lying
 behind the retina.  When, therefore, parallel rays, as for
 instance those from a star, fall upon the eye, they do not
 unite until they have passed beyond the retina.   Persons
 so affected, however, are generally able, by adjusting the
 eye, to move the  image forward,  and thus to see distant
 objects distinctly.  Near objects however they  see in- 
74
THE FIVE SENSES OF MAN.
distinctly because they cannot sufficiently adjust the eye
so as to  form the image upon the retina, therefore the
near-point for them must be at some distance from the eye.
Long-sighted persons consequently use  convex  glasses
to enable them to see near objects distinctly, for a con-
vex lens converges  the  rays, and  brings them sooner to
a focus, and therefore moves the image forward.  When
a long-sighted person wishes to read,  he must either put
on his  spectacles or hold the book at some distance till
it reaches his near-point, whilst on the other hand a short-
sighted person can  read  quite well without spectacles,
if  he only  holds the book near  enough  to his eyes.
                        Fig. 16.

Short-sighted persons can see objects when placed close
to the eye  even better than those who  enjoy normal
sight, because their near-point lies nearer to the eye, and
all objects apparently increase in size as they approach
the eye.
   Another less striking defect, which is seldom met
with in a normal eye, but is often strongly developed in
cases of illness,  is caused by  the unsymmetrical forma-
tion of the interior of the eye.  In looking at the con-
centric  circle (fig. 17) with one eye,  we shall  observe
that the  lines are never all distinct at the same time,
but, as we adjust the eye,  two opposite sections will
alternately appear  clear and distinct as their  positions 
THE SENSE OF SIGHT.              75
change.   From this it follows that  the curvature of the
eye is not exactly the same in a horizontal and vertical
direction.  Consequently, rays from vertical lines have a
different focus, or point of convergence, to rays from hori-
zontal lines.   Thus, for instance, in the adjustment for
                        Fig. 17.

horizontal  lines, vertical  lines at the same distance will
appear indistinct, and vice versa.   This is thej:ase in the
figure of the concentric circle, since the horizontal and
vertical directions of the lines merge into each other.
    One other circumstance  may be mentioned which 
7^           THE FIVE  SENSES OF MAN.

takes place in the refraction of light in the eye.   It is
well known that inferior opera-glasses and telescopes
represent objects with coloured edges, and, in order to
avoid this defect, so-called achromatic lenses are made
use of, which are constructed so as to prevent the resolu-
tion of light into coloured rays.   Every common lens
gives images with coloured edges, because the points of
convergence of the coloured rays, of which white light is
composed, do not coincide,  the  refrangibility  of each
colour of the spectrum  increasing from red to blue.
                        Fig. 18.

Thus we  cannot at  the  same time see a red line and a
contiguous blue  line distinctly;  for  when  the eye  is
adjusted for  the red line the image of the blue one lies
in front of the  retina, and when the eye is adjusted for
the blue line the  image of the red one  lies behind the
retina.  Now the eye is not a perfect  achromatic instru-
ment, although objects  do  not appear with  coloured
edges in ordinary vision.  If we  look at the sharp  edge
of a dark object against a white  surface, as, for instance,
the horizontal bars of a window frame against  a cloudy
sky, the uppei  edge  will appear of a yellow tinge and 
THE SENSE OF  SIGHT.
77
the lower one of a blue tinge, particularly if we do not
carefully adjust  the eye for it.   The coloured rays are
seen here, as in optical instruments, because the resolved
rays are not  hidden by  the edge of the window frame,
whilst within a  white  surface the  rays  overlap and
reform white light.
    From  an insufficient power of  adjustment bright
objects, if  seen against a  dark  ground, at a certain
distance from the eye, will ap-
pear  to  be  surrounded  by a
coloured fringe, which causes
the  light surfaces  to appear
larger than  they  really are.
This  phenemenon   is  called
Irradiation.   If, for instance,
we  look  at the  two  equal
squares in fig. 18,  the black
one on a light, and the light
one on a dark  ground, at a
little distance from  the  eye the  latter will look larger
than the former.  In fig. 19 the two white squares, when
seen from a  little distance, will appear to run  into each
other, and to be joined together by a white bridge, since
the resolved rays overlap.  It is a well-known  fact, that
people look  larger in light clothes than in dark, which
may also be explained as the effect of irradiation.   This
circumstance is  not unknown in the art of dress, but is
carefully studied.  That  a black dress contributes to an
elegant figure, ladies know very well.
 
78           THE FIVE SENSES OF MAN.
                   CHAPTER III.

 Recognition of the Interior Portions of the Eye—The Ophthalmoscope.

The  retina is the point where  the  physical  process  of
vision passes  into the physiological process.   Until  it
impinges upon the retina, the  light which penetrates the
eye has  only undergone physical  changes,  consisting
chiefly in refraction, the last perceptible result of which
is the production of the image upon the retina.  From
this  point  the  process  passes  from   our  immediate
observation,  and  the  difficulty  of  discovering   its
character increases at  each step.   The image upon
the retina is reversed,  and  yet  we see every object
in   the field  of vision  upright.    This is  the  result
of  the  experience,  which   we  have   acquired   from
childhood, in  the exercise of the organ of sight.  The
point  A  (fig.  io), which  is on  the  right, is  imprinted
upon  the left  portion of the  retina, and we  therefore
know by experience that a ray coming from the  right,
must strike  the left portion of the retina; and because
we always imagine the objects we see to be  external to
ourselves, we must do so by unconsciously following the
line a  A, through the optical centre k.  In this manner
the eye  projects a  uniform   field of vision,  which  is 
THE SENSE OF  SIGHT.             79
obtained by drawing, from every point of the retina out-
ward, straight lines  through the  optical centre of the
eye, which  lines  will  terminate  upon a  convex sur-
face.
    This is really the manner in which the eye interprets,
in all cases, its sensations of sight. For luminous appear-
ances may  be produced, without  our perceiving  any
external object,  but  merely  a part of the  eye or an in-
ward irritation ;  and yet, in the same manner, we imagine
them to be external to ourselves.
    If we shut the eye and press the head of a pin  upon
the outer edge  of the eyeball, we shall see in the  dark
field of  vision a white or coloured spot of light, which
has the same form as the compressing body :  it will be
seen upon the left side of the field if the  right side  is
pressed, and upon the upper half if the lower is pressed,
and vice versa.   The retina, therefore, extends as far  as
the part which  projects beyond  the socket of the eye,
 and can be irritated by pressure.  It is well known that
 when the eye is struck, a cloud of sparks is seen, which
 is caused by the mechanical  concussion of  the retina.
 These  luminous  images, often perceived involuntarily,
take, speaking  scientifically, the form of the body pro-
ducing the pressure ; at the same  time we observe the
relation between the  position of  the irritation and the
 position of the sensation of sight.  We transpose a  point
 on the  left side  of the retina to the right, because we
 imagine that a ray of light has penetrated  the eye from
 the right, which must fall upon the left half of the retina
    We are also able to perceive particles within the in-
 terior of the eye, which are found in the transparent media
 There are  many persons who always see round particles
              5 
8o
THE FIVE SENSES OF MAN.
 or filaments,  which seem to float  about in the field of
 vision.  They may be  more distinctly seen when look-
 ing upon a bright surface—a cloudy sky, or through  a
 microscope.  They follow every motion of the eye, and
 have, moreover, a  peculiar motion of their own.  These
 particles are produced by filaments and cells, which may
 be found floating about in the narrow space between  the
 hyaloid membrane  and  the  retina.   They cast their
 shadow directly upon  the retina, which then,  from  ex-
 perience, refers them to external objects.
    It has  also been discovered by more careful obser-
 vation that the refracting media of the eye are not abso-
 lutely transparent, but that a kind  of cloudiness is seen
 in places which throws a  shadow  upon  the  retina.   If
 we look at  the sky through a small hole in a  sheet of
 paper, held a  short distance from  the eye, the  hole will
 appear to be  surrounded by a coloured  fringe.   This is
 caused partly by a  cloudiness  in the vitreous  humour,
 and partly  by the peculiar radiating formation  of the
 lens, already described.  All such phenomena are called
 entoptic, because they deal with the perceptions of the  in-
 ternal portions of the eye.  They are produced by the
 incident rays of light casting shadows of these  particles
 upon the retina.  They are best seen when an isolated
 pencil of light, like that admitted through a small aper-
 ture, is allowed to fall upon the eye ;  for, in that case, the
 shadows produced are distinct, whilst they are generally
 obliterated  in  ordinary vision, because  the light pene-
 trates the eye from  all sides.
    One of the most  interesting entoptic phenomena is
the Arborescent Figure, discovered  by Purkinic.   If to-
wards evening, we place ourselves opposite a dark wall in 
THE SENSE OF  SIGHT.
81
a dark room, and move a lighted candle to and fro before
our eyes, looking, however, fixedly at the wall beyond, we
shall then, after a little practice, see this arborescent figure,
whose intersecting branches cover the whole of the dark
space, and which is unmistakeably caused  by  the  blood-
vessels in  the  interior of the eye.  The field of vision
assumes  a reddish appearance, upon which   the veins
stand out  in dark shadows.  The trunk of the figure
rises a little on  one side  of  the centre, where the optic
nerve enters the eye, and thence branches out after the
manner of blood-vessels, which is undoubtedly a proof
that in this experiment we see  the blood-vessels of the
retina itself.   One spot alone is  free from vessels: the
yellow spot, which  is the most  sensitive to light of all
parts of the retina.  If now the candle is  moved  to and
fro, the figure will also move, and  follow the direction of
the light.
    All these observations lead to the conclusion, that we
are thus enabled to perceive the shadows of the vessels of
the retina.  That these vessels cast a shadow behind them
is clear, but that the shadow should be sufficient to cause
a perception, leads to the very important and interesting
fact, that  the elements of the retina which receive the
impression of  light, must lie behind the  blood-vessels.
The diagram in fig. 20 will explain how the  shadow of
a vessel can  produce an image.   If the light  is  placed
at a, its image will be depicted upon the  retina at  b.  At
this particular spot no vessels will be seen, because the
light is too dazzling.  But the image at b forms another
source of light, and if  there is a  vessel at v, then  its
shadow will be thrown upon c.  Now, the retina projects
the  image  perceived at  c, outwards, through the  optical 
82
THE FIVE SENSES OF  MAN.
centre k, to d, where  the vessel appears  in  the  field of
vision.  If the light is now moved from a to a?, then the
image will move  from b to b', the shadow from c to c',
and the image of the vessel from d to d', thus performing
the same movement as the light.   We do not, however,
generally perceive these retinal vessels, because usually
the light falls upon the retina  from all points  of the
                       pupil, and therefore no  distinct
                       shadow  can be  produced.   In
                       the experiment  just  described
                       the light proceeds from a single
                       point  only,  b,  and  produces  a
                       distinct shadow.   Moreover, the
                       light is  an  unusual  one,  and
                       throws the shadows  upon places
                       which are not  accustomed  to
                       receive it.  This  latter circum-
         Fig. 20.         stance seems to be of some im-
                       portance, for if the  light is held
perfectly still, the figure gradually fades  away, because
the sensitiveness of the parts of the retina upon which
the shadow is becomes blunted; it appears  again, how-
ever, if the light is moved from side  to side, so that the
position of the shadow is changed.
   A  considerable amount of light penetrates the eye
through the pupil,  which is quite sufficient for the repre-
sentation  of the external world, but none of this light
seems to be reflected.   The pupil  of the  eye  generally
has a dark appearance, so that we  cannot  see  further
into the  eye than the  iris.  It  is, however, possible  to
illuminate the eye  in such a manner, that all  the parts of
the retina may be seen.  This was first done in a satis-
 
THE SENSE OF SIGHT.
83
factory  manner  by the  celebrated physicist Helmholtz,
the discoverer of the Ophthalmoscope.   Before describ-
ing this apparatus and  its functions, we  must discuss
the fact of the dark appearance generally presented by
the pupil.
    The amount of light reflected by the background of
the pupil cannot, of course, be very great; for the re-
tina alone is able to reflect light, and as it is very trans-
parent,  and  has,  moreover, a  dark layer of pigment
immediately behind it, which  absorbs all  the light that
has penetrated  to  it, the reflection must necessarily be
weak.   We know  how difficult  it is to see through  a
window into a room from the street.  This is due to the
small amount of light which comes through  the  win-
dow  in comparison to  that which penetrates the  eye
from  without, so  that the eye is not  sufficiently sen-
sitive to perceive the weaker  impression ;  moreover,
the reflection from  the  panes of glass  considerably in-
creases  the difficulty of perceiving objects in the interior
of the  room.  If, however, the room is lighted up at
night, we can see the  interior very distinctly from the
outside, although  the  illumination of  the  interior  is
weaker than it was in the day-time.
    These circumstances also apply to the eye ; but there
is another circumstance which adds to the difficulty of
examining  the interior of the eye.    The same  fact
makes it impossible to see the  background  of  a camera
obscura through the lens, even when it is white. Accord-
ing to  the  laws  of refraction,  both  the incident  and
emergent rays in  the eye, or  in a camera  obscura, have
a fixed  direction, whilst the light which proceeds from a
room through  the  window is  diffused, that  is  to  say 
84           THE  FIVE SENSES OF MAN.
 emits rays in all  directions.  Let  us suppose an image
 of a lighted candle  to  be thrown upon the retina, then,
 as far as the refracting media of the eye are concerned,
 this image may be regarded as a second object, the rays
 from which will take an outward, and therefore opposite
 direction.  Now this will be precisely the same as the
 path of the incident rays ; for if, at the point where an
 image of an object  has been formed by a lens, we place
 an exactly similar object  instead of the image, then an
 image will be  formed in the exact position of the first
 object, and  of  equal size.   We see from this  experi-
 ment therefore, that  the rays of light, which are emitted
 by an image formed upon the retina, must return to the
 object from which  they originally proceeded.
    If, therefore, a light  is placed before any eye which
 we wish to examine,  the  rays will all be reflected by the
                          eye  into the light, and  we
                          are unable to intercept them
                          by our own eye, because we
                          should  hide  the  light  by
                          placing  ourselves  between it
                          and  the eye  under exami-
                          nation.  By means, however,
                          of a  transparent plate of
                          glass, this obstacle may be
                          overcome, and  the eye ex-
                          amined  when illuminated, in
                          the manner  represented in
fig. 21.  C is the eye under observation, B the observer's
eye,  and the  plate of glass S forms an angle of 45° with
the line between the two eyes.   The rays emitted by the
lighted candle A strike the glass plate S, and are partly
•\
Fig. 21. 
THE SENSE OF  SIGHT.
85
reflected into the eye, which they illuminate.   The rays
reflected by the eye C, again strike the glass plate, which
some of them penetrate, and pass into the eye  of  the
observer, and the remainder return to the  light A.  The
pupil of the eye C, may now be seen brightly illuminated,
and even the illuminated retina can be seen more or  less
distinctly.  The'rays emitted by the image formed upon
the retina, which  pass  through the glass plate, would
form an image at  a, which is at the same distance from
the glass plate as  A. The rays are, however, intercepted
by the observer B, who is  thus  enabled  to  examine a
part of the retina.
    In  fact, a piece of window-glass placed in an oblique
position, as described above, is the simplest form of an
ophthalmoscope, and  may  easily be arranged by any-
one who wishes to make the experiment for himself.  An
ordinary piece of glass is sufficient for the purpose, if
placed in the same position, relatively to  the eye under
observation and the light, as that shown in the figure. It
is well  to place a screen between the light and the person
under  observation, to  prevent  any  annoyance  arising
from the intensity of the light.   The observer  must then
place himself close in  front of the person whose eye  is
under  observation, hold the glass in the manner  de-
scribed, and  move it  about  till  the reflection  of  the
light falls  upon  the  eye.   The  illuminated  pupil  will
then be seen through the glass, and appear of a reddish
colour.
    But, in order to see the separate parts of  the retina
distinctly, it is necessary to make use of lenses adjusted
to the  sight of the observer, and  the refractive power of
the eye under observation ; and the result  of  such a 
86
THE FIVE SENSES OF MAN.
combination is a  perfect ophthalmoscope.    The glass,
again,  has been  replaced with advantage  by a mirror,
generally a  concave  mirror, with an aperture in  the
centre, through which the observer looks.  Fig. 22 shows
the method of using this  apparatus   constructed  after
Ruete's  plan.   The light  is placed  near the person
under  observation, A.   The rays emitted fall upon  the
                       Fig. 22.

concave mirror d, which reflects them into the eye under
observation.  The observer B, looks through the aper-
ture in the concave mirror, and moves the two lenses
m and / till they are in such a position that a  distinct
image of the retina appears.
   We are now in a position, with the aid of the ophthal-
moscope, to make a thorough examination of the retina.
Fig. 23 gives a tolerable representation of all that we are 
THE SENSE OF  SIGHT.             87
able to distinguish of the image.   The background  of
the whole  is of a dull red, whilst the  point where the
optic  nerve enters is distinguished as  a round,  bright
spot, and we may see rising out' of its midst the  retinal
vessels, arteries  a, and veins b, which extend over the
entire retina.  The yellow spot also, the point of most
                        Fig. 23.

distinct vision, may be  distinguished  as  a small  bright
spot.
   The ophthalmoscope has  become an  instrument  of
incalculable value to the oculist.   Many changes in the
retina and interior of the eye, which are due to disease,
can be observed and examined by means  of the ophthal-
moscope ; and, in fact, the medical treatment of the eye
has made an  immense advance since the discovery  of
this instrument. 
88           THE FIVE SENSES OF MAN.
    The eyes  of many animals,  those  of cats,  for  in-
stance, exhibit a peculiar brilliancy, which is particularly
remarkable in the dusk.  It was formerly thought that
the eyes of such animals emitted light independently, as
it was also thought that light could be emitted  by the
human  eye, under the  influence  of passion.   This bril-
liancy, however, in the  eyes  of these animals is caused
by a carpet of glittering  fibres, called the tapetum, which
lies behind the retina,  and  is a  powerful reflector.   In
perfect  darkness  no light is observed  in  their  eyes, a
fact which has been established  by very careful expe-
riments ;  but,  nevertheless,  a very  small amount  of
light is sufficient to produce  the luminous appearance in
them. 
CHAPTER IV.
Formation of the Retina—The Blind Spot and the Yellow Spot— Position
            of the Perception of Vision in the Retina.

The retina is  the nervous organ of the  eye, a delicate
and highly complicated apparatus, whose mechanism has
hitherto been but little understood, whose formation has
long been the object of laborious investigation, and is so
still.  Different layers can be distinguished in the retina
when a fine section of it is examined with the microscope.
The innermost layer, which lies next  to  the surface of
the vitreous humour, consists  of nerve-fibres, in which
the optic nerve loses itself.  It  penetrates all the mem-
branes of the eye at a point a little to one side of the
centre, where the blood-vessels also  enter  the eye, which
can be  seen with  the ophthalmoscope.  At the point of
entrance of the optic nerve, therefore, the retina consists
only of the fibres of the  optic nerve, which from  this
point radiate outwards.   One  spot, however, which lies
in the centre of the retina,  is free from nerve-fibres, and
surrounded by them in a circular form.   On  account of
its colour it is  called the yellow spot.  This is the part
of the retina with which we see  most distinctly;  we will
therefore speak more fully of its structure 
90
THE FIVE  SENSES  OF MAN.
   The layers of the retina are represented in fig. 24 as
they have been described  by  Max Schultze.  In  the
Fig. 24.
interior there is a fine  limitary membrane, 1 ;  then to-
wards the exterior follows—the layer of nerve-fibres, 2 ; 
THE SENSE OF  SIGHT.             91
the layer of nerve-cells, 3, which consists of cells similar
to the ganglion  cells  of  the  brain ; then  the granular
layer, 4, which consists of a grey indistinct mass of fine
granules ; then the inner granular layer, 5, consisting of
little round grains ; then the  intermediate granular layer,
6, consisting of  a fine granular  mass with small fibres ;
then follows the outer granular layer, 7, which is exactly
like the inner one; and ultimately after a second fine
membrane, 8, is  found a layer of small Rods and Cones, 9,
which has a very peculiar and interesting structure.  For
the most part  it consists of small unconnected trans-
parent rods, which are placed close together like palisades,
at right angles to the surface of the retina.   From time
to time a small  rod is seen between them, which expands
at the end, and  is called a cone.  These cones lie very
close  together  only in the midst of the yellow spot,
where there is a small depression in  the  retina.  In  the
yellow spot they are very  numerous, but decrease in
number towards the edge of the retina.
    The rod and cone layer, on account  of its regular
arrangement, gives us a starting point for  conjectures
upon the action  of vision.   The light  which enters  the
eye must undoubtedly pass  through all the given  layers
of the retina, and ultimately reach the  rod and  cone
 layer which is covered externally by the black pigment
 of the choroid,  10.  Light can penetrate no further, since
 it is here absorbed by the black pigment.
    The rods  and cones, judging from' their appearance,
 have all the properties which are characteristic of sensory
 organs.   On the surface of the retina  they form a con-
 tinuous  covering,  which  closely  resembles a regular
 mosaic, and each part, therefore, of the retina seems to be 
92           THE FIVE SENSES OF MAN.
provided with a special
          Fig. 25.

first, and from this reason
tiveness, imparted by a rod
 or cone, while at the same
 time all points are  united
 by  an  intimate,  and,  as
 it were,  geometrical con-
 nection.  These properties
 agree so closely with  the
 mathematical distinctness
 of our power  of  vision,
 that they immediately give
 rise to  the conjecture, that
 the  action of vision must
 take  place in this  mosaic-
 like formation.  If we draw
 a line to the retina, from a
 point of the field of vision,
 through the centre of  the
 eye, it  falls upon either  a
 rod or a cone, is delineated
 on a little square of mosaic,
 and   is  perceived   by its
 means alone.  Thus we can
 imagine the entire field of
 vision to be  formed as  a
 fine mosaic upon the retina,
 in the  same  manner as  a
 picture worked upon a piece
 of tapestry.
   Of the layers of the re-
 tina  the light-rays  strike
 the fibres of the optic nerve
e might be  led to suppose 
                THE SENSE OF SIGHT.             93

that these fibres must be sensitive.  This, however, is not
the case.  The celebrated physicist, Mariotte, has already
shown that the optic nerve, without the retina, cannot
perceive a trace of light, for there is a spot on the retina
which  is quite  blind, and it is just at this spot that the
optic nerve enters (fig. 7).
    Suppose in  fig. 25 that we fix the right eye upon the
small white cross, whilst the left is closed, and hold the
book at about the distance at which a good eye is gene-
rally able to read.  If, now, the book is moved slightly
to and fro, then we  shall find a  distance at which the
large white circle to the  right disappears entirely.   The
eye must, however, always be firmly fixed upon the white
cross.   As soon as the eye is turned away from it  in
either direction, the white circle reappears.
    In this experiment the white  circle, under the given
conditions, falls exactly upon the  point where  the optic
nerve enters, which  is situated in  both eyes at a slight
distance from the centre of  the retina, or, in other words,
the yellow spot.  It is  a remarkable fact that at this
point we by no means perceive a  hiatus  in the field  of
vision ;  but the points which are seen by the edges of the
blind spot move towards each other, and so fill  up the
hiatus.  The black ground on which the white circle lies,
appears therefore  quite  continuous, and  this would also
be the case if the ground were white and the circle black.
In ordinary  vision, also, we perceive no dark hiatus  in
space, although it is  easy for practised persons to perceive
that objects,  which  fall upon  the blind spot,  can  be
forced to  disappear entirely ; even the sun itself can  be
blotted out of  the heavens, if  its  image is allowed  to
fall  upon the  blind spot.   In all these experiments one 
94
THE FIVE SENSES OF MAN.
eye must be  closed, since it is impossible for an object
to fall upon the blind spot of both eyes at the same time.
   This remarkable experiment shows us, moreover, that
the fibres of  the optic nerve are not  excited  in  the
smallest degree by  light,  and hence  we may  conclude
that the nervous layer of the retina is also penetrated by
light, without any action taking place in it.
    The yellow spot of the retina presents other remark-
able properties, which we will proceed to consider.
    When we fix  a point with the eye  exactly, then the
ray of light passes  from this point through  the middle
of the pupil, and through  the centre of the  lens, and
falls upon the retina  almost  at  its centre, where the
yellow spot is found.
    In the yellow spot, therefore,  the power  of  vision
possessed by the retina is the greatest, and in it lies the
depression of the retina, with which we are able to  dis-
tinguish clearly a single point in the field of vision.
    In reality the formation of the yellow spot explains
its peculiar properties.   It  has already been mentioned
that the fibres of the optic nerve in their expansion
upon the  retina, surround the yellow  spot in  a bow-
like form, apparently  for the  purpose of not  depriving
it of any part of the light which falls upon it.   This
again  proves that  it  is not  necessary to our  power
of vision  that the light should  fall upon the  nerve-
fibres.
    In the yellow spot the cones are very delicately formed,
and closely packed  together.  We must, therefore, con-
sider them as special elements of the sensation of light.
 In addition to the cones, ganglionic cells are found in the
yellow spot, whilst  the other layers appear thin and  fine; 
               THE SENSE OF

small  pigment-cells may also
be  seen,  which  produce  the
yellow colour.
    If,  therefore,  as has  been
shown,  the  fibres of the optic
nerve  are not themselves sen-
sitive to light, and the action
of the light-waves takes place
in the rods and  cones, then a
connection between the nerve-
fibres and these organs appears
highly probable.   Towards this
connection,  therefore,  micro-
scopists  have  turned  their
attention  for a  considerable
time,  and  later  investigations
have  established  the fact  of
its  existence.   The  connec-
tion, indeed,  only exists  by
means of very  fine filaments,
which  penetrate all the layers
of the  retina, and  are often
difficult to detect, but which,
 thanks to unwearied investiga-
 tion, have  been  observed with
 certainty.  Fig.  26 shows how
 the nerve-fibre 2 is connected
 with the ganglion cells 3, the
 granules of the corneous layers
 5  and 7, and lastly, with  the
 rods and cones 9.   And  this
 is the path by which the irrita-
SIGHT.              95
Fig. 26. 
9^           THE  FIVE SENSES OF  MAN.
 tion caused by light, which takes place in the rods, passes
 to the optic nerve.  The optic nerve  ultimately conveys
 it to the brain, and there awakes the sensation of light.
     The trunk of the optic nerve, just like any other
 nerve, can be excited directly without the intervention of
 the retina, and by every  such  excitement the sensation
 of light is produced.  If, in operations upon the eye, the
 optic nerve is  severed, the patient has  no sensation of
 pain, but of a brilliant flash of light, upon which follows
 perpetual darkness.  We  may also excite the optic nerve
 by electricity,  if we pass a current through the socket
 of the eye and  the cranium, when flashes  of light  are
 always observed  in the field of vision.
    Investigations  in  the domain  of  nerve  physiology
 have led to the result, that  all our nerves are merely in-
 struments  for  the  conveyance of one  and the  same
 action—the irritation of the nerve, which passes along
 them, in accordance with similar laws.   The nerves of
 touch, sight, hearing, taste and smell  all agree in their
 nature.  The  optic nerve no  more conveys the light-
 waves to the brain, than the auditory nerve conveys the
 sound-waves.   But  both nerves  at  their  terminations
 possess different kinds of  apparatus, which are called the
 sensory organs, and by means of which they can be ex-
 cited.  The terminal apparatus of the  optic nerve is the
 eye, or, more strictly speaking, the retina, which has the
 property of being  excited  by the light-waves,  and of
 communicating this excitement to the optic nerve.  The
 terminal apparatus of the auditory nerve is found within
 the inner ear, and by its vibration excites the auditory
 nerve.  Now the action is identical in the  two sensory
organs as soon  as it has reached the  nerve, and  carries 
THE SENSE OF  SIGHT.
97
with it no trace of a musical sound or of a ray of light.
The two actions produce in our brain, however, sensa-
tions of so different a nature, because each nerve has a
special centre in the brain, where it terminates.  The
centre of the optic nerve has peculiar properties different
from those of the centre of the auditory nerve.  The first
only arouses the sensation  of light as soon as it is set
in action, the latter only produces sensations of sound.
The action of  both centres  is quite mechanical, and
therefore they can each only produce one and the same
effect.   From this reason  light  is perceived, even if  no
light enters the eye, as soon  as the optic nerve is  torn,
severed, or excited by electricity, because the optic nerve
communicates its excitement to its  own  centre, which
replies with a sensation of  light.  The same is the case
with the  auditory nerve, whose  centre can only produce
the  sensation of sound, and never anything else.   If,
under  accidental circumstances, the auditory nerve had
been attached to the eye, and the  optic nerve to the ear,
then every ray of light would produce a sound, and every
sound in our ear would produce an appearance of  light
in  our imagination ; we  should then see a symphony,
and hear a picture.
    Returning to  the action of the sensation of light  in
the  retina, we may imagine it to be as follows :—
    The  light-waves, in a  manner not yet quite under-
stood, give rise to a process in the rods and cones of the
retina which very  probably consists of intermolecular
motion.   The rods and cones communicate this internal
motion,  by means of  the  fibrous prolongations  already
mentioned, to the  upper layers  of the  retina ; and thus
from a particular cone, the motion reaches the nerve-fibre 
9§           THE  FIVE SENSES OF MAN.
with which it is connected.  The motion then produces
an excitement of the nerve, which  excitement produces
in the brain the sensation of light.
    By a very ingenious calculation Heinrich Miiller has
shown that the rods and  cones are the point where the
perception  of  light takes place.   For this purpose  he
makes use of the arborescent  figure of Purkinje.   If, for
instance, the flame is  moved to and fro (fig.  20), the
figure moves also, as is shown by the construction.  Now
this figure is formed  by the shadow of the vessels of the
retina, and from the  construction given  above, the posi-
tion of the shadow  can be calculated in the following
manner.   If we measure the angle a k a' round which
the candle flame moves, and which is equal to the angle
b k b', we  can find  the  distance b U upon the retina.
Now the distance of the vessel v from b U can be deter-
mined approximately, therefore the angle b v U can also
be determined, which is equal to the angle c v c'.  Now
c and cf are  the shadows of the vessel, which apparently
moves in  the field of  vision  from d to d'.  We, there-
fore, measure  by observation  the angle d k df, which  is
equal  to c k cf, calculate  from it  the line c d', since we
know k c, and now, in the small  triangle c v d', which
may be regarded as  an isosceles triangle, we know the
base c cf and the angle c v cf.  This gives us the distance
of the vessel v from the point where its shadow  falls ;
and, in fact, we find  that the shadow falls exactly upon
the rod and cone layer.
    Now, what is the nature of the action of light upon
the rod and upon the shadow ?  With our present know-
ledge  of the  properties of light, we  can imagine some
possible answers.  For instance, the  action  may be a 
                THE  SENSE  OF SIGHT.              99

chemical one.  This conjecture is most probable, because
we  know that light, by the  chemical  decomposition of
iodide, or chloride of  silver, can produce a picture on a
photographic plate.  We may, therefore, suppose that in
the rods and  cones a substance is also present, which
undergoes chemical change  by the  action of light, and
that, at  the same time,  a  real substantial  picture is
formed upon the  retina.   Of course, this picture would
not be a permanent one like  that of a  photograph, for it
disappears  as soon as the light ceases to fall upon the
eye, and we must  suppose that the nutritive  action, or
circulation  of the blood,  always  destroys it.  Such  a
chemical picture would be able to  excite the termina-
tions of the optic nerve, for we know that nerves can be
excited by chemical agencies.  Still, all this is  mere con-
jecture, and similarly  with  the possibility that  the action
of light  on  the  retina may be due to electrical  action,
which we know takes place in  nerves  and muscles.   In
short,  it still  remains a problem for science to solve,
what  it  is which gives the rods and cones the remark-
able property of exciting  the optic nerve  by means of
light. 
100
THE FIVE  SENSES  OF MAN.
                    CHAPTER V.

 The Colours of the Spectrum — Combination of Colours—The Three
               Primary Colours—Colour-blindness.

 The light which we perceive in nature is by no means
 of  the same kind, but we  distinguish a number of kinds
 of  light which  we term colours.  All objects possess a
 certain colour, due either to  the light which they reflect,
 or which they transmit.   We speak, therefore, ordinarily
 of  coloured  light,  although physics tells us  that colour
 cannot be separated from light, as if it were like a colour
 which an artist lays on a picture.  The kind  of light
 which we call white, can be decomposed, by the prism,
 into a number of colours, which comprise  all the simple
 colours,  and  which, when combined,  produce  all  the
 colours which occur in nature.   If a sunbeam is allowed
 to fall upon a prism, as is shown in fig. 27, the beam is
 decomposed  into a number of colours, whose  sum is
called a spectrum.
    A ray falls through the opening  b, upon the prism s.
 If the ray continued without interruption, a bright circle
d, would fall upon the opposite wall. By refraction, how-
ever, the ray is turned aside, and since the coloured rays
which it contains possess different degrees of refrangibi- 
THE SKNSE OF SIGHT.
IOI
lity,  a coloured stripe  r v—a spectrum—is  formed, in
which the  red, the least refrangible, is found at r,  and
the violet, the most refrangible, at v.  Fig. 28 contains the
Fig. 27.
colours of the spectrum in their order.  The spectrum
also contains dark stripes, Frauenhofer's lines, which are
peculiar to sunlight, and are caused by  the absorption
of light by the gases present in the outer  layer of the
sun. The spectrum commences with red, which gradually
     H         G          FE      DCBA

    II         I          I    II      I    HI
                        Fig. 28.
changes into orange; then follows a smaller stripe of pure
yellow  at  the line D, and then green at E.  The blue
which follows is of considerable extent, and we, there-
fore, distinguish blue at the line F, and indigo at G.  The
spectrum finally terminates with violet.
    The colours in the  spectrum are not sharply sepa-
rated, but pass gradually into each other.   We can dis-
tinguish as  quite pure  colours,  red, yellow, green, blue,
violet,  each  of  which  produces  a sensation entirely 
102         THE FIVE SENSES OF. MAN.
 different to the others.  The nearer, however, two colours
 are situated in the  spectrum the greater relation they
 seem to bear to one another.  Thus the red seems to
 be more nearly related  to the yellow than to  the green
 or blue;  and the impression  which yellow and green
 produce  is  more similar than  that produced by yellow
 and  blue, or violet.  A direct change, however gradual,
 from red to green, would seem to us unnatural, whilst the
 intervening yellow offers a natural connection between
 the two colours.
   Physics teach us that the  rays  of a spectrum con-
 sist of light-vibrations with different wave-lengths, which
 diminish from the red to the violet.   Since the different
 kinds of light are transmitted with equal velocity, violet
 makes more vibrations in the same length of time than
 red light;  the  former  about  66y  billions,  the  latter
 456 billions in a  second.  We learn, further,  that all the
 light-rays of a spectrum differ solely in the wave-lengths
 of their vibrations, and possess no other special mark of
 distinction, nothing which has the  slightest resemblance
 to the colour we perceive in them. Imagine the propa-
 gation of  long  and short waves upon the surface  of
 water ; this phenomenon will be somewhat analagous to
 what occurs in the red and violet rays.   Water-waves of
 different lengths,  however, are quite similar in  regard to
 the impression which they make on our eyes, whilst the
 light-waves, on  penetrating  the eye, cause sensations
 which  essentially differ  in kind.  Thus  red  is  widely
 different from green and blue.
   The spectrum has no defined limit at either end, but
 passes gradually into black, the  transition being more
gradual at the violet than at the red end.   Nevertheless. 
THE SENSE OF SIGHT.
103
physical  and  chemical  action  is  still  present  beyond
these visible boundaries.  The invisible, or ultra-red por-
tion, gives evidence of considerable  heat, which  in this
part of the spectrum reaches its  maximum, and gra-
dually diminishes in the visible part.  The violet end
has the power of exerting strong chemical action upon
certain compounds, such as  iodide and chloride of silver ;
therefore a photographic spectrum extends considerably
beyond the violet.   It has, however, been  shown by
Helmholtz, that, under  certain conditions, we  can also
perceive the ultra-violet end of the  spectrum with the
eye.  If we separate the ultra-violet  part of a spectrum,
thrown on a screen in a darkened room, by  allowing  it
to pass through a slit  in  the  screen, and if it is then
allowed to  pass  a second  time through a prism, which
purifies it from all strange rays by  refraction, it produces
upon the eye the sensation  of a glimmer of light, which
has a lavender grey colour.
    The eye, as  is  well known, is  only  to a  certain
extent  sensitive  to  light-vibrations.   Physicists  con-
clude  that all  the  rays  in the  spectrum,  visible as
well  as  invisible,  which are situated  beyond  the red
and  violet,  consist  of vibrations  of the ether,   which
differ only in  their wave-lengths,  and are otherwise
quite similar in form  ; of their vibrations, however, only
those of intermediate length  act upon the  retina, while
it is  insensible to the vibrations of the ether of  greater
or less  wave-lengths.  What we, therefore, distinguish
as light and colour  arises  from  a subjective property
of the retina, inasmuch as it  only  reacts on  certain
ether-vibrations.   We  might,  therefore,  imagine the
existence  of eyes, which could  not  perceive the  inter-
          6 
104         THE FIVE SENSES OF MAN.
mediate parts of the spectrum, as ours can, but only the
rays situated at  the invisible ends.   To such eyes the
world would  have quite a different aspect.
   All the colours which are found in nature or are pre-
pared artificially, simple or compound, can be composed
from  the  colours  of the spectrum.   Further, white  is
formed  by the combination  of all the colours of the
spectrum, in  the proportion in which they occur in the
spectrum.  The constituents, therefore, of all the . light
which the eye can see are contained in the spectrum.
    In order to discover what  impression  the mixture  of
colours would have on the eye, it was formerly thought
sufficient to mix  together colouring matters, such as are
used  in painting, or coloured liquids.  It was, however,
an error to suppose that by this means the true  mixture
of colour was obtained ; that is to  say, the  same colour
which our eye would perceive, if the  two simple colours
acted upon it simultaneously.  If, for instance, we mix
together a blue  and yellow  powder, we obtain green.
We should, however, never produce a green if we mixed
together blue and yellow light-rays.  The cause of this
difference is the following.   A colour, such as is used  in
painting, consists of minute particles, which are transpa-
rent, inasmuch as they transmit a certain coloured  kind
of light, and absorb the  remainder.   If we now consider
light falling  upon a greater number of particles, then a
small part of it, consisting  of white light, is reflected
from  the  surface; the greater part, however, penetrates
the outer layer, and is reflected from  the surface of par-
ticles which are situated at a greater depth.   This light
is coloured, i.e. the residue  of the white light is absorbed.
A yellow  powder,  therefore, absorbs  all  the  rays with 
THE SENSE OF SIGHT.             105
the exception of the yellow ;  yet this is not absolutely
the case, for  rays  which are situated  near the yellow,
among others some green, are  transmitted also.
   The case  is the same with  the blue powder.  It also
transmits some green, as well as blue, rays, and absorbs
the rest.  Now mix  the small blue and yellow particles
together.  The blue rays will be absorbed by the yellow,
and  the yellow rays by  the  blue  colouring matter, so
that the blue and yellow  almost disappear in  the mix-
ture of the colours, and only  the green  remains, which
is reflected, since it  is transmitted to a considerable ex-
tent by both bodies. The action would be exactly the
same if we looked  through a blue and a yellow glass
placed  side by  side. The colour seen is  green, for the
blue glass only transmits blue and  some  green, but no
yellow, while  the yellow absorbs the blue completely, and
only transmits green.
    The phenomena, are, however,  entirely  different, if
we allow two colours to act on  the eye simultaneously,
which  may be done  in  the following manner.  A spec-
trum is thrown upon   a screen  in which two slits are
made; through  these  two  slits two  rays of  simple
colours pass,  and can be united  by a lens.  This is the
most perfect  method of combining  colours.
    An easier method of combining  colours is the Colour
Top.
    This instrument consists  of a disc, which can be
made to rotate  round  a vertical axis,  as represented in
fig. 29.  One hand holds  a  handle, which  supports the
upper  end of the axis* and the other forcibly draws off a
string  which  is wound  round  the axis, thus setting the
top in  rapid  rotation.  On the top a paper disc has been 
106         THE FIVE SENSES OF MAN.
fixed, sections of which, as may be seen from the figure,
have been painted with the colours which are to be com-
bined.  The images of the colours are so  rapidly super-
posed upon the retina, that the impression produced by
the colours on the  mind, is that  of a  mixture.  If we 
THE SENSE OF  SIGHT.            107
have black and white sections in the disc, the result is a
grey colour; and by making use of other colours, we can
produce mixtures of every possible shade, which can  be
made darker by the addition of black.
   The following is a very simple method of mixing two
colours together, which can be  done without any instru-
ment.  Two coloured wafers, b and c (fig. 30), are placed.
on the table a certain distance  apart, and a small plate
of glass is placed before the eye, so  that  the wafer b is
seen directly through it, and at the same time a reflected
           — i         3           c
                        Fig. 3°-

image of the wafer c, so that both images are superposed,
and their colours  combined.   In this case also we pro-
duce a true mixture of colours.
   Now all  these  methods give  concordant results, al-
though the second method rests on a different  principle
to the two others.  In the  latter the different colours
were mixed objectively before they entered the eye ; by
the coloured top, however, the  differently coloured rays
were not mixed, but they only fell upon the retina in very
rapid succession,  the  combination of the colours thus
taking place on the  retina.   This,  however,  shows  us 
108         THE FIVE  SENSES OF MAN.
that a  mixture of colours does  not produce a special
excitement  on the  retina,  but  one composed of  the
excitements, which each colour creates independently.
    If,  now, all the colours  of the  spectrum are  repro-
duced upon the colour top with the  greatest possible
exactness,  and  in the proportion  of their  brightness,
then their mixture will reproduce white, which, on account
of the imperfect clearness of the  colours employed, will
have more or  less of a grey tinge.   Exactly the  same
grey, however, can be produced on a  colour top, by  the
combination of black and  white;  and  since  black is
nothing more  than the absence of light, the combination
of the colours corresponds exactly to the white which is
contained in this grey.
    The combination of two simple  colours of the  spec-
trum  has produced  a very remarkable  result.  It has
been found  that there are  several  pairs of colours the
combination of which produces white.   We include
amongst them yellow and indigo, though a mixture of
two colouring matters  of this tint would produce green.
Such pairs are called complementary colours, and besides
yellow  and  indigo,  red and greenish-blue,  orange and
blue,  greenish-yellow   and  violet are  complementary
colours.
    Now if we look for these  complementary colours in
the spectrum, we find that two complementary colours
always  lie   a  certain  distance apart.  Two adjoining
colours  can  never be  complementary, neither  can the
two ends of  the spectrum, red and violet, the  combina-
tion of which produces purple.  It seems, moreover, that
one of the complementary colours must always be situ-
ated somewhere  near  green, as greenish-blue, sky-blue, 
THE SENSE OF  SIGHT.
109
yellow and greenish-yellow. On the other hand, pure green,
as it appears in the spectrum, has no simple complementary
colour, and it is necessary  to mix with the green  two
colours, red and violet, to produce the sensation of white
light.  These three colours, red, green and violet, are now
received as primary  colours, because they are  the  only
three pure colours in the spectrum which, when combined,
produce a nearly perfect white.
    We will now proceed to determine what influence is
to be ascribed to these three primary colours in the action
of the sensation of colour.
    The vast number of colours and  shades  of colour
which we meet with in nature have, through the discovery
of the colours of the spectrum, been reduced to a certain
number  of simple  colours.   But  between the simple
colours of the spectrum  there  are a  great number of
transition colours, which produce many different shades.
Each point of the  spectrum differs in colour  from  the
point next to it; this change being due  to difference in
wave-length of the light-vibrations, and therefore it seems
as if the number of colours and the transitions between
them, which  can be  seen in the spectrum, must be  end-
less.  The question  has, therefore, been asked, how  is it
possible that the retina of the eye should be irritated in
so many different ways, for each colour must produce a
particular irritation ?
    To consider this question more closely, we must return
to our conceptions of the physiology of the nerves.   We
know that the irritation of light  takes place in the rods
and cones of the retina, and that they transmit the irri-
tation to  the fibres  of the optic  nerve.  If, for  example,
a ray of light falls upon a cone,  then some kind of  irri- 
110          THE FIVE SENSES OF  MAN.
 tation must take place in  the  fibre which  is in connec-
 tion with the cone, to  provide us with  some means of
 deciding that we  have to do with  red  light,  else the
 brain would be unable to recognise the sensation of red.
 When a blue  ray of light falls upon the cones, the process
 of irritation must clearly be of a  different kind in the
 same nerve-fibre, so as to inform the brain  that the light
 causing the irritation is blue.   With green  light the pro-
 cess in the nerve fibre must again be different;  in short,
 we must assume that in one and  the same nerve-fibre,
 the irritation produced by each colour is  special and dif-
 ferent in kind.
    This result is, however, directly at variance with  our
 conception of the process of the irritation of the nerves.
 According to  this  conception all  nerves  were the same
 in  their nature,  and in the action  of their irritation.   It
 is possible to join together a sensory nerve and a motory
 nerve, so as to form one nerve, and in this case an irrita-
 tion of the  sensory nerve is directly transmitted to  the
 motory nerve, and  causes  a contraction  of the  muscles
 belonging to it.   In both kinds of nerves the process is
 the same, and if the irritation of a motory  nerve during
 life produces motion only, and that of a sensory nerve
 sensation only, the sole reason is that  the former is con-
 nected with the muscles, the  latter with the centre  of
 sensation in  the brain,  producing in this complicated
 organ different kinds of phenomena.  We  have already
 learnt that the optic nerve differs in no respect from the
 other nerves of the body.  If  it were possible, we might
place any motory nerve between  the  eye and the brain,
 and the perception of light would  not be  destroyed in
the least.  The  irritation of such  a  nerve would only 
THE SENSE OF  SIGHT.
Ill
produce a sensation of light in  the brain, because  the
central organ of this nerve, when irritated, never produces
any other sensation in the brain than  that of light.  If,
now, the action is exactly the same in the optic nerve as
in all other kinds of nerves,  how is it possible  that, in
the same nerve-fibre, red light  should  give  rise -to  a
different action to blue,  and green light different again
to yellow ?
    There is only one way of getting out of this difficulty,
namely, accepting as a fact that  every sensitive  element
of the retina is connected with several nerve-fibres, each
of which is sensitive to a  particular colour.  Let us sup-
pose, for example,  a  nerve-fibre  to terminate in a cone,
which, through its  physical or chemical constitution, is
only  affected  by  red rays; then  this nerve-fibre will
transmit the  irritation to  the brain, and the brain thus
receives an intimation that the impression has been made
by a certain  kind of light, which it  recognises as red.
Let us also suppose the same cone to be connected with
another  nerve-fibre, the end of  which can only be irri-
tated by a green ray, then the brain,  if the  irritation of
this nerve-fibre  has been conveyed to it, becomes con-
scious of the presence of a different kind of light, which,
from  experience, it will call green.  We can  thus picture
to ourselves the existence of several kinds of nerve-fibres
in the optic nerve,  which  differ from each other only in
their  terminal organs within the  rods and  cones, each of
which can  be irritated by a particular kind of light alone.
At first it would  be supposed that a vast number of fibres
must exist in a sensitive element  of the retina.   It would
be a great temptation  to claim  for every colour in  the
spectrum a separate nerve-fibre ; but it is quite sufficient 
112          THE FIVE  SENSES  OF MAN.
 if we reduce the number of fibres to three, in accordance
 with the number of primary  colours, red, green, violet.
 In fact, all the phenomena of the sensation of colour may
 be perfectly explained on the  supposition that, in  each
 point of the retina, three kinds of nerve-fibres terminate,
 one of which is sensitive to red, another to green, and the
 third to violet.
    Exactly as white light is produced by the  combina-
 tion of  red, green and violet, all other shades of colours
 may be formed  by the combination of these primary
 colours.   If* white light falls upon the  retina, then all
 three kinds of fibres,  those  sensitive to red, green and
 violet, are irritated, and this simultaneous  irritation pro-
 duces the sensation of white.   If the retina is illuminated
 by red  light, then the fibre sensitive to red is irritated
 most strongly.  It is, however,  very probable that the
 two other kinds of fibres are irritated at the same time,
 though  in a less degree ; first, the fibre sensitive to  green,
 because green lies nearer to the red in the spectrum, and
 then that sensitive to violet.
   According to this theory,yellow light irritates equally
 the fibres sensitive to red and to green, and only slightly
 that sensitive to violet. Yellow, therefore, is not a primary
 colour, but, physiologically speaking, a compound colour ;
 because it is due to a  combination  of  the  sensations of
 red and green.
   Green light irritates principally the fibres sensitive to
green, and very slightly those sensitive to red and  violet.
   Blue light  irritates simultaneously the fibres sensitive
to green and violet in an equal degree, and very slightly
those sensitive to red.  Blue,  therefore, physiologically
considered,  is  also a compound colour. 
THE SENSE OF  SIGHT.
H3
    Violet light irritates very strongly the fibres sensitive
to violet, and the other two only slightly.
   This theory of the perception of colours was first put
forward by Thomas Young, and has more lately been
developed by Helmholtz.  In accordance with this theory,
the action of the spectrum  colours upon the retina has
been  illustrated  by  Helmholtz by the  accompanying
    The horizontal lines, I, 2, 3, represent the three kinds
of nerves :—1, the fibre sensitive to red ;  2, to green ;  3,
to violet.  The letters under the line 3 correspond to the
colours  of  the spectrum in  their  natural order, red,
orange, yellow, green, blue, violet.  A curve is described
above each line, showing the strength of the irritation
exercised by all the colours of  the spectrum upon each
kind of nerve.  Curve 1 attains its greatest elevation be-
tween red and orange ; curve 2, at green; curve 3, between
blue and violet.  If, now,  we  draw a  perpendicular line
through  the three curves from each colour of the spec-
trum, then  the perpendicular section of the curves will
show us the  relative strength  of the irritation  of the 
I 14          THE  FIVE SENSES OF MAN.
nerves.   We  see  that yellow  exercises  a moderately
strong irritation upon 1 and 2 ; green irritates 2 strongly,
and 1 and 3 only slightly; blue  again produces an irri-
tation  of medium  strength  upon  2 and  3 ; and violet
irritates 3 almost exclusively.
   A very interesting observation  has proved the fact,
that in the retina there must be particular elements sen-
sitive to  red.  Thus it  can readily be proved, that  the
outer edges of the retina are insensitive to red.   If we
take a red body, for instance a stick of red sealing-wax,
in the hand, and move  it to one side beyond the field of
vision, looking straight before us all the time —if we now
move it slowly forwards till it is just distinguishable on
the edge of the field of vision, it will no longer appear red
but black, and the red  colour suddenly reappears if we
move it forward.   This is not the case with blue, which is
perfectly distinguishable at the edge of the field of vision:
    The  retina,  at its  edges, is, therefore, blind  to  the
colour red, a fact  most  simply explained  by supposing
that the  fibres sensitive  to red are here wanting.   Since
these parts of the retina are  perfectly sensitive to blue,
they are not devoid of  the fibres which are apportioned
to the other  parts of  the retina  for the  sensation of
colour.  If there were but one nerve-fibre for the  trans-
mission  of all sensation of colour, it is incomprehen-
sible why it should not be sensitive to red at the edge of
the retina.  It follows, therefore, that there must be par-
ticular nerve-fibres for the sensation of red.
    This peculiarity of the edge  of the retina  in  the
normal eye passes, in the eyes  of  many, into red-blind-
ness.   That is to  say, there are a  great  many persons,
almost one in twenty, who  are incapable of distinguish- 
THE SENSE OF  SIGHT.
"5
ing red  colours  distinctly.  They know, indeed,, from
ordinary conversation, that a certain colour  is called red,
and by experience  are enabled to use this expression.
They call blood red,  because they know  it is  generally
called so, and other objects in the same manner ; so that
it often happens that they themselves are  unconscious of
their imperfect sensation of colour.  But  sooner or later
an occasion arises  which shows  their inability to select
a red object  from  similar  objects differently coloured.
They confuse especially red with dark green and yellow.
If, now, a spectrum is  shown  to people  suffering from
this  red-blindness,  they  will distinguish  two  principal
colours,  which they  will call  blue and  yellow.  They
imagine that the spectrum is shortened, especially at the
red end,  and the extreme red they do  not see at all.
   This  peculiarity may be explained by  supposing that
in such  persons the  nerve-fibres  sensitive to red  are
either wanting or insensible to  irritation.   The world
must appear to them quite differently coloured to what
it appears to us.  What looks to  us white, must to them
have a greenish-blue  appearance, because red is wanting
in it, and yet  they call  it white, because it comprehends
the whole  of their series of colours.  There are, more-
over, many degrees of red-blindness, so  that  sensitive-
ness to red is present in a greater or less degree.
   This  fact is a proof that red  is one of  the primary
colours, since  it can be wanting altogether, and it is also
a substantial confirmation of the  theory  of Young and
Helmholtz,  according  to which these  three  primary
colours  have  special  nerve-fibres in  each point of  the
retina. 
u6
THE FIVE  SENSES  OF MAN.
                   CHAPTER VI.

Incidental Images—The Phantascope—Positive and Negative Incidental
         Images—Incidental Colours—Harmony of Colours.

IN  ordinary  vision  a picture disappears  as  soon as
the object seen  is withdrawn, or it has ceased  to  be
illuminated.  In some well-known phenomena, however,
it  may be observed  that the impression of light  lasts
for an appreciable time  after the  light is withdrawn.
If a burning stick is whirled quickly round in a circle,
we  have the impression, not of a point, but  of a fiery
circle.   Similarly, a  rising  rocket  and  a  falling star
produce  the  impression of a line of light.   Moreover,
if, in the night, the darkness is illuminated by a flash of
lightning, we always  have the impression that the flash
has lasted for some time; although, in reality, it is of such
momentary duration,  that  if a railway train, as it rushes
by, were illuminated by lightning  it would  appear to
be standing still.   The persistence of these impressions
is unpleasant if we have looked at a powerful light or
the sun.  We then perceive spots  of light for some time,
even when the eyes  are  closed,  which  greatly incon-
venience our sight.
   These prolonged   impressions of  light are called 
THE SENSE OF SIGHT.             117
incidental images.  These  images may be  observed of
all bodies, which are not too feebly illuminated, if the
eye  is  sufficiently  sensitive ;  especially  of  a  bright
window, when the eyes are opened  and closed rapidly.
After  a  few  seconds a  faint image  of  a window is
observed,  which gradually disappears.  The  existence
of incidental images has  already been pre-supposed in
the mention of the rotating disc, which contained black
and white,  or  coloured  sections.    If the  rotation is
sufficiently  rapid we no  longer  perceive  the  single
sections, but a combination of their colours.   We have
here to do with the phenomenon of incidental images,
for the image of the black section  is superposed upon
the  image of the white  one with  such rapidity,  that
the  incidental image of the black section has not yet
disappeared ; the images,  therefore,  of the  black and
white  sections are  mentally conbined, and  the result
is  a grey.   The revolutions must,  therefore, be  made
with a  certain  velocity,  for  the  grey colour  appears
when  white  follows  black  about  thirty  times  in  a
 second.   If.the revolution is less  rapid  it produces  a
 flickering between  the black and white, which produces
 an  unpleasant and fatiguing impression upon the eye.
 This sensation is still more troublesome with a flickering
 light, since as the light diminishes the eye each time has
 a short  rest, and is irritated with  increased  vigour by
 the light when it breaks out again.
     The  persistence of these incidental  images is the
 basis  of a  well-known  instrument,  the phantascope, or
 magic disc, on which various figures are  seen  in motion,
 such  as dancers,  horsemen,  gymnasts,  etc.   Fig. 32
 explains this phenomenon by a simple example.  Upon 
n8
THE FIVE SENSES OF  MAN.
a disc a pendulum is represented in different positions
of its oscillation.  If we look, from the other side, through
the holes I —12 upon a mirror, and turn the disc with
                         tolerable rapidity, the pendu-
                         lum is seen first in the position
                         of 1, then in 2, 3, etc.  The
                         images follow each other very
                         rapidly, so that the pendulum
                         appears to have made  an  os-
                         cillation.  Here the incidental
                         image  of one pendulum  re-
                         mains  a  sufficient length of
                         time for the image of the next
                         to take its place.
                            From  the  kind of  inci-
                         dental  images we  have just
                         considered,  must be  distin-
                         guished  those  which  are
formed  when  we look for a  long time  at a  bright
object.  Look,  for instance, through a window at the
bright sky for  from half to a whole minute, without
moving the eyes, so that the window frame  may  be
depicted upon  a definite part of the  retina,  and then
turn the eyes quickly towards  a  wall, or a white sheet
of paper.  A  dark  window  will now be  seen  with a
bright frame, so  that in this  incidental image  all the
bright parts of the picture appear  dark, and the dark
bright.  On this account  it is called a negative inci-
dental  image,  while the  former  kind of  incidental
image is called positive.
   Negative incidental images are caused by the retina
being fatigued  at the  spot where  the light has acted
 
THE SENSE OF SIGHT.             I 19
strongly  upon  it.  The bright  surface of the window
will  have excited  the  retina  for  some  time, in con-
sequence  of which a  definite portion of it will  be
fatigued, whilst that part upon which the dark window
frame  was depicted will  not  be fatigued.  If the eye
is now turned upon another surface, the fatigued  parts
will  be more feebly  excited  by this surface  than the
rest, and consequently  a dark  window with  a  bright
frame  will appear.
    A similar experiment may be very well performed
in  the following  manner.  A  small  square  of  black
paper  is  placed  upon  white  paper, and  the  eye fixed
upon it  for  some  time.  If the eye is now suddenly
turned  upon the  white surface of the paper, a  bright
square is seen upon it which  moves  about  with the
eye, and after some  time gradually  fades  away.   It
is  characteristic  of this  phenomenon  that  it follows
the direction of the eye, and thus  proves that we have
not to do with a  reality, but with an optical illusion,
caused by an action within the eye.  The most curious
part of it  is that we, nevertheless, imagine its cause
to  be  external to ourselves, since we have been  uncon-
sciously taught so by experience.
     Negative incidental images can, however, be per-
ceived with closed  eyes.  If we  close  the  eyes after
 looking at  a bright object we perceive the same  ob-
ject in a darker tint.   This fact seemed for a long time
 inexplicable, and  at variance with the theory of inci-
 dental images mentioned above.  The instructive  in-
 vestigations, however, of Purkinje, at the commencement
 of this century, had  already shown that, for our eyes,
 absolute darkness did  not exist.   In  the densest dark- 
 120         THE FIVE  SENSES OF MAN.

 ness, even when, in the darkest night,  every trace of
 light is  artificially excluded  with  the utmost care, the
 eye has still  a perception of light  of its own.   The
 sensibility of the eye increases in this darkness  in  an
 extraordinary  manner,  and  fantastic clouds of  light
 pass  over the  field  of vision, moving  up and down,
 disappearing  and  reappearing.    It  is  very  probable
 that  this is  due to some internal  excitement, caused
 by the circulation  of blood  in the  retina.
    This peculiar power of the retina remains even when
 the eyes are closed, apart from the diffused light which
 penetrates the eyelids in the day-time, and  the negative
 incidental images which are seen with closed  eyes are
 sufficiently explained  by  supposing that the fatigued
 parts of the  retina are  less sensitive to  this  feeble
 sensation of  light.
    The incidental colours  also, which are formed in the
 eye, are most interesting. It is well known that there are
 combinations of colour which are pleasant to the eye, and
 some which  are unpleasant  or  even  ugly.   Pleasant
 combinations  are blue  and  yellow,  red and green  in
 all  shades ; while green and blue, yellow and green, and
 their accompanying  shades, are  distasteful.   We speak
 therefore of harmonious and, unharmonious colours, thus
 drawing a comparison from music.
    If, with reference to this point, we consider the posi-
tion of colours in the spectrum, we find that harmonious
colours  are nearly  complementary colours, whilst un-
harmonious colours are situated in the spectrum more or
less near to each other.
   This  does not, however,  scientifically  explain the
cause of  the harmonious  or unharmonious  relation 
THE SENSE OF SIGHT.             121
        between colours, which is first  satisfactorily explained
        by the following experiment.
           If we look for a long time at a green surface and then
        direct the eye upon a white one,  it appears for some mo-
        ments to be  of  a red  colour.  This  observation occurs
        with tolerable frequency  in  ordinary life.  In order to
        give it a scientific form, lay a  small square of green
        paper upon a sheet  of white  paper, and look at it very
        closely for some time with one eye.   Then look at the
        white  paper,  and a red square is seen upon  it, which
        follows the direction of the  eye and gradually fades
        away.
           Now what is the cause of this incidental colour ? The
        following is  the  simple  explanation : green  light  does
        not excite all the elements of the retina which are sensi-
        tive to light,  but  only  those  sensitive to green, and by
        looking for a long time  at the green  paper  these have
        become fatigued.  If we then look at the white surface,
        this excites all the sensitive elements of the retina ; the
        fatigued parts are least excited,  so that the complemen-
        tary colour appears in which the red predominates.
           Incidental colours are always seen in complementary
        colours.  If we had been looking at a red paper, then
        the white surface would look green ; the action  of blue
        produces a yellow incidental image, and vice  versd.  In
        short,  the  colour which  is seen,  and the  incidental
        colour, are always of such a kind,  that conjointly, they
        produce white.
           These facts give important support to the theory of
        colours just mentioned.  They explain why red should
r       excite the  nerves sensitive to red more readily than the
        rest, and green and violet the nerves  corresponding to 
122         THE FIVE SENSES  OF MAN.
them, and why the excitement  of  all  three  together
should produce white.
   The following is an observation which belongs to this
part of the subject and which is frequently made in ordi-
nary life.   If we  gaze for a moment at the sun, very
strong incidental images appear which last for some time.
They are  always coloured and frequently change their
colour.  This arises from the fact, that the colours of the
incidental image of white  sunlight  do not  disappear
simultaneously.  When one colour has  faded the image
is no  longer white, the remaining colours appear, which
gradually fade away after many variations.
   Colours were very early compared to musical sounds,
and we have  already spoken of the  harmony of colours
in this sense.  This comparison is, however, scientifically
satisfactory only to a certain extent.  The impressions
made by a mixture of  colours and a chord in music are
very different in character.  In a chord a practised ear
can hear  the different notes, and  separate it  into its
component parts. A mixture of colours, on the contrary,
makes an impression as a whole, and can only be sepa-
rated into simple colours by a practised eye, to a certain
extent.   It is impossible, however, for the eye to distin-
guish the primary colours in white, although we may be
quite certain that it contains them ; while in every com-
bination of tones each tone can be recognised. 
CHAPTER VII.
The Movements of the Eyes—Binocular Vision—Simple  and  Double
      Vision—The Identical Spots of the Retina—The Horopter.

The  eye  is  endowed  with great mobility  within  its
socket.   Since it has a spherical  shape and the hollow
in which it is situated has a spherical shape  also, it is
evident that this gives  the eye  the power of turning
in every possible direction.   The eye, as  we know from
experience, can  be moved  with  great rapidity, which
enables us  to  direct our attention rapidly  to different
consecutive objects.   We  should  appear  much more
clumsy, therefore,  if our eyes  were fixed firmly in our
head, and we  were always obliged to move the  head
from  side to side.
    Fig. 331 represents the situation of both eyes in their
sockets, together with the muscles which give them their
power of motion.   The socket is enclosed within walls of
bone, which, at the back,  contract to a  funnel-shaped
form  as far as the aperture  through which  the  optic
nerve passes.   The socket is filled up with a mass of fat,
in which the eye-ball  is embedded as in  a  socket-joint.
It encloses nerves, muscles and blood-vessels.  The figure
1 Helmholtz, ' Optics.' 
124         THE FIVE  SENSES OF MAN.
also  shows the passage of the optic  nerve (n) through
the bony aperture (o), and, before its exit,  the cruciform
shape of the combination of both optic nerves (m), which
is called the chiasma.  The muscles which are intended
for moving the  eye, are attached to it like the bridle to
a  horse's  head.   They  almost  all spring  from the 
THE SENSE OF SIGHT.
125
osseous wall at the point where the optic nerve enters,
and extend through  the  entire length of the socket to
the eye-ball.   There  are  four optic muscles which  pass
directly to the eye, of which one is situated above and
one below, one on  the  outer and  another on the inner
side.
    It is clear that the upper muscle directs the eye up-
wards, making it revolve upon the axis D, the lower one
downwards, the  inner one  inwards, and the outer  one
outwards.  Since, in  ordinary vision, we always fix  the
same  point  with both eyes, we therefore move  them
simultaneously according to  fixed laws.   If  we  look
upwards or downwards with both eyes, then the corre-
sponding muscles are always brought into action. If, on
the contrary, we look with  both eyes  to  the right,  the
outer muscle is brought into action for  the right eye, and
the inner muscle for the left, and vice versa. If, however,
we direct our eye inwards towards  a   near object, then
the two  inner  optic  muscles contract; if the eyes  now
look at  a more distant object, the two outer muscles
bring  the direction of the  eyes more nearly  parallel.
We  are  unable to turn  both  eyes at the same time
further apart than when their axes are parallel.
    Thus we see that the contractions of optic muscles  are
connected in many ways. The symmetrical and similarly
named muscles frequently contract  simultaneously,  and
the opposite muscles  frequently have a common action.
All these combinations, however, are intended to enable
us to fix  the eyes upon  the same point, so  that the
optical axes,  drawn from the yellow  spot through the
centre of the pupil, may meet in the  point upon which
the eyes are fixed.  We are never able so to move the 
126         THE FIVE SENSES OF MAN.
eyes that the optical axes shall  not  meet.   We cannot,
for instance, look with  one eye upwards and the other
downwards, or  with one eye to the left and the other to
the right.
    Besides the muscles named there are  two other
oblique muscles which  are  attached to the eyes in an
oblique direction.  The position of one is superior  and
internal (/) and  has a very curious course.   For instance,
it commences  at  the posterior aperture through which
the optic nerve enters ;  it does not then pass forwards
directly to the eye, but through a ring, u, like a cord which
runs' over a pulley,  then turns  round and  is attached
obliquely to the  upper surface  of  the eye-ball.  The
second oblique  muscle is situated upon  the lower side of
the eye-ball, and  is not shown in  the figure.  It com-
mences from the inner wall in the socket, passes  on-
wards  under the eye, and is attached (at r) opposite to
the upper oblique muscle.  The two  oblique  muscles
give the eye the power of performing movements which
are impossible with the aid of direct muscles alone. It is
easily seen in the figure that the oblique muscles can roll
the eyes round  an axis  (b),  which approximates to the
optic axis, in opposite directions.
    The various directions in which the eye can be moved
by means of the combined activity of the muscles named,
and the precision  of the motions described,  not only
allows  the picture of the outside world to be depicted
upOn definite parts of the retina, but also  gives  expres-
sion and life to our  countenance.  It is chiefly the eye
which  betrays  in our face  the  state of our mind  and
thoughts ; and this is done, for the most part, by the move-
ment and position of the eyeball, associated with which 
TFIE SENSE OF  SIGHT.
127
are,  of course, the action of the facial muscles, of the
eyelids, as well as the  power  possessed  by the eye  of
accommodating itself to  a change of circumstance.   A
troubled look  lowers the eyes; an  animated one raises
them;  and  thus the  mind,  while it derives  mental
nourishment from without through the eye,  reveals its
inner actions through the same organ.
   Monocular vision is incomplete.   The entire field  of
vision  is depicted on the retina as a plane surface, or like
a picture, without presenting any means of distinguishing
the various  distances of the objects from our eye.  With
a single eye we only, in reality, perceive a bright surface
with different lights, shadows and colours, upon which we
see  objects  in a single  plane.  But  practice  gives  us
indirect means of distinguishing the distance  of objects.
Objects whose size we know, we  should consider distant
if they looked small, and  near  if  they looked large.   In
monocular vision we also use the  laws  of perspective,
from  which objects gain  the appearance of solidity.
Again, we have  to adjust our sight much  more  to  see
distinctly objects which are near,  than when  they  are
distant, and by this  means we  are enabled  to determine
their distance from us.  But in reality, with one eye we
still  see merely  a plane  surface, we obtain no idea of
solidity, for it  is only  the experience  gained  in  life
which  impresses  upon us the fact that we have to do with
a world of  space.   Where this  experience fails us, we
not  unfrequently, in monocular vision, fall into extra-
ordinary illusions.   For instance,  if we are  looking  at
the sky, and a small insect flies past, so near to one eye
that it is not seen by the other, we imagine we have
seen a great bird in the sky.   The following experiment
               1 
128         THE FIVE  SENSES OF MAN.
 can easily be made.   Suspend a ring by  a thread,  and
 let some one hold it before you  at a certain distance
 so that the thin edge only of the ring is visible.   Then
 try  to  pass  a small stick  through the ring, and it is
 astonishing how difficult it is  to aim  correctly.  If the
 other eye is opened, it is done with ease.
    Binocular vision  reveals to  us  the third dimension
 of space, enables us  to look into the depths of  the
 surrounding world.  Even with a single eye, by means of
 the  touch, and  the  movements we  make  in space, we
 should be able to arrive at some notion of it, but a clear,
 immediate view would be impossible under any circum-
 stances.
    We will proceed to  consider binocular vision  in its
 simplest relations.  If we fix our  gaze upon a certain
 point,  the corner of the table opposite us,  then in both
 eyes the corner falls upon the yellow spot  of the  retina,
 and both optical axes, when prolonged, meet in the corner
 of the table.  Although  an image  of  both points is
 formed in both eyes, yet we see a single point,  and  not
 two  points.   This is  the first  enigma which we meet
 with in  this  part  of our subject:—Why  do we  see a
 single object, and not two objects, with binocular vision ?
   A definite answer  to this  question  cannot yet be
given, but we may say that the union of both images
in one is an act  of the  brain.   From  an  idea that  the
chiasma  of the optic nerve has some influence on  single
sight, it has been thought that a union of the nerve-fibres
takes place there.  Nevertheless, this is not the case, and
we must confine ourselves to the idea, that the brain is
able, under certain conditions, to unite the excitements 
THE SENSE OF SIGHT.
I29
of the two nerves, since it transfers the cause of them to
the same spot in the outer world.
    Thus if  we  fix  our gaze upon  the  corner of  the
table when  it is  at  a certain distance, the two optical
axes intersect at this point and form an angle with each
other, which  is called the  angle of  convergence or  the
optic angle.  If we  now allow our gaze to  move further
along the edge of the table from the corner, the angle of
convergence gradually diminishes.  We have, of course,
no means of knowing the  size of this  angle without
scientific measurement, but we have  a delicate sense of
the position of the eyes by means of the muscles  which
move them outwards or inwards, and this sense enables
us to determine the  distance of  the  objects seen.  This
sense is called the muscular sense, which is  more or  less
developed in all our limbs, and  by  means of which we
measure all  our movements, maintain our  equilibrium
when walking or standing, and which gives us much of
our dexterity.
    Thus, while we allow the point of intersection  of the
optical axes to wander over space, we become conscious
of its extent, and of the distances of the objects contained
 in it.  But this is not the only advantage which we  gain
from binocular vision.  We have, hitherto, only spoken of
 the observation of the single point fixed by the eye, which
 is depicted upon the yellow spot of the retina.  The whole
 retina, however, takes part in the act of vision, though
 with diminishing distinctness towards its edge.  Thus, in
 our eye, all the pictures on the retina are combined in
 a peculiar  manner, giving an  appearance of solidity to
 the objects  seen.   If both eyes could only perceive the 
13°         THE  FIVE SENSES OF  MAN.
 point fixed by them, we should still be able to obtain  a
 knowledge of the nature of a body, by allowing them to
 wander over its surface.  This might be done  perfectly,
 and with tolerable rapidity, though we should  miss  the
 idea of solidity,  which  immediately  takes  place  in  bi-
 nocular vision.  For  if we fix our eyes as steadily as
 possible upon  a certain point of an object, the  corner of
 a table, for instance, we perceive the entire object  at the
 same time, since  the  image of the whole  table  is im-
 printed upon the retina.
    Before we  enter further into the question of our per-
 ception of solidity, we must consider a property of the
 retina,  which  has hitherto only  been ascribed to the
 yellow spot.   If a point in the field of vision falls upon
 the yellow spot of  both  eyes, we then  perceive  a  single
 image of it.  This power of combining the visual image,
 however, is possessed by the entire retina, for in ordinary
 sight the whole field of vision presents a single,  not a
 double  appearance to us.
    The parts of the retina  whicn  are able  to  combine
 the  images depicted on them are called  identical  or
 correspojiding   points.  It  is  easy  to see  the  relative
 situation  of these  points.   If we take a point in the
 field of vision to  the right,  this point is  depicted  in
 the right eye upon the inner  half of  the  retina, but  in
 the left eye, on the contrary,  upon the outer half of the
 retina.  If, therefore, it is possible to see this point as a
 single and not as a double one, then in the outer half  of
 the left retina,  and the  inner half of the right, there must
 be  points  which  mutually correspond.   It  is  evident,
therefore, that the upper and lower halves of the retinae
must correspond as well as the  two right  and the two 
THE SENSE OF  SIGHT.
131
left halves, for the entire upper half of the field of vision
is depicted upon the lower  half in both eyes, and  the
lower upon the upper; the left upon the right, and  the
right  upon the  left.   Hence the corresponding points
are found by imagining the retinas of both eyes to be
superposed  in  their natural position.  Points  which
coincide, correspond as the points a, b, c (fig. 34), on
                        Fig- 34-

the  two semicircles which represent the right and left
retinae.
    It  can easily be  shown  that,  in reality,  we do
perceive  double  images,  if  they  do  not fall  upon
corresponding parts of the retinae.  If we hold a finger
at some distance from the  eyes,  so that we can see a
more distant object beyond it, the window or the lamp,
for instance, then the finger will appear double as  soon
as we fix our eyes upon the distant object, and the latter
will  appear double  when  we fix our eyes upon  the
finger.
   In fig. 35, both eyes are  represented  in  the position
in which they would be fixed  upon the finger / the
image of which is depicted upon the yellow spot and is
seen singly.   On the other hand, the distant object g
would  appear double,  for it  is  depicted at g1 and^-2,
two points, one of which lies to the right and the other to
the left of c, and, therefore, cannot correspond.  We  then 
132         THE FIVE SENSES  OF MAN.

see the image of g on each side  of /at G ! and G2, and
indistinctly as long as/ is distinct.  If we shut the right
eye then the image  G2 on the same side disappears, if
the left eye is  closed Gl  disappears.   The  two  points
                             on the retinae^-1 and g2
                             can,  under  no  circum-
                             stances,  enable  us  to  see
                             a  single  point in the field
                             of vision as such.
                               If, now, we fix the eyes
                             upon  the  object g,  the
                             finger  immediately  ap-
                             pears double (fig. 36) ; g
                             now  falls  upon the cor-
                             responding centres c, and
                             is seen as a single point.
                             In the right eye the image
                             / is formed to the right of
                             c  at/2, and in  the left to
                             the left of c at f\   We
                             transfer both images to
                             the opposite  sides  at F 1
                             and  F2.   They fall upon
                             points of the retinae which
                             do  not  correspond,  and
                             consequently appear to us
           Fig. 35.        .    double.   If we close the
                             right eye, then the  left
 image disappears, and vice versa.
    From  the  position of corresponding points in the
 retinae, we can find for  every  position of the eye, by
 construction, whether a point of the field of vision  falls
 
THE SENSE OF SIGHT.
*33
upon corresponding  points or not, whether, therefore, it
will appear single or double.
   There is only one case in which every point of the
field of vision is seen single, and that is when it is at a
very great distance.   When we look, for instance, at the
C       I        c
                        Fig. 36.

sky, which is at an immense distance from us, and  fix
the eyes upon a single star, then all the other stars ap-
pear single and not double. 
134          THE  FIVE SENSES OF  MAN.
    While the two rays(i)  which proceed from the star
looked at are parallel to each other, the  rays (2) which
reach the eye from a star situated to the right are also
parallel  (fig.  2>7)  and  meet the  retina at a a.    Since
these points are situated at an equal distance from, and
                             upon the  same  side of,
                             c, they cause the percep-
                             tion of  a single image.
                             Objects on the  horizon, or
                             distant  landscapes,  when
                             seen from  the top  of a
                             mountain, produce nearly
                             the same effect, since the
                             angle of convergence in
                             these cases  is insignifi-
                             cant.
                                 As soon, however, as
                             we  direct our  eyes  upon
                             near objects after viewing
                             distant ones, then the con-
                             ditions  under  which the
                             visual perception is single
                             or  double  become  com-
                             plicated.   A particularly
                             interesting  case  of  this
kind was first accurately examined by Johannes Muller.
If we (fig. 38) fix our eyes  upon  a point C,  situated in
middle distance, then the optical axes converge, and C is
depicted upon the centre c.   A circle is now described,
which passes through C  and the  optic  centre k of both
eyes, from which it can be shown that all points in space
which lie upon the circumference of this circle are seen
Fig. 37- 
TFIE SENSE OF SIGHT.
135
singly.  Take a point A, and draw from it the imaginary
lines A k a to both eyes, meeting the retinae at a, then the
angles c k a in both eyes are equal, since they are  equal
to the angles at the circumference A k C.  Thus, in both
eyes,  the point a is equidistant from  c, and on the same
side ; hence, the point A must be seen singly.
    We can  convince ourselves of the truth of this by the
following experiment.
    Take two quill pens, and hold one in the position C,
firmly before the eyes, fixing them steadily upon it. Now
move the other pen along the  line x y;  we shall then
perceive that a double image  of it is  seen when it is
moved towards x and y, and that  there is one point A at
which it is seen single.
   This imaginary figure in space, in which all points are
seen single, is called the Horopter, which, in the cases  ob-
served, always forms a  circle. It has also been discovered 
136         THE FIVE  SENSES OF MAN.
that a straight line can be drawn through the point C per-
pendicular to the circumference, upon which all  points
are seen single also.
    For other positions of the eye complicated Horoptery
figures have been constructed,  which we  will not enter
into further.  The only other interesting case is when we
look into the distance, standing in an erect position. As
Helmholtz has shown, the ground then forms our Horop-
ter.  When  we think how safely we step  forwards with-
out fixing our eyes on the ground before us, this property
of the eye will not appear  unimportant,  for we should
stumble  much  more  frequently if  we fixed our eyes,
when walking, upon an object  at an equal height with
our eyes. 
CHAPTER VIII.
The perception of Solidity—The Stereoscope—The Telestereoscope—The
  Pseudoscope—The perception of Luminosity—The struggle between the
  Fields of Vision.

ALTHOUGH we can prove  from experience  that, in bi-
nocular vision,  double images really exist, and can show
that only limited portions of space can  be seen  single,
yet we are quite unconscious  of  the  existence  of such
double images in  our general vision.  A hasty  glance
round the room, or through the window upon the street,
convinces us that we have  seen no  object  double, but
everything single.  We have learnt, further, that we are
looking into space, where objects are situated at different
distances, and not upon an even surface of any kind. In
binocular vision we gain an idea  of space,  and an im-
pression of solidity.
    The  conditions under which  we obtain an idea  of
space with binocular  vision are easily  discovered by  a
simple consideration.  If, for instance, we fix  the eyes
upon an object in the room so that we see the  wall be-
hind it, it will  then conceal from view a certain portion
of the wall.  The portion concealed is not, however, the
same for both eyes, but it  lies  more  to the left for the 
138         THE FIVE  SENSES OF MAN.
right eye, and to the right for the left eye,  and  if we
close quickly first one eye and then the other, the object
in front of the wall will  appear to move from  side to
side.  Hence, it is evident that the images on the retinae
of both eyes cannot possibly be the same ;  the  objects
seen must appear rather to merge into each other.
   The difference  between the  two retinal pictures of
course is not determined by the will but by a  definite
law.   Suppose that we have looked at an object from
two different points of view; then we shall have received
two  images, which bear  a  strong  resemblance to each
other.   This is the case, in a less degree, with binocular
vision, for the right eye has a different  point  of  view in
the head to  the left.
   In a special case  mentioned above, we have  already
made use of the law under which the displacement of
objects seen takes  place.   As  a general  rule it  will be
found that with two objects, placed one behind the other,
the nearer one is displaced by the right eye to the left,
and by the left eye to the right.
   With a large field of  vision,  filled with a  number of
objects, it becomes very difficult to find by construction
the direction and amount of displacement for both reti-
nal images.   With a single body,  however,  the  result is
much more easily attained. Let us take, as an example,
a truncated  pyramid of four sides, which is viewed from
above.  The appearance will  be similar to that repre-
sented  in fig. 39  (p).  If,  however, we  look only with
the right eye, then the summit of the cone will  seem to
be displaced towards the left,  as in r,  and towards the
right,  as in  /,  if  we look only with the left eye.  Of
course it is here supposed that the head is held  perpen- 
THE SENSE OF SIGHT.
139
dicularly over the pyramid, so that the centre of the head
is perpendicular to the centre P.
   Now if we look at the pyramid with both eyes, it is
clear that the image r is depicted upon the right eye, and
3     _

                       Fig. 39-

the image / upon the left.  These images are different,
yet experience shows us that both are combined so as to
produce the impression of solidity.
    The celebrated philosopher, Wheatstone, was the first
to state that the perception of solidity depended upon
the dissimilarity of the images  depicted upon  the retinae
of the eyes.  He constructed a stereoscope of a very sim-
ple kind, in which two drawings were viewed  in the fol-
.©    .0
Fig. 40-
lowing manner:  The two eyes, R and L (fig. 40), look at
two mirrors, s and s, which are placed obliquely to the 
I40         THE FIVE  SENSES OF MAN.
eyes, and at right angles to each other.   At r is placed a
drawing, one of the pyramids for example, as it appeared
to the right eye ;  and at / the corresponding picture  as
seen by the left eye.  According to the laws of reflection,
each eye now sees the image intended for it in the pro-
longation of the optical axes, where both images are com-
bined so as to produce an image in relief.   We think we
see a single object  simply because each eye  sees the
same object which it would see  if the  rays  proceeded
from a real object.
    Wheatstone's  stereoscope has been superseded  by
Brewster's, which is known to all.  There is no need  to
                         describe  its  outward  form.
       ■-----tt----p       The  most important part  of
           / \             this  stereoscope  consists  in
           / \            two lenses (fig. 41) s and  s,
          1   \           through which  the  two eyes
         ,'    \           R and  L look.  The pictures
         '     '                              1
     a  I     \   a       r and  I respectively  corre-
     j   /      \  !        spond  to the  right  and left
     j   /       \ |        eye.  The ray a b  from the
     !  /        \ I        right-hand picture is refracted
     ! /         \ |  •     by the  lens, so that it appears
                         to come from c, and the same
                         is  the  case  with the  corre-
                         sponding ray  for   the  left
                         eye.  Thus both pictures are
                         united  in the point p,  where
                         each  eye seems to see its cor-
                         responding  picture,  and the
two pictures therefore unite to form one picture in relief.
   Photography, as is well  known, has been  of great
Fig. 41. 
THE SENSE OF SIGHT.             141
service in the production of stereoscopic pictures.   Two
photographic cameras, instead of the eyes, will produce
pictures exactly similar to  those we see in  reality, and
this result is not only attained  with single  objects, but
with a space filled with a great number of objects, which
may be grouped entirely- according to taste.   The im-
pression of relief, which the stereoscope always produces,
is, therefore, very striking.
   It is also possible to see stereoscopic pictures without
any instrument, if the axes of the eyes are placed paral-
lel to each other, so that the right eye is fixed upon the
right picture, and the left eye upon the left.  We must
then look as if we wished to see a distant object through
the picture.  To do this, make a small hole in  the centre
of each picture, and hold the paper in such  a position
that each eye looks through the hole at one distant ob-
ject.   If we now try to fix both eyes  upon this object,
and to see it distinctly, which is easily done, we see that
the two pictures have combined and produced a stereo-
scopic picture.
   In looking through a stereoscope, we allow the  eyes
to wander over the field  of vision  just as we do  when
looking at ordinary objects, and the appearance of relief
will be most distinct at the point of intersection  of the
optical axes.   The eyes converge when we look at a near
object, and diverge when we look at a more  distant one.
If, for example, we look at the bases a b and ax bx of the
pyramid (fig. 39), and then at the nearer edges c d and
cx dx the optical axis of the right eye moves from ax bx  to
cx dv whilst the optical axis of the  left only moves over
the much smaller space from a b to c d, proving that the
two axes must have converged.  This accounts for the 
142         THE FIVE SENSES  OF MAN.
feeling, that the object of our vision is at a greater  dis-
tance from us than it was before.
    This movement of the point fixed by the eyes, which
is situated at the intersection of the optical axes, affords
an important aid towards the determination of distance,
and towards the comprehension of the objects seen in
space, and evidently  plays an important part in enabling
us to see stereoscopic pictures.   This might  lead us to
suppose that our  perception of solidity arose from an
investigation of the entire space by the rapid movements
of the eyes, and from our being thus enabled  to  gain a
knowledge of its  dimensions.  But, however  important
this fact unquestionably  is it is, quite possible  to  see
objects in relief without moving the eyes at all.  We see
objects in relief, for instance, during the momentary illu-
mination caused by a flash of lightning, or  by an electric
spark,  although the eyes have had no time to make  any
movement.
    This has been  still more clearly proved by  Dove's
experiments, in which stereoscopic  pictures were seen
under  the momentary flash of the electric light.  The
impression of solidity was generally quite perfect within
the range of distinct vision in  the centre  of the field,
whilst as to the outlying parts of the  field  of vision, no
decision could be made concerning them on account of
their indistinctness.
    It is very remarkable that in these experiments double
pictures were not seen any more than in  ordinary vision,
although they must have been present.  Double images
can only be seen when we look at objects long and atten-
tively.   The former  is impossible under a momentary 
THE SENSE OF SIGHT.
143
illumination, and our attention is otherwise occupied  in
ordinary vision.
    From this it follows that in the perception of solidity
the double images are united in a single  one, not, how-
ever, through any property  of the retinae,  or the optic
nerve, but  through the will alone; for we  can see the
double pictures if we wish, especially if we have time  to
direct our attention to it.   Neither is  the combined im-
age due to our neglect of the image depicted upon the
right or left eye, for both are united in the  imagination.
Here, again, we are brought into contact with a mental
act, which, as yet, we have not been  able to  explain
physiologically.  Still, we must add that this imaginative
power can  only be attained by experience, and  that we
should certainly never possess  it if we had  not acquired
a knowledge of the dimensions of space by constant prac-
tice.  This experience is chiefly acquired  through the
unceasing movements of the eyes.
    The combination  of the double images is a substan-
tial aid towards  our  perception of solidity.  For, since
the two images are dissimilar, and we see with both eyes
a larger portion of an object than we do  with one eye,
the object  seen cannot have the appearance of  a  plane
surface, and our experience tells us that it is a solid  ob-
ject.
    Stereoscopic pictures have given us  a closer insight
into the law  which governs our perception of  solidity.
The dissimilarity of  the two pictures explained why the
four-sided pyramid was seen  in  relief.   From this it  is
easy to discover the appearance the picture must present
if we were  to look from the base into  the interior of  a 
144         TIIE FIVE SENSES OF MAN.
four-sided pyramid. We have only to change the position
of the two pictures, as in fig. 42.  The right eye sees the
picture r, in which the point  is displaced to the  right,
	-J d/
	*"\
                        Fig. 42.

and the left eye sees the figure / where the displacement
is towards the left.  The stereoscopic impression results
from our appearing to look into a hollow body.
   It is precisely the same with  the following figures
(fig. 43).  The first has the appearance of a  truncated
cone, the  summit  of which is  turned towards us; the
                       Fig. 43-

second has the appearance which such a cone would pre-
sent if we looked into its interior from the  base.  If, by 
THE SENSE OF  SIGHT.
145
a simple contrivance, we transfer the inner circle in such
a drawing from position 1 to position 20 we can observe
directly the change of the one body into the other.
   From  this experiment it  is clear  that the  change
of the position of any two  stereoscopic pictures would
always be attended with a similar  result, i.e., all the  ele-
vated parts  would be changed into depressions, and vice
versa.   An  instrument arranged so as to produce these
inversions with simple bodies is called a Pseudoscope. It
changes convex  into  concave,  alto-relievo into basso-
relievo.  But the inversion  of a complicated picture—a
landscape, streets, etc., produces an impression which  is
perfectly bewildering.   It seems as if all the  objects-
men, trees, etc., had been placed in a depression of the
earth, and yet everything remains in its place. Therefore,
the nearer objects appear very large, because we imagine
them to be at a great distance,  and the more distant
objects smaller, because they seem to be nearer.
    The action of Helmholtz's Telestereoscope is very in-
teresting.  By means  of this  instrument we  obtain a
stronger stereoscopic view of distant objects than we are
able to do with our own eyes.  Let the two eyes r and /
look into two  mirrors s  (fig. 44),  which,  as in Wheat-
stone's stereoscope, are placed obliquely to the eye.  At
a little distance on either side, and  almost parallel to
them, are placed  two other  mirrors s\   The rays from a
distant object fall upon these  two latter mirrors,  then
upon the first  pair, and from these reach the eye.   The
same effect, however, is produced as  if our eyes were
placed in the position of the two  mirrors s1,  and were,
therefore, at a greater distance from each other.  Let us
suppose a pair of eyes to be so placed in a gigantic head, 
146         THE  FIVE SENSES OF MAN.
it  would at once be  evident that they would be able to
see a larger portion of a distant object than we  do with
two ordinary eyes, and this would produce a more power-
ful stereoscopic effect.  We see from the figure, that in
looking at the ball K, the eyes would only see the section
                        Fig. 44.

a c a, but, with the help of the instrument, they would
see the larger section, ba cab, because the rays from b
are reflected by the mirror sl upon the mirror s.   Look-
ing at a landscape in this manner, it will seem to have 
THE SENSE OF SIGHT.             147
been brought nearer, because it is only when objects are
near to us that we see so large a  portion of their surface
as we do  by means of this instrument; and  since the
apparent size of the objects remains the same, the im-
pression is produced of  a  model, in which the different
bodies stand out in high relief.
   The study of stereoscopic phenomena  has  also ena-
bled us to explain luminosity. Those bodies are lumi-
nous which can reflect light, but are not perfectly  smooth
and level.   Water, when quite smooth, reflects  perfectly,
but is not luminous, becoming so as soon as its surface is
ruffled by ripples.   Silk is luminous because  each par-
                        Fig. 45.

tide reflects light, though altogether they do not form a
reflecting surface.   Now it has been observed that if two
pictures are viewed through the stereoscope, one of which
is white and the other black, as in fig. 45, this gives rise
to an impression of luminosity.  This phenomenon has
been explained in the following manner.  A given point
upon a luminous surface can never appear equally bright
to both eyes, because it does not reflect the same amount
of light in every direction.   We know this, in a general
way,  from the fact that such  luminosity  changes its
position with the movement of the  head.    It is to  this 
148         THE  FIVE SENSES OF  MAN.
peculiarity that luminosity, in a great  measure, owes its
existence.  Now the same effect is produced by looking
at the two pictures, one of which  appears black to one
eye, and the other white to the other eye.  They do not
combine so as to form an equal  tint of grey, but a strug-
gle ensues between the fields of vision, the colour seeming
to be first black, then white,  and producing the sensation
of luminosity.  The change  from  dark to light upon the
same point of the retina, produces an unpleasant quiver-
ing or  twinkling  light, which is  caused by  weariness.
But, on the other hand,  the alternating changes  from
dark to light in both eyes, which produces luminosity, is
a pleasant sensation.   In this case no  unpleasant irrita-
tion of the retina takes  place,  such as is caused  by a
flickering light, but our attention moves from one field of
vision to the other, and it is  the alternation, apparently,
which pleases us.   The pleasure which  luminous bodies
give, and which  it is natural for all men to  feel, only
lasts while the luminous body transmits those changing
lights to the eye ;  it ceases as soon as the surface becomes
perfectly smooth, and, as a looking-glass,  reflects  to  us
the naked truth.
    It is very interesting to observe how faithfully this lu-
minosity is reproduced in photographic stereoscopes. The
gleam of the setting sun upon the  rippled surface of the
sea, the brightness of columns of  marble,  appear in the
pictures just as they do in reality, and yet, when we look
at each single picture with the unaided eye, we can only
see the dark and light spots, which could never give the
impression of luminosity.  But when we look  closely we
see that many spots in the one picture are dark, which 
THE SENSE OF SIGHT.
149
are light in the other.  It is the combination of the two
which produces luminosity.
    The struggle between the fields of vision, which has
been mentioned above, is still more remarkable, if, instead
of black and white, we  choose two colours, blue and red
for instance.   We do not then  see a single mixed tint,
as we might be led to suppose, but a hazy uncertain pas-
sage from one colour to the other, which wanders hither
and thither.   Here and there a mixed tint is seen, and
the whole has a somewhat luminous appearance.
    The struggle  between  the  fields  of vision is also
called into action, if two differently coloured glasses are
placed before the eyes, when looking at a white surface.
The interchange of colours is quite irregular, one colour
appearing in the  midst of the other without any rule.
Some experimenters, however, assert that they see one
colour or the other at will, from  which it would appear
that we are able  to give special attention first to the
image on  the right retina and then to that on the left.
    The union of the two retinal pictures  into a single
picture in relief still  remains one of the most wonderful
phenomena of the senses, in spite of all these  observa-
tions, which some regard as quite a sufficient explanation.
We must be satisfied for the time in fixing the  physical
and physiological conditions of these phenomena.   The
representation of the  material  world  is  a distinctly
mental act which takes  place in  the  brain, and as such
will, for a long time, escape scientific research. 
150         THE  FIVE SENSES OF  MAN.
                   CHAPTER  IX.

Optical Illusions-Apparent Size of the Moon—Intuition—Illusions of the
          Sense of Colour—Mental and Corporeal Vision.

However perfect may be the  optical apparatus  with
which nature has provided us, there are cases in which
we see things differently to  what  they are in reality.
Such phenomena we call optical  illusions,  and they are
the more interesting because they give us a closer insight
into the act of perception.
    We can judge with tolerable accuracy as to whether
two lines are  parallel  to each other or  not.  Zollner,
however, has remarked that we are subject to  a  curious
deception if the  parallel lines  are crossed  by  short
oblique lines which slant inwards as in fig.  46.   The
lines 2 and 3 appear at first sight to widen downwards,
the lines I and 2 upwards, and  yet they are exactly paral-
lel to each other  when measured.   The oblique strokes
on  the lines 2 and  3  would  intersect each  other if
extended  downwards, and the result is that  the  lines
themselves look as if they would intersect each other if
extended  upwards.  Thus the apparent convergence of
the lines always  has an  opposite direction to that of
the oblique  strokes. 
THE SENSE OF SIGHT.
151
   This  phenomenon may  be explained as follows :—
The  oblique strokes produce an  error  of judgment
by leading  us  to imagine  that,  because they  would
intersect if  produced in an up-
ward  direction,  the  lines must
do so if produced in  a down-
ward direction.   The delusion is
stronger if the lines are placed
in a horizontal position. •
   This bewilderment  of  the
judgment in many  cases gives
an apparent motion to objects.
The  most  familiar example  is
the giddiness which  is felt after
we have turned the body round
rapidly.  For a short time, after
the motion  has  ceased, the  ob-
jects  round us seem to move  in
an opposite direction.  A some-
what  similar phenomenon takes
place if a number of objects are
moved   rapidly  across  a  sta-
tionary background.   In look-
ing, for instance, at a  waterfall,
through which the rocks are visible, we are, after a time,
seized with the  impression that the rocks  are moving
upwards.
   In these  cases we transfer some of the  rapidity of
the object in motion to the object which is  at rest,  but
in the  opposite direction.   This phenomenon  is still
more  striking if, when sitting in a train which is  not in
motion, we look at one which is passing.  We are very
Fig. 46. 
152         THE FIVE SENSES  OF MAN.
frequently impressed with the idea that we are moving
ourselves, and the  other train is standing  still, and we
can only undeceive ourselves by looking at the  motion-
less  objects through  the other window.   In  this case
we  transfer  all the rapidity of the  other  train to our-
selves, and thus receive the impression of motion.
    It seems as if the motion of the  eyes played a con-
siderable part in many of these illusions.  When looking
at Zollner's figure, our eyes may easily be displaced from
their ordinary position, by rotation round their axes, into
an oblique position, from looking at  the oblique lines, so
that the  vertical  planes , of the optical axes  are  at an
oblique angle  to  each other instead  of being  parallel,
and this  may contribute towards the error  in  our "judg-
ment as to the direction of the parallel lines.  In support
of this view  Helmholtz mentions  the fact that, in the
momentary  illumination of the electric light, the illusion
is absent or  at least much weaker, because in the moment
of vision there is not  sufficient time for the eyes to make
any movement.
    The  movement of the eyes can also assist in pro-
ducing giddiness.   For  whilst we are turning round,
the eye tries to fix for a time every  object seen, and in
our revolution passes  in a backward direction to the next
object.   In this manner a backward  motion of the eyes
is produced  which will continue for  some  seconds after
our own motion has ceased, and it is probably this which
causes the apparent motion  of objects.  At least all the
motions  of the eyeball, which are made by the muscles
of our eyes, independently of our consciousness or will,
give rise to  an  apparent  motion  in  the surrounding
objects.  This is  the  case  in the rapid motion of the 
THE SENSE OF SIGHT.
153
eyes in illness, which  produces a feeling of giddiness.
This  apparent  motion  of  objects  is  observed  if
we  displace the eye-ball  with  the  finger, and  move it
gently  backwards  and  forwards.   Objects  will  then
appear to move in an opposite direction to the motion of
the eye-ball, because we are unconscious of the motion
of the eye-ball in the reverse direction.
    A  somewhat similar effect is produced by looking for
some time at objects in motion.  In looking at a water-
fall the eye endeavours to follow the falling mass for a
certain distance, and  from continually  returning to the
first point, acquires a backward motion.  If we continue
looking for some time, we shall notice that the stationary
objects round us appear to be moving upwards because
we  are  unconscious   of  this downward motion of the
eye.
    This confusion of our judgment on horizontal, upward
 and downward directions is of frequent occurrence.   Let
Fig. 47-
us suppose, for instance,  that  when  walking down a
gentle incline  b  on 'a road (fig.  47), we see at some
distance before us a road a, which has exactly the same
inclination, and which  joins b.   We shall  find that we 
154         THE FIVE  SENSES OF MAN.
generally exaggerate the incline of a and  are  surprised
at finding it much less steep than we had supposed.
This illusion seems to arise from our taking the road b,
upon which  we are walking,  as the base  by which the
incline a is to be estimated.   Our imagination identifies
b more or less with a level surface, which is our ordinary
standard, and, therefore, the road a, as may be seen in
the dotted figure (a' U), appears to be steeper than it  is
in reality.

             —.----------------------1

                        -----------— a

                            -----—_ b'
                       Fig. 48.

    Sometimes, in  mountain  districts, we  meet with
another remarkable illusion: a brook will  appear  to set
the laws of gravitation at  defiance,  and to  flow uphill.
If we are walking downhill upon a road a, near which a
stream b flows, which has a more  gradual  fall than the
road, or, perhaps, is nearly horizontal, we  shall imagine
that the stream is flowing uphill  in the direction  of the
arrow.   In this case, again, we imagine the road to be
horizontal, since we have been accustomed to employ it
as the base by which  we  estimate the  position of sur-
rounding  objects ; and,  therefore,  as  a!  U shows, the
stream appears to be  flowing in an  upward  direction.
The apparent uphill  flow of the water  is  still  more
striking if it flows in a trough over the downhill road.
    We are also liable to many errors in our estimation 
THE SENSE OF SIGHT.
155
of the size of objects.  It is a well-known trick  to  ask
a person to fix the spot on the wall to which a hat would
reach if placed against the wall on the floor.   When the
hat is placed against the wall, the height to which it
reaches is found to be considerably less than was sup-
posed.  The reason of this illusion is  that we always see
the wall foreshortened, but the real  size of the hat is
impressed upon our  memory, and, therefore,  we  always
exaggerate  in our  minds the height  upon the wall to
which we imagine  it would reach.   Moreover, in the
uniform surface  of the  wall  we have no  measure by
which we can estimate its distance, and in such  cases it
is always observed that we  imagine the distance to be
greater than it is in reality.   Upon a papered wall where
the lines act as a guide to our judgment, the  illusion, of
course,  is not so great.
    We can give these observations a more definite form
in the following manner.  Imagine two squares of equal
1
                       Fig. 49-

size composed of parallel lines (fig. 49), vertical in  the
one case and horizontal in the other.  We shall find that,
at the first glance,  in the latter case the height appears
to be greater than the breadth, as in fig. a, while in  the
former (fig. b),  the  breadth is, apparently, greater than 
156         THE FIVE SENSES OF MAN.
the height, and both  appear to occupy a greater space
than the square c, of  equal  size,  but without lines.   In
this case we base our  estimate of the size of the squares
upon the number of intervals between the lines, the sum
of which gives us  an idea of the size of the square, which
we do not  obtain  when we look in a horizontal direc-
tion along the lines.   The  sum  of  the  parts appears,
therefore, greater than the whole, because, in the  former
case, we obtain an idea of the size of each part, whilst the
undivided whole  does not produce such a vivid impres-
sion of its contents upon our imagination.   We imagine,
therefore, that the empty square  cannot  contain  the
other two, although,  in reality, its size is  exactly  the
same.
    Our vision, again,  is subject to various illusions when
exercised in cases in which it has had no experience.   If,
for instance, we see people  walking on a small hill  the
summit of which  is sharply  defined against the sky, and
if  their figures stand  out  against the horizon, they
appear to be of a gigantic size, although, when seen close
at, hand, they are not above the usual  height.  This
error in judgment is due to  the fact that the figures  are
standing out against the horizon, and that, according to
our ordinary experience, all  objects upon the horizon are
at a considerable distance from us, which has accustomed
us to consider this distance  in our estimation of size, and
to imagine that all objects seen upon the horizon are
necessarily of a considerable size.  It is this experience
which, under a change of circumstances, leads our judg-
ment into  error, for  in this case we  imagine that  the
persons seen are  at a greater distance than they are in
reality. 
THE SENSE OF SIGHT.
157
    There is a common question :  What is the apparent
size of the moon in the sky ?  Some will say as large as
a plate,  others  as large  as  a  florin, and others, again,
hardly as large as a sixpence.
    It is clear that  neither the question nor the answer
has any strictly scientific  meaning. For those who know
that the moon is a great sphere, whose diameter is 2,160
miles, and which travels through  space at a distance of
237,600 miles from the earth, will  never compare its size
with that of objects upon  the earth, but will always have
an  idea of a very large and very distant sphere.  But to
the inexperienced eye the stars in  the heavens appear to
be  all  fixed  at  equal distances from us on a surface,
which stretches over the earth like a vault. The heavens
appear to have the form of a rather low bell placed like
a lid upon the horizon, which seems to be at a greater
distance than the  zenith, although  we never form  any
definite idea  of this distance.   Here it  seems as if the
disposition of the moment, or our individual conception
played the principal part, and that on this depends the
object with which  we compare  the apparent size of the
moon.   If we cut  a small circular  hole about the size of
a sixpence in a sheet of paper we  shall find  that the
moon can be seen perfectly through  this hole, when it is
held at a short distance of some steps from the eye.   But
if we make a hole  as large as a plate, it must be held at
a distance of several hundred yards to allow the moon to
fill  it exactly. The object, therefore, with which we com-
pare the  apparent size of  the moon is quite an arbitrary
matter.
    Another circumstance  however,  which is not  arbi-
trary, deserves scientific  consideration.   Everyone has 
i58
THE FIVE SENSES OF  MAN.
 remarked how much larger the moon appears to be when
 sinking  below, or just  rising above the horizon, than
 when it stands high  in  the  heavens.   In  this case we
 have not to deal with  the refraction of rays of light in
 layers of air of different density ; for although refraction
 causes  an apparent elevation of the heavenly bodies
 above the horizon, so  that the sun and moon appear to
 be flattened out in a perpendicular direction, no real in-
 crease in the apparent diameter can be produced in this
 manner.
    Two reasons may be given for the cause of this illu-
 sion.  In the  first place the heavens  present to us the
 appearance of an ellipsoidal bell. We, therefore, suppose
 its edge upon the horizon to be at a greater distance
 from us than the  highest point in the zenith ; and since
 we imagine the moon to be placed upon the  even surface
 of the  heavens,  we  imagine  it  to  be at  a greater
 distance when  upon  the horizon than when in  the
 zenith.   But we know  from experience that the apparent
 size of an  object decreases  as its distance  from  us
 increases;  and that the greater the  apparent size of a
 distant object,  the greater shall we imagine its real size
 to be.  Now since the moon  seems to be more distant
 when on the  horizon  than  when  at  the  zenith, its;
 apparent size still remaining  the same,  we suppose its
 real size to be greater when on the horizon than'when
 at the zenith.
    But a second cause  of  this optical illusion is the fact,
that when the  moon is on the horizon, we compare its
size with the size of objects on the earth. If, for instance
the moon is near a house, we see that it occupies a much
larger space than  the  house ; and  we involuntarily try 
               THE SENSE OF  SIGHT.            159

to imagine how much larger such  a ball would be than
the house if it really stood near it.  In this  manner we
arrive at an idea of size which is comprehensible to  our-
selves, and of  considerable importance for terrestrial
objects.  We  cannot make a comparison of this kind
when the moon stands high in the heavens ; and, as the
space which it then occupies appears to us very insignifi-
cant,  when  compared  with the entire  surface of the
heavens, it follows that we are  inclined to  compare  it
with smaller objects.
    The cause of many optical illusions may be traced to
the fact, that, in  many  cases,  we have no  competent
standard for estimating the size of the objects seen.
Fancy then  comes  into  play  and  often causes very
curious illusions.  If, for example, we see a weathercock
upon a steeple in the position  shown in
fig. 50,  we  have  no  means  of judging
whether  it  is turned  away  from  us or
towards  us.   Indeed,  we can  imagine,
through  an  effort of the will, the  one in
the drawing to  be turned first one  way
and then the other.   We are, however,
generally enabled to judge of its  true posi-
tion by means of certain accessories, such
as  light  and  shade, or  its motion.  But
when deprived of such aid, we are often at
a loss as to its position, or merely by an effort of the will
can suppose one or other to be the true direction.  This,
of course, is only the case when we are at such a distance
from the steeple that we are unable to judge of the dis-
tance of the shaft and the free  end of the weathercock,
which is  exactly reversed in the two positions.
 
i6o
THE FIVE SENSES OF  MAN.
Fig. Si-
   In all such cases, where our conception of the object
seen allows of several interpretations, our imagination
accepts one  of these  interpretations and neglects the
                      other, though  at  the same  time
                      we are able to  pass from one in-
                      terpretation  to the other.   This
                      property of the imaginative faculty
                      has been  called Intuition.   The
                      following  example  will serve  as
                      an explanation of this property.
                      We can form  two  different con-
                      ceptions  of  the   accompanying
fig. 51,  first as two equal triangles, the  sides of which
intersect each other.   This would be the  most natural
idea.   But it is also possible  to view the figure  as  a
                        hexagon,  the sides  of which
                        are set with small triangles.
                        The impression made by the
                        figure  in  the two cases varies
                        with our idea of its origin.
                            It  is  the same  with  the
                        drawing in  fig.  52.  We can
                        imagine it to represent either
                        a staircase against a wall, or an
                        overhanging portion of a  wall,
                        the lower part of which has
                        been   removed,  and   whose
                        under  surface  has taken the
                        form of steps.   Our idea of its
                        form changes quite arbitrarily.
   Passing to  our sensation of colour,  illusions  again
occur which  are caused by  errors  in  our judgment.
Fig. 52. 
THE SENSE OF SIGHT.             l6l
Place a small square of white paper on a large piece of
green paper, and cover the whole with a thin transparent
piece of tissue paper so that the underlying paper is seen
through it.   The  small  square will  then appear red,
while the  surrounding surface seems whitish.  If shown
to an unprejudiced  person, who does not  know what is
under the tissue paper, he will immediately  pronounce
the small  square to be red, and will hardly  notice any
colour on the  surrounding surface.   Under similar cir-
cumstances a white square upon a red ground will appear
green, a white upon blue, yellow, and vice versa.
    We might, in this case, be led to imagine that we had
merely to deal with the ordinary phenomena of comple-
mentary colours, which  are  produced when the eye,
after having been fixed for some time upon a coloured
surface, is turned suddenly upon a white one.  The white
surface will then look red, if we  have wearied the retina
by gazing for some time at a green light;  or vice versa.
But, in the case we are speaking of, the phenomenon can-
not be explained  by fatigue of the retina, because the
colour of  the larger surface is of so  little consequence
that  it is scarcely recognised, and, therefore,  cannot
fatigue the eye by its colour.   If the tissue-paper  is  re-
moved, we immediately  recognise the small square as
white, although the fatigue produced by  the coloured
surface would be still greater.
   Phenomena  like  those  just described have  been
termed  Simultaneous Contrasts by  Helmholtz.   They
are explained by our having made  an erroneous judg-
ment on what  we  call white light.   We  call  a  body
white when it reflects  all the colours of the spectrum in
the proportions in  which they are contained in sunlight. 
162         THE FIVE SENSES OF MAN.
But we  are  accustomed  to  slight variations  in these
proportions, and call such variations white also.   This is
the case  in the experiment under consideration, in which
we suppose the  coloured  surface, which  is covered with
white paper, to be white.   The result  is that we do not
recognise the true white of the small square as such, but
as a white  which inclines to the complementary colour.
    It is  very probable that  the kind of light which  we
call' white' would not remain the same if the proportion
of the colours in the light of the sun were to alter; and
since we suppose even the sun  and its light may not
remain the same  for ever, it is quite possible  that our
descendants may have a perfectly different idea  of white
to that which we now have.
    It is in these illusions of the organ of vision, and, also,
of the other sensory organs, that  we are best able to  re-
cognise the important part which the Mind plays in the
perceptions of the senses, for it is  the mind alone which
is able to alter the perceptions of the senses, and, thereby,
in many cases to create illusions.  We see, further, that it
is by the activity of the Mind alone that the Sensations
of the senses are converted into Perceptions of the senses.
For the sensation of the senses, i.e. the excitement of the
sensory organ and the  passage of this excitement  to the
brain, does not at all imply the connection of this sensa-
tion with a perception  of an  object or occurrence  in the
external  world.  In regard to animals, and especially in
pigeons,  the observation has been  made, that upon  the
removal  of the cerebral hemispheres, in which state they
may live for some time, they still possess a sensation of
light, which penetrates the eye, and causes a contraction
of the pupil;  an  action which can only be caused by the 
THE SENSE OF  SIGHT.
163
central  organ of the optic  nerve in the brain.   But a
comprehension of the objects seen—i.e. a true percep-
tion of the senses—is  no longer possible to these animals.
They behave  like blind  animals, run against every ob-
stacle, and no longer possess the power of recognising
the objects seen as belonging to the external world.
   We cannot help conjecturing that the process must be
the same in man: that  the perception of the external world
is essentially an act of the Mind, which has its seat in the
cerebrum and is connected with this organ ; and, further,
that the sensory organ with its nervous connections only
affords the brain the  material which it converts into a
Sensory Perception.
• 
            PART  III.

THE  SENSE  OF  HEARING.
                   CHAPTER I.
General construction of the Organ of Hearing—Sound as a Tone, a Note,
              and a Noise—Musical Instruments.

The organ of hearing is not so fully exposed to our
view as the eye.   The latter exposes nearly the half of its
surface to the light; the former, on the contrary, conceals
its most important parts deep in the  solid  structure  of
the head,  and  only exposes  to view a very subordinate
part, the Pinna or External Ear, which, on this account,
in ordinary language,  has received the  unmerited title
of the  ear.  The barbarous punishment of cutting off the
ear, which was customary in olden  times, and among un-
civilized nations, has clearly proved that the external ear
is not  essential to the power of hearing.  Moreover, most
birds  have  no external ear,  and yet they  have a very
good, and partly  musical  sense of hearing.
    The relation of the organization of the ear to that of
the eye is the same as the nature of the range of vision
to the  range of hearing.  Our range of vision is limited 
THE SENSE OF  HEARING.           165
by an opaque body ; in a transparent medium, however,
it extends to the infinite distance of the stars, when the
ray of light penetrates the eye in an almost straight line
to its posterior surface.   The extent of our perception of
sound, even in our atmosphere, is very limited, but sound
can penetrate the thickest walls and travel by the  most
circuitous paths, so that in the deepest mines where no
light can penetrate, the ear gives us certain information
of the existence of an exterior world.  Thus, the inner
parts of the ear are hidden in a deep cavity in the skull,
which  is provided with  many  wonderful  labyrinth-like
passages.  Sound does not penetrate these passages in a
straight line, but passes  along a  very complicated path,
composed of tubes, membranes, and the ear-bones.
    In fig. 53, the essential parts  of the  organ of hearing
are represented, in  their natural size, after  Helmholtz.
We see the Auditory Canal,  D, which terminates within
                        Fig. S3-
with the  Tympanic Membrane, c c.   This membrane is
circular and stretched obliquely, and is followed with-
in by a hollow, filled  with  air, B, the  Tympanic  Cavity, 
166         THE FIVE SENSES OF MAN.
whose  walls  are  formed of bone.   In it  are situated
the Ear-bones, three in number, called  the Hammer
(Malleus), the Anvil  (Incus),  and the Stirrup (Stapes).
The hollow opens inwards and leads to  a canal,  the
Eustachian Tube, E, which is extended in a trumpet-like
form and opens into the Pharynx.   On the inner wall of
the tympanic cavity there are two  openings closed with
membranes, an oval aperture, o, Fenestra Ovalis, on which
the base of the stirrup rests,  and a circular aperture, r,
Fenestra Rotunda, the membrane of which is free.
    The two openings lead to the Labyrinth, A, a peculiar,
coiled, bony cavity, the walls of which are clothed with
membrane and remarkable organs, and the interior filled
with the  labyrinthine fluid.   One  side of  it is  distin-
guished  by the name of the  Cochlea, and the other, the
Semicircular Canals.
    From this rapid view of the essential parts  of  the
ear, we  find  that the  Auditory Canal, the Tympanic
Membrane, and the Tympanic Cavity with the Ear-bones,
only serve to receive the sound from without and to con-
vey it to the interior, that it must there be communicated
to  the Labyrinthine  Fluid, and that in the Labyrinth
the proper physiological process of hearing  commences,
since at this point the  apparatus  for the transfer of sound
terminates.  In fact, the Auditory Nerve here penetrates
the solid structure of  the skull, and through its  con-
nection with  most curiously formed terminal organs,
changes the dead sound into a living sensation.
    Although  we can form  no conception  of sound, as
such,  except  through the  excitement of our  auditory
organs, nevertheless its origin and propagation in nature
were recognised by physicists long before anything  was 
THE SENSE OF  HEARING.           167
known of its physiological action. In this respect physi-
cal acoustics were, to a certain extent, just as indepen-
dent of physiological  knowledge, as was the case with
optics ;  since, in the  latter, the undulatory  theory  was
developed, without any definite knowledge of the con-
stitution of the retina, and of the excitement of the nerves
situated in it.  It  must be granted that the exclusively
objective  physical study of sound and light must neces-
sarily have made some progress to enable us to consider
the functions of the sensory organs physiologically ;  and
it has  been  shown, that many of the inventions which
have been laboriously made by the ingenuity of man
during the last century, are to be found in the greatest
perfection already  existing  in the  Sensory  Organs.
Nevertheless, in the  present  century, the  science  of
physics has reached the point  at which  it may be conj
siderably  advanced  by the  physiological  study  of the
sensory ■ sensations,  which  study may provide fresh
material for future investigations.
    Among the sensations of sound, there is  one kind of
sensation  different from all others, and  which  has a
dennite character of its own.  This is the  sensation of
musical sound, which is  distinguished by the terms tone
and note.  All other impressions of sound  which do not
possess this character  may be included  under the term
noise.
   A tone is  produced as soon as any elastic body is set
in rapid vibration.   If, for instance, a thin piece of
metal, or a knitting-needle, is fixed firmly at one end and
its free end struck, it  is then set in vibration, which is
accompanied by a kind of hum or tone, and the  shorter
the part in vibration, the higher is the tone.  The tone 
168         THE FIVE  SENSES  OF MAN.

of a tuning-fork is produced in precisely the same man-
ner.   Imagine the tuning-fork, in fig. 54, in vibration.
The branches  of  the fork  at each vibration approach
first each other and then recede, an action which is not
apparent  to the eye, in the tuning-forks used for musi-
cal purposes, since the  vibrations are  too  small  and
follow each other too  rapidly.   By means of the follow-
ing contrivance the vibrations can be made self-regis-
tering.  On one branch a pencil b is fixed, which makes
                        Fig. 54-

a mark upon a plate B B, drawn rapidly before  it.   An
undulating line is  produced which gives the number of
vibrations, if the  rate of the motion of the  plate is
known.  Fig. 55 represents an apparatus, the Phonau-
tograph, with which  such an experiment can be per-
formed.  The point, r, of the tuning-fork writes upon a
cylinder, round which  is rolled a sheet of paper covered
with lamp-black.   The  cylinder can  be  set  in  rapid
rotation  by  means of a handle, a screw, at A, giving
it  a gradual forward motion during  the revolutions.
Two wires lead  from the fork and  the cylinder to  a 
THE SENSE OF HEARING.
169
strong induction coil, which, by  means  of  clockwork,
gives an electric spark  every second, which makes  a
hole in the blackened paper.  By this means the  number
of vibrations in a second can easily be counted.
    Physicists have  agreed to understand by the word
vibration,  a forward and backward  movement  of  the
vibrating body, so that a single vibration  is represented
by a curve of the undulating line, first in an upward, and
                        Fig. 55-

then  in  a downward direction.  The body  makes  an
excursion or elongation from its  position of rest first to
one side,  then swings past its  position of rest to  the
other side, and then returns to its position of rest again,
after which the movement is periodically repeated in the
same  manner.  The time occupied by the  entire vibra-
tion is called a period of vibration,  and its extent the
amplitude of the vibration. 
170         THE FIVE SENSES OF  MAN.
    In all musical instruments the tones are produced by
some such action as  this.   A  stretched  string, which
produces the tone in pianos and stringed instruments,
vibrates  in the  manner shown  in  fig. 56, by  springing
first to one side  and  then to the other.  A glass plate or
a bell sounds when it  is struck,  in consequence  of  the
vibrations, which its  particles make at right angles to its
                        Fig. 56.

surface.  Such vibrations, which are made in a direction
perpendicular to the length of  the body, or  better,  per-
pendicular to the direction of the propagation  of the
vibrations,  are called transverse vibrations.  Longitudinal
vibrations, however, can also produce tones ; for instance,
metallic rods when struck upon their  terminal surfaces,
or wooden  rods when rubbed  in  the  direction of their
length.
    In  the  examples given  the tone  is caused  by the
vibrations of a solid elastic body.  A tone, however, can
be produced directly by vibrations of  the air, when, for
instance, we blow across the mouth of a hollow ball, of
a bottle, or of a hollow cylinder.  An  instrument  which
rests on this  principle  is the mouth-pipe, which can be
used in the  organ, or  as  a flute.  Fig. 57 represents
two such organ-pipes.   Air streams up from below  into
the chamber K from  bellows,  and  is directed by the
triangular  piece  of wood  d, through a small cleft  c,
against the lip a b, by which it is made to  blow against
the column  of air contained in the  cylinder R r,  and
throws it into vibration.  In this action the  air  is com- 
THE SENSE OF  HEARING.           171
pressed by the first shock, then expands, comes again in
contact with  the  current of air, and is  thus  made  to
vibrate periodically, which  vibra-
tions consist of condensations and
expansions.
    The vibrations of the air con-
sist of condensations  and expan-
sions, which follow each other with
great rapidity, and a tone is always
produced if the vibrations follow     ||J!
each  other with  periodical  regu-
larity.  A tone can, therefore, be
produced if  a continuous current
of air is interrupted with great ra-
pidity and regularity.  An instru-
ment   invented by  Seebeck, in
which this  takes  place,  is called a
Siren, and in its simplest form  is
shown in  fig. 58.   It consists of
a disc, perforated with holes ar-
ranged in circles,  and which  can
be set in rapid rotation.  Air  is
blown through the tube B against
the holes.   The  more  numerous
the holes,  and the more rapidly
the disc  is made  to  revolve,  the
higher will be the tone produced.
Each  puff of air then produces a -       Fig. 57.
vibration of the air. The siren affords the means of deter-
mining the number of vibrations for any particular tone.
If  the disc A is provided with four series  of holes, the
number of  which, counting  from  within  outwards, is
5450468�24016809 
172          THE  FIVE SENSES OF  MAN.
respectively 8, 10, 12, 16, and if we blow upon  them  in
this succession, we  hear the recognised  chord,  which,
starting from the note c, may be distinguished by c, e,g, d'.
                        Fig. 58.

The quality remains  the same for every rate of revo-
lution, provided the rate is constant, only the absolute
pitch and character of the tones change with the rapidity.
If we had a disc which was provided with rows of holes,
corresponding to an entire scale, we  could then produce
perfect melodies upon it.
    In this instrument the tone is  caused by the sudden
expansion  experienced  by  the  current  of  air,  which
issues from the tube as soon as a  hole comes  before  the
opening of the tube, and by its sudden  condensation as
soon as the hole is past.   This action is then transmitted
through the air  to  our ear in the  form of waves of
sound.
    A more complete  form of siren, constructed by Dove,
is  shown in fig. 59.   A current of air is blown through
the tube,  B B,  into the  chamber, A~A.   The  latter is
provided with a lid, which is perforated by a number of 
THE SENSE OF HEARING.           173
holes;  close above it is  a  disc S S, which revolves upon
a  perpendicular  axis, and is perforated by  the same
number of holes in an oblique direction.  The holes of
                        Fig. 59-

the lid, also, can  be inclined  in  the  opposite direction.
The current of air in its passage through the instrument,
strikes against the sides of the holes in  the disc, and 
174          THE FIVE SENSES OF MAN.
sets the disc in rotation, the rapidity of which gradually
increases.   The interruption of the current of air causes
a tone, the height of which gradually increases, and on this
account produces the unpleasant impression of a howl.
Since the number of revolutions can be read off, by means
of a toothed wheel with a dial and pointers, t z z, as soon
as the toothed wheel is put into gear with the screw, t,
by the handle,  h, we are then  able  to discover the
number of puffs of air, that is to say, vibrations for each
particular note.
   There is another method  of producing tones which
is employed in music, i.e. by means of Reed-pipes, which
are used in organs, clarionets, oboes,  and bassoons.  In
the instruments which have been mentioned hitherto, the
tone is produced either by  a  fixed  body in a state of
                        Fig. 60.

vibration, a string, a tuning-fork, etc., without any current
of air reaching it, or by the interruption of a current of
air, without it setting any elastic* body in  vibration—as
in the siren. In reed-pipes, on the contrary, both methods
are employed, and together they produce a peculiar note
of their  own.  The  solid  vibrating body here consists
of the so-called  Tongue,  fig.  60.  An elastic strip of 
THE SENSE OF HEARING.           175
metal z z, is fixed at one end and is allowed to  vibrate
freely in the orifice b b, of  a frame a a.   The  tongue
forms  one  side  of a  closed  tube,
which  is  open above, and which can
be  placed in  a wind-chest  (fig. 61).
The current of air which  is blown
through the lower tube of the wind-
chest can only escape  through the
orifice between  the tongue   and the
frame.   The  air   first  presses the
tongue  into the tube, and escapes
by  the upper opening, to   which  a
funnel-shaped tube  should*be fixed
to strengthen the sound.  The tongue,
however, by reason  of  its elasticity,
springs back again, covers the open-
ing, and  periodically  interrupts the
current of  air by  its movement  to
and fro.   A tone  is thus produced,
the pitch of which  is determined by
the number of  vibrations made by
the tongue.  In some instruments the
tone can be changed by a moveable
wire,  the  position  of which may be
changed so as to increase or diminish
the length of the vibrating tongue.
    Trumpets  and horns are wind in-
struments, in  which the lips of the
performer play the part of a tongue.
The extended lips are set in vibration, while the tube of
the instrument serves as  a  resonator, and  considerably
increases the intensity of the tone.   There  is, therefore,
Fig. 61. 
176          THE  FIVE SENSES OF  MAN.
a peculiar art in playing these instruments, since no tone
whatever is produced, however strong the current  of air
may be, if they are placed in the open mouth.
   The formation of the human larynx is exactly similar
to that of the reed-pipe.   The tongue is formed  by the
vocal-chords, two elastic  membranes, whose free  edges
enclose a narrow slit, which, in fig.  62,  is artificially
imitated by means of sheets of india-rubber.  If we blow
into  the tube which is meant to represent the wind-pipe,
a tone is produced which is not unlike the human voice.
The sheets of india-rubber are made to separate, and the
slit is thus opened ; their elasticity brings them together
again, which gives rise to a periodical interruption of the
current of air.  Such an apparatus is called an artificial
Larynx. 
CHAPTER II.
Hearing through the Air and through the Skull—The External Ear and
  the Auditory Canal—Recognition of the Direction of Sound—Arrange-
  ments for the Protection of the Ear.
THE  proper sensation of hearing commences at the ex-
pansion of the auditory  nerve  in  the labyrinth, whilst,
hitherto, the only action has been the conveyance of the
sound-waves.  The first  process in hearing is, therefore,
the conveyance of the sound to the fluid of the labyrinth,
the waves  of which have the power of exciting the ex-
panded nerves.   The sound  is generally conveyed  from
the surrounding air to the labyrinth through the auditory
canal, the tympanic cavity, and the ear-bones.  For, every
body which vibrates in the air, throws the air which sur-
rounds it into similar vibrations, which extend outwards
from  the point of their origin, in the form of a circle,
while their intensity decreases with  the  square of the
distance.   The vibrations consist of condensations and
expansions of the air, which follow one another, like circles
of waves upon the surface of water, as is represented in
fig. 6^.  Sound, however, can pass through solid bodies
also, without the intervention of the air, and  reach the
labyrinth  in this manner.  If  we strike a tuning-fork 
178          THE  FIVE SENSES OF  MAN.
gently, and hold it before the open mouth, we hear  no-
thing ; but as soon as it touches the teeth we hear its tone
quite clearly, since it is now conveyed directly  to  the
labyrinth through the skull, without passing through the
                        Fig. 63.

tympanic cavity, or the  ear-bones, at all.  We can also
hear the ticking of a watch if we touch it with the teeth,
or press it upon the bones of the skull just behind the ear.
Moreover, there is a well-known children's game, which
depends chiefly upon the conveyance  of sound to the ear
through bone.  A metallic spoon is fastened to a string,
which  is wound round the first finger,  and the finger
pressed firmly into the auditory canal.  The spoon is then
struck  against the edge of the table, and the  sound pro-
duced  resembles that of  a large bell, for the vibrations
of the  spoon are communicated to the labyrinth, through
the thread, the finger, and  the  bones of the skull,  with
increased intensity.
   The communication of sound through bone is of no
value to man under ordinary  circumstances, since we
do not  generally bring the head into close contact with the
sounding body.  In fishes, however, it plays a very impor-
tant part, for they possess neither- external ear, auditory
canal, or ear-feones, but only a labyrinth, which is entirely 
THE SENSE OF  HEARING.           179
closed by walls of bone, or is only covered with membrane
on  its outer surface.  The sound-waves  of the  water,
therefore, are transferred directly to the fluid of the laby-
rinth through bone.  This is possible because the sound-
waves of water, which is well known to be  incompres-
sible,  are  similar to  those of a  solid  body, and  do
not, like those of the air„ consist of expansions and con-
densations.  Sound-waves of  the air,  on the contrary,
are transferred to solid bodies and to  liquids with  great
difficulty ; and to effect this, ingenious contrivances are
required, which  are found  in  their greatest perfection
in our ears.   Such contrivances are  not required  by
fishes, for they have only to hear  in water ;  indeed,  an
apparatus filled with air, if placed between the water and
the labyrinth of their  ear, would considerably diminish
their power of  hearing.  In case of illness the communi-
cation of sound through the bones can be of considerable
importance to us.  If  the apparatus for the communica-
tion of sound is diseased, and can no longer perform  its
duties, in spite of deafness,  sound can still be communi-
cated  by means of bone, which then gives the physician
a very important diagnostic sign that the  labyrinth and
the auditory nerve are in a sound condition.
    The regular action with which we have to do is the
conveyance of sound through  the air, beginning with the
reception of the sound-waves  by the  external ear and
the  auditory canal, and ending with  the transfer of the
same to the labyrinthine fluid.  In this path there are
many ingenious contrivances, which are deeply interest-
ing.   Although there is much in this action which is not
yet explained with sufficient certainty, still, since the in-
vestigations of  Helmholtz  upon the  tympanic cavity, 
i8o
THE FIVE SENSES OF MAN.
 many discoveries have been made which have opened the
 way for new enquiries.   Of course enigmas still occur,
 as is generally the case in nature.
    A consideration of  the  curiously coiled  folds,  fur-
 rows, and hollows of the human external ear might  lead
 us to suppose that nature has  created  all these details
 for some definite purpose, as well as for the beautiful
 form  which  may accompany  them,  and  which nature
 often puts to  some  practical purpose.   Nevertheless, it
 would be very difficult,  from  a  scientific point of view,
 to establish such a supposition, although formerly it had
 not a few adherents ; and we arrive at  a  very different
 conclusion, if, as is frequently the practice in physiology,
 we pass from human anatomy  to  the animal kingdom,
 and consider the form of the  external ear we  meet with
 there.
    The external ear, together with the auditory canal, cer-
 tainly forms a funnel-shaped ear-trumpet, which, in horses
 for instance, we find in a form which answers this purpose,
 as  a  true funnel with  tolerably  smooth  walls.  The
 marked  strengthening  of the sound  produced by  a
 funnel-shaped ear-trumpet, which can easily be imitated
 by placing a roll of paper in the ear, is especially remark-
 able if we close the other ear and then  listen  to  the
 general rustle in the  air; or, still better, if we endeavour
 to hear the ticking of a  watch placed upon the table at a
 slight distance, with  and without the tube. The sound-
waves which fall upon the wide opening are reflected in-
wards by the sides of the tube, and this causes a continual
increase  in  the  intensity of the sound-waves,  i.e.,  the
amount of the condensation and expansion.  While  this
function is fulfilled to a  considerable extent by the ears 
THE SENSE OF HEARING.          l8l
of a horse, and all  the  more from  the  ease with which
they can  be moved in any direction,  the  human  ear
appears to be  but little  adapted for this purpose.  For,
in the first place,  its form deviates considerably from that
of a funnel, which  is only partly assumed by that part
which joins the auditory canal, D (fig. 53); and, secondly,
the external ear in  man is almost unmoveable, although
there are  muscles which can give  it an upward, back-
ward, and forward  motion.  Man, however, seldom pos-
sesses any command over the muscles  of the ear, such,
for instance, as that possessed by the celebrated physio-
logist, Johannes Miiller, so  that ' moving the ear'  is a
peculiar art.
    This want of  mobility can be explained by the fact
that the motion would be of no special use, and we have,
therefore, no cause  for such  useless motion, so that ulti-
mately  the will  loses its  influence over these  muscles.
Whilst this connection of phenomena points to the small
importance of the human external ear as an ear trumpet,
this conclusion may be confirmed by the following expe-
riment : after a small tube has been placed in the  audi-
tory canal, the entire ear may be filled up with a mass of
dough,  without  any diminution being  observed in  the
acuteness of hearing.
    Nevertheless,  the external ear has some influence on
the hearing, for a marked  difference is  produced by an
artificial increase in  the size  of the external  ear  by
means of  the hand, which is a general practice with deaf
persons when  they  wish to  hear more  distinctly.  The
endeavour has also  been made  to  discover whether  the
angle which the surface  of the  ear  makes with the side
of the head may  not have some effect on the acuteness 
182          THE  FIVE SENSES OF  MAN.
of hearing, and it has been  supposed that an angle of
400 is most suitable for this purpose.  Still, most persons
might well object, on aesthetic grounds, to such a large
angle, especially when the change of the position of the
ear with regard to the head  makes scarcely any percep-
tible difference to the hearing.   It has also been supposed
that,  in consequence of its  being composed of elastic
cartilage, it has the power of receiving the sound-waves
from the air, and conveying them through its mass to the
auditory canal and tympanic membrane, and that its sur-
face is increased for this purpose  by such a number of
folds.  But such an action can only be very unimportant;
for if  the ears are entirely closed with wax, we are ren-
dered quite deaf to all sound which is conveyed  by the
air, and  the external ear renders us no perceptible assist-
ance.   Nor do we hear the  ticking of a  watch,  if it is
placed on the cartilage  of  the ear when the auditory
canal  is closed, while  it may be heard very distinctly if
placed on the bone behind the ear.  We must, therefore,
consider cartilage as a bad conductor of sound.
    The next question is, whether the external ear ren-
ders us any assistance in  recognising the  direction of
sound.  Doubtless we are able to  recognise the direction
of sound, but we  also learn  from experience that in this
recognition we are liable to many errors of judgment.
We recognise the origin of  the  sound-waves  by  the
fact  that the  sound is heard most  distinctly when
the auditory canal is  in the same straight line with  the
direction of the sound-waves.   For this purpose we turn
the head in different directions until  the  sound appears
to us  to be loudest.  In this case only a single ear acts,
and the sound must have sufficient duration  to  enable 
THE SENSE OF  HEARING.
183
us to determine its direction.   Very frequently, however,
the direction of a sound may be recognised directly with
the aid  of both ears,  without any further  examination,
since the sound appears louder when heard  by the "ear
which is turned in the direction of the sound than when
heard by the other. In this case, hearing with both ears
plays a not unimportant part, which may be compared in
some respects with binocular vision, although here  we
have to do, not with the estimation of distance, but only
of direction.  But in hearing with one ear we have to make
use of the movements of the  head, just as we do in mo-
nocular vision,  when we  wish  to  estimate  distances.
Nevertheless, we cannot exactly determine the distance
with both ears ; therefore, in  a careful search for the ori-
gin of sound, we almost always turn the head.
    In this action the  external ear performs a  really im-
portant service.   When, for instance, we turn the head
towards the origin of a sound, then the external ear is
situated in about the most favourable position for the
reflection of the  sound-waves  into the auditory  canal,
less favourably if the sound comes from the front, and
most unfavourably if from  behind, since  those  sound-
waves which fall directly upon  it will be reflected out-
wards. We can then easily form an opinion as to whether
■the sound comes from before or from behind,  especially
if we have  already gained experience  of the usual in-
tensity  of sound, and, still better, if we have made com-
parisons by turning the head.
    However, the supposition has also been made by Ed.
Weber, that the external ear has the power of perceiving
the direction of the sound-waves by its being set in vi-
bration itself, and that we can determine whether the 
184          THE  FIVE SENSES OF MAN.
sound-waves have fallen upon the anterior or posterior
surface.  In this  connection the following  highly inter-
esting observation of  Weber's should be mentioned.
If we are observing a sound which reaches us from the
front, and place both hands, turned backwards, upon the
head  in front of the ears, so that the hollows of the hands
represent the exterior portions of the ears, then we expe-
rience the illusion that  the sound comes  from behind.
Weber supposed that while the hands played the part of
the exterior ear, they were  also capable of perception,
that their backs  received the shock of the sound-waves,
and on that account we fancied that the sound came from
behind.  If this explanation were correct, then we should
be forced to conclude that the tactile nerves of the exte-
rior ear, and of the hands, were able to feel the movement
of the sound-waves in the air, for special nervous termi-
nal apparatus are not to be found in them.  Moreover,
the tactile sense is much  weaker in the external ear than
in the hollow of  the hand, and we certainly do  not feel
sound-waves as such with the  hand; for instance, if we
close  the upper end of an open mouth-instrument with
the hand while it is sounding.   It is therefore improbable
that the skin of  the external ear is able to perceive the
direction of sound-waves.
   The power of determining whether a sound comes
from before or behind is much more simply explained by
the diminution which sound suffers when it comes from
behind.   In Weber's experiment, therefore, in which the
hand  is held before the  ear, the sound  is  deadened by
the hand much  in the same way as when it comes from
behind.
   Weber has also observed that  it  is  very difficult to 
THE SENSE OF HEARING.
185
distinguish whether the sound comes from before or be-
hind, if we place the external  ear flat  upon the head ;
which can easily be explained in both ways.   According
to Weber, because the posterior surface of the external
ear is not affected by the sound-waves, and according to
our view because the means for the conveyance of sound
are very similar in both directions.
    Finally, the external ear  has to be  considered  as a
means of affording protection to the inner ear.   Just as
the eyelids prevent the entrance of many injurious sub-
stances into  the eye,  so the external ear prevents the
entrance of dust and  small particles, which might easily
be blown through the air, into the auditory canal; and to
prevent the entrance of insects, not by a closing move-
ment, but, in the human ear, by means* of its peculiarly
coiled form, which makes the entrance difficult to find.
The latter, also,  is more  or less covered with minute
hairs, which  serve to  catch  any dust which penetrates
without perceptibly deadening the sound.
    The auditory canal, the first part of which  is com-
posed  of cartilage, and the inner part of bone, as we
have seen from the figure, is  not a straight tube of equal
width.   It is contracted at the opening, which contraction
turns inwards and upwards;  then  it expands and termi-
nates at the tympanic membrane, the surface  of which
is placed at an oblique angle to it, and is directed in-
wards and downwards.  On  the sides a fatty substance,
the tvax of the ear, is secreted by small glands,  which is
intended  to keep the sides,  and perhaps the tympanic
membrane itself,  in a supple condition, and to protect
it from dryness.   An  excessive secretion of it may be
injurious and produce deafness, by closing the passage. 
186          THE FIVE SENSES OF MAN.

   We must not assume that the inequalities in the au-
ditory canal have any particular influence on the convey-
ance of sound, for if  we hear a sound through an india-
rubber tube  it  makes but little difference whether the
tube is straight or bent, since the reflection from the sides
allows the sound to pass along it with scarcely any dimi-
nution in intensity.   The irregularities, therefore, cannot
have any special acoustic influence;  perhaps  they only
serve as a protection, by keeping  any dust which  may
enter the ear from reaching the tympanic membrane.
   The auditory  canal has been proved by Helmholtz
to have a special resonant action.  Since every enclosed
volume of air gives  its fundamental tone, when we blow
across it, as with the mouth-pipe, a bottle, or a hollow
globe, the auditory canal, also, must possess such a funda-
mental tone. The  pitch of this tone is tolerably high,
so that tones of the same pitch, in consequence of this
strong resonance, seem shrill and unpleasant to us ; as
for instance, the very  high tones of a violin or  the cry of
a bat.  Perhaps this  is also-the cause of the unpleasant
sensation produced  by the  screech caused by scratching
glass or porcelain with certain metals.  Upon the whole,
the resonant action  of the auditory  canal  strengthens
the high tones in  some degree, and deadens  the  low
ones; this  can  be  varied by  placing a small  tube of
paper in the auditory canal, and by  this artificial elon-
gation lowering the pitch of fundamental tone. 
                  CHAPTER III.

The Tympanic Membrane, the Tympanic Cavity, the Ear-bones, and the
  Eustachian Tube—The Vibrations of the Tympanic Membrane and of
  the Ear-bones.

The tympanic membrane has  not  an even surface,
but it shows a depression, which is directed towards the
tympanic cavity.   This is caused by the  union of the
membrane with the handle of the hammer, which is firmly
attached  to  the  tympanic membrane' throughout its
entire length, its  end corresponding with  the centre of
the depression.  In fig. 53 this relation is roughly repre-
sented ; fig. 64, however, shows us  the  tympanic mem-
brane, after the view of Helmholtz, in its connection with
the hammer, as seen from the tympanic cavity ; fig. 65
shows the three ear-bones (ossicula  auditus).  It shows
that the tympanic membrane is contracted  inwards into
a funnel-shaped depression by the handle of the hammer,
and  that the  centre of  the depression is  found  be-
neath the middle of the  handle.  The tympanic mem-
brane, therefore, has a concave form, from the edges to
the central  depression ;  but as seen from  the auditory
canal, it bulges inwards like  a sail.  The  edge of the
tympanic membrane is inserted into a bony groove.
   Within the membrane is found a layer of muscular 
188          THE  FIVE SENSES OF MAN.

fibres, part of which extend in a radiating, and part in an
annular direction.  In the  figure other details  may be
observed—the  head of  the  hammer k, its long pro-
Fig. 64.
cess /, and  a point *, upon  which  a tendon  of the
Tensor Tympani muscle is fixed.  We see also  a liga-
ment below the head, which fixes the hammer and the
mouth of the Eustachian tube t.
    In fig. 65 we have a representation of the form and
connexion of  the  ear-bones magnified  four  diameters.
The head of  the  hammer, which extends above the
upper  edge of  the tympanic  membrane,  articulates
with the anvil bone Am.   Besides the long process  /,
there is  a short process close  under  the neck of the
hammer-head, not seen in the drawing, which is directed
outwards and  presses  against the  upper edge  of the
tympanic membrane, and  is here slightly turned up in
an outward direction.
    The general form of the anvil'is that of a bicuspid
tooth, whose upper surface articulates with the hammer-
head.  Two processes descend  from it  like roots, the 
THE SENSE OF HEARING.           189
shorter of which Am. k, is directed towards the posterior
side  of the tympanic  cavity, and is attached to it by
means of ligaments ; whilst the longer  one, Am. I,  pro-
jects into  the interior of the tympanic cavity, and, by
means of a small joint at its termination, articulates with
Am.       Am. k
the head of the stirrup s.   The stirrup, whose form is at
once  suggested  by its  name, has an oval-shaped foot-
plate,  which  points inwards  and presses against  the
fenestra ovalis of the tympanic cavity.
    The tympanic cavity is a moderately small cavity,
which extends above the  upper edge of the tympanic
membrane.  It may be compared to a drum, which is
only covered on one side  with a membrane—the tym-
panic membrane.  Still it differs  from the form of a
drum chiefly because it passes downwards, forwards and
inwards into the Eustachian tube.
    This canal, which leads to the pharynx, serves  an
important purpose.   The  tympanic  cavity  is  a space
situated within bone and filled with air.   If this air were
entirely shut off from  the  atmosphere,  it would* soon 
19O          THE  FIVE SENSES OF MAN.
 change its composition, and would probably be absorbed
 by blood and replaced by liquid secretions,  if a  con-
 tinuous renewal were impossible.  This renewal can take
 place through the Eustachian tube, which, from time to
 time, allows of an exchange of air ; but  this canal per-
 forms other functions than a  mere renewal of the air; it
 also  maintains  equilibrium  of pressure  between the air
 within the tympanic cavity and the atmosphere, which is
 of great importance for the proper action of the tympanic
 membrane.  For, as soon  as the pressure of the atmo-
 sphere is greater than the pressure within the tympanic
 cavity, the membrane  will be compressed inwards; and
 if it is less than the pressure within the cavity, then the
 membrane will bulge outwards.  Both would be injurious
 to the movements of the membrane, and are prevented
 by the Eustachian tube, which, at proper times, allows the
 entrance and exit of air.   It does not, however, remain
 long open, but is generally closed, and is only opened in
 the act of swallowing—an act which is performed not
 only during eating, but frequently, at other times, for the
 carrying away of secreted saliva, and thus its function as
 an aid to the ear is sufficiently fulfilled.
    Valsalva, a prominent  anatomist in  the seventeenth
 century, has shown by the following experiment how the
 Eustachian tube is opened during swallowing.  The nose
 is  closed, and the  cheeks blown violently out with the
 mouth shut.   If, now, under these  conditions,  we go
 through the act of swallowing, without  allowing air to
 escape by  the mouth—which is easily done after some
practice—a peculiar pressure is immediately felt in both
ears, which is caused by air having been  forced through
the Eustachian tube into the tympanic cavity, and by the 
THE SENSE OF HEARING.'          191
membrane being thus pressed outwards.   This sensation
lasts until we go through the act  of swallowing  again,
which allows the excess of air to escape.
   In a similar manner the air in  the tympanic  cavity
may be rarefied.   If we close the mouth and nose, rarefy
the air in the pharynx by a strong inspiration, and per-
form  simultaneously the act  of swallowing, air is then
drawn from  the  tympanic  cavity, the membrane bulges
further inwards,  and  a similar sensation of tension is ex-
perienced in the ear,  from which we are freed in the next
act of swallowing.   But  during this experiment of Val-
salva  our power of hearing is deadened  and weakened,
whence it follows, that in the condition of  abnormal
tension, the tympanic  membrane does not  perform  its
usual function.
   The Eustachian tube  is opened by means of muscles
which are attached to  its cartilaginous walls, and which
move during the act of swallowing.  In connection with
this action, the question arises:—why the tube is not open
continuously, but is  generally closed, and only momen-
tarily opened to be closed again directly ?   If this tube is
intended for the exchange  and renewal of air, it would
surely be much simpler if it remained always open.
   A tolerably  decisive  answer to this question  can be
given.   The idea at  once arises that sound-waves, also,
could be conveyed  through the  open  tube into the
tympanic cavity.  This can be shown to  be possible  by
placing  a tube in the  Eustachian tube and allowing a
sound to pass through it.   Therefore, if the  Eustachian
tube remained open,  our  own  voice would sound like a
roaring  noise, and even those persons who have a great
predilection for  hearing  their own voice, would find  its 
192         THE FIVE SENSES  OF MAN.
 continuance, under such an  increase in intensity, dis-
 agreeable.
    Moreover, if the Eustachian tube remained open, the
 passage of air inwards and outwards during respiration
 would cause movements of the air within the tympanum,
 and a consequent  bulging  inwards and  outwards of the
 tympanic membrane.   In  short, we  perceive that the
 closing of the canal is of great practical  importance, and
 that'a peculiar  mechanism is required to open it from
 time to time.
    Further, there is the possibility of the  open Eustachian
 tube changing the vibrations of the tympanic membrane,
 and, of course, the resonance of the air within the tym-
 panum.  For when a drum is struck, the tone is changed
 as soon as an opening is made in its sides.  Nevertheless,
 a theoretical and experimental examination is  required
 before a similar influence can be proved for the Eustachian
 tube.
    Finally,  the tube provides means  for carrying away
 the secretions, which are produced upon the  membrane
 of  the tympanic  cavity.    The  passage towards  the
 pharynx is, therefore, much more easy than in the oppo-
 site direction, in which the  walls approach each other in
 a valve-like  manner.  During a  cold the tube may be-
 come impassable, and  then, through want of a renewal
 of the air  in the tympanic cavity and the collection of
 mucus in it,  a greater or less degree of deafness is pro-
 duced, on which account aurists introduce a small  elastic
 tube into the  Eustachian tube as a protection against
this evil.
   Among the  solid bodies,  whose vibrations are able
to  produce a tone, may be placed a stretched membrane, 
TFIE SENSE OF HEARING.           193
for instance, the membrane of a drum, which produces a
tone when struck.   The  vibrations which are caused in
it are transversal, and its action  may be represented by
supposing  each diameter of  a  circular membrane  to
vibrate  as if it were a wire.  An essential part of the
drum is the cylindrical-shaped volume of air enclosed in it,
which considerably increases the  intensity of the sound
by resonance,  if its fundamental tone corresponds with
that of the membrane. A membrane stretched over a ring
without the drum gives a tone  of very moderate intensity.
    The tympanic membrane of the ear is not made to  "
vibrate  by an impulse derived  from solid  bodies, but
from the sound-waves of the air.  The action here takes
place in the reverse direction to  that  of the drum.  In
the latter case the drum-stick causes the skin of the drum
to vibrate, and the skin transmits the vibration  to  the
enclosed air, and, at the same time, to the  surrounding
air.  In the ear, on the contrary, the sound-waves strike
against  the  stretched membrane, which in turn  strikes
against  a very ingeniously formed drum-stick, the  ear-
bones, which transmit the vibrations further.
   The manner in  which the ear-bones perform their
vibrations  in conjunction with the tympanic membrane
and the membrane  of the fenestra ovalis is  highly inte-
resting.    As to the tympanic membrane,  we can have
no doubt that, owing to its power of dilatation  it can
only  perform  transverse vibrations,  since  it is  set  in
motion  by  the sound-waves.  The  vibrations  of the
tympanic membrane, as  we.  shall see, are not really so
simple as those of a freely-stretched,  even  membrane ;
still they agree in form upon the whole.
   Further, the  ear-bones are solid bodies, partly  irregu- 
194          THE FIVE SENSES OF MAN.
lar, partly of a rod-like form, and  their vibrations must
be assumed to be very irregular also. A rod fixed at one
end can, as we know, produce  transverse  vibrations, if
the  free  end  is  drawn aside and suddenly  let loose.
It can also produce longitudinal vibrations, if it is rubbed
in a longitudinal direction.   Now it has been observed
that solid bodies are very good conductors of sound.  If,
for instance, we  place our ear upon a horizontal trunk
of a tree or upon a wall, and  strike a distant point of
it. gently,  then  we  shall  hear the  sound  distinctly,
although through  the  air we can  hear nothing at all.
Moreover it is known'that we can hear the distant tramp
of horses  much more distinctly through  the medium of
the earth than through the air.   All  sound-vibrations
which are conducted by solid bodies are,  for  the  most
part,  longitudinal vibrations.  The entire mass of the
tree, for instance, does not vibrate first to one side  and
then to  the othei ; it  does  not bend to and  fro like a
firmly fixed  pen in vibration,  but the  vibration take?
place in its particles, which,  for  the most  part, move to
and fro  in  a  longitudinal direction.  Real longitudinal
vibrations  of the particles are only  produced by the
friction of thin rods in a longitudinal direction.
   We  can now  imagine the  existence of such  longi-
tudinal vibrations in the substance  of  the ear-bones, for
bone is  a  very good conductor  of  that  class  of vibra-
tions, as we have  already observed,  when treating of the
conveyance of sound to the  ear through  the bone.  This
idea has found many supporters, although the ear-bones.
as Helmholtz  maintains, are  of so small  a size com-
pared with the length of a sound-wave, that this action
is quite  out of the question.  The  average  wave-length 
THE SENSE  OF HEARING.
195
of the medium tones in the air varies from one-half to a
whole metre (20 to 40 inches), while in solid bodies it is
still greater, since they transmit sound with greater  ra-
pidity.  Moreover, the ear-bones are  by no means im-
movably fixed, and from their small  mass are light, so
that an impulse acting upon one end  of them must  set
the whole chain  of bones  in  motion.  When, therefore,
a sound-wave acts upon the hammer, it  is improbable
that the motion should be transmitted to the stirrup, like
a wave through the heavy trunk of a tree, for a sound-
wave  is so long that all parts of the three bones must lie
upon  a  single point of the  entire wave almost simul-
taneously, and it must therefore produce a general move-
ment  in the same direction.
    We must return, therefore, to the  only possible con-
clusion, that the ear-bones vibrate in a transverse direc-
tion, and a closer examination of the manner in which
they are connected will  show us how this is produced.
    The handle of the hammer is attached to the tympanic
membrane throughout its entire length, whilst its  head
projects above the edge of the membrane into the  tym-
panic cavity.  The neck  of the hammer,  its long and
short  processes, are fastened by elastic ligaments, which
form an axis round which the hammer vibrates to and fro.
This  axis passes inwards through the neck of the ham-
mer, and is distinguished in  fig. 66 by the point a.   All
the points of the hammer, which are situated below  the
axis,  upon a vibration  inwards of the tympanic mem-
brane, will be driven inwards, while all the  points which
lie above the point a will be driven outwards, as is shown
by the arrows.   Now, whilst the" handle of the  hammer
vibrates inwards, and its head outwards, the anvil will be 
196          THE FIVE SENSES OF MAN.

set in motion by its articulation with the hammer-head,
indeed just as if the joint formed quite a solid connection.
The  latter is of a peculiar  form, which, from Helmholtz's
researches, may be compared in the mechanical action to
toothed-wheels.   These work so closely into each other,
that the body of the anvil moves with the hammer-head ;
the long process, however,  swings upwards and inwards in
the same direction as the hammer-handle.  This gives,
Fig. 66.
as we see, an impulse inwards to the stirrup  also, whose
foot-plate strikes against the fenestra ovalis, and  by this
means the sound-wave finds its entrance to the labyrinth.
   It is evident that in the backward swing  all move-
ments must follow in  the  reverse direction.  We have
only to mention that the teeth of the hammer  and anvil-
joint work into each other.  When, therefore, under certain
circumstances—for instance, a strong pressure of air in
the tympanic cavity—the membrane is pressed outwards,
it communicates this motion to the hammer-handle, the
hammer-head is moved forcibly  inwards, and the articu-
lated surfaces separate, which prevents the stirrup from
being detached from the fenestra ovalis.
   The system of the  ear-bones, therefore,  performs a 
THE SENSE OF HEARING.
197
simultaneous motion round a common axis. Such a motion
may be compared to  that of a lever ; indeed  we have
here to do with a two-armed  lever,*such as the bent lever
used in bell-work, in order to pull the wire in a different
direction.   One arm of the lever, upon which the power
of the vibrating tympanic membrane acts, is the ham-
mer-handle ; the  other is the hammer-head,  with  the
anvil and the stirrup, which  set the entire labyrinthine
fluid in vibration.
    The vibrations of  the  ear-bones are therefore, in
reality, transversal, although  they are in  no way analo-
gous to the vibrations of a stretched cord, or  a  fixed
pen ; for the ear-bones are by no means firmly fixed,  and
do  not vibrate by reason of their elasticity, but resemble
very light  and movable little  levers,  which are set in
simultaneous motion  by the  vibration of the tympanic
membrane.
    In a human ear which has been dissected it has also
been observed that the ear-bones  do really vibrate in
the above manner. If a small bright point in  them is
observed under the microscope, while a sounding organ-
pipe is connected with the auditory canal, it appears to
be  elongated in the direction of the motion into a  small
bright streak.
    The ear-bones, as we have seen, act as a lever which
connects  the  tympanic  membrane  and the  fenestra
ovalis, and transfers the force from the  former to  the
latter.  While they are set in motion  by the membrane,
they strike like drum-sticks against the membrane of the
fenestra ovalis, only with this difference,  that  they  are
as firmly attached to this  membrane as they are to  the
tympanic membrane, and  cannot be detached from it. 
198         THE FIVE SENSES  OF MAN.
                  CHAPTER  IV.

The Function of the Tympanic Membrane—The Telephon—Flame Mano-
   meter—The Importance of the Funnel-shaped Form of the Ear—The
   Muscles in the Tympanic Cavity.

If we strike a membrane, which is  stretched over the
mouth of a tube or a ring, a tone is produced—its funda-
mental  tone—which increases  in  acuteness with the
tension  of the membrane, and  diminishes  with an in-
crease in  the  size of the membrane.  If we sound the
same tone in the neighbourhood  of the membrane, which
may be  done most simply with the voice, the membrane
is  set in vibration, just  as a  window-pane rattles when
its fundamental tone is sounded loudly.   If,  however,  a
different tone is sounded, the membrane is  unaffected,
and only when we approach  its  fundamental tone is  it
set in vibration, or in other words, the resonance begins to
increase in strength.  The resonance of such a membrane
may very  readily be  recognised  by strewing  upon  it
some fine sand, which is thrown into  a dancing  motion
by the vibrations.
   It might be supposed that the  tympanic membrane
also possessed  such a fundamental tone.   It would, how-
ever, be  very injurious to our hearing, especially to our 
THE SENSE OF HEARING.
199
perception of music,  if the tympanic membrane behaved
like an evenly stretched membrane.   For, in this case, we
should hear  the fundamental tone  of  our  tympanic
membrane sounding  with great intensity, and other tones
with rapidly decreasing intensity, so that we should only
hear the greater number of tones very faintly.
   Thus the  tympanic  membrane,  as we learn  from
experiment, has the very remarkable property of answer-
ing  equally well to tones of any pitch, in  the range
of tones  perceptible  to  our  ear, of about 60  to 4,000
vibrations a second, which is impossible with an ordinary
stretched membrane.
   There are some artificial means by which membranes
can  be made  to vibrate with a large  series  of tones,
and  which  therefore show  some  similarity  with  the
tympanic membrane.  We have here to mention a very
interesting instrument, the Telephon of Reiss.   It is able
to transmit  tones by telegraphic  means, so  that  we
are able to telegraph a melody quite correctly.  The
construction  is  as follows :—A  box, A (fig. 6j), has a
tube, R, through which we can sing into the box.  On the
cover there is a membrane made  of dry bladder, which
has only a slight tension, and which, on that account, has
a very deep fundamental tone.  At one spot  there is a
strip of sheet tin which  extends  from the  edge to the
middle, where  a light metallic  plate, P, rests upon it,
which leads to the conducting wire d.  On the other side
a  wire  leads  to the battery, B,  and from the battery
another wire leads to the apparatus K, which can  be
placed at  a  distance.   This apparatus  consists of a
sounding-box K,  in  which  a thin  electro-magnet, E,
lies  surrounded  by  a coil  of wire,  through which the
               10 
200          THE FIVE SENSES OF  MAN.
current passes.  Such an electro-magnet has the property
of producing tones,  the pitch of which depends upon
the number of currents which follow each other with ex-
traordinary rapidity.   The number  of vibrations  of the
tone is then equal to the number of currents transmitted.
    When we sing into the box A, the membrane is set
in vibration.  The little plate, P, dances up and down on
W,r,w^w,>W,w^.,,,w^^
                        Fig. 67.

the strip of tin, and, therefore, contact is made and broken
just as many times in a second as the tone sung makes
vibrations.  This  produces exactly  the  same  number
of currents of brief duration in a second, and the electro-
magnet gives the tone sung, which is strengthened by the
sounding-box.
    Now this apparatus works tolerably well within the
compass of an ordinary voice ; the membrane, therefore,
vibrates with a large number of tones, similarly to our
tympanic  membrane.   This is explained  by the fact 
THE SENSE OF  HEARING.
201
that the membrane is not  stretched quite equally, and
that it possesses  a very deep  fundamental tone.  The
result of this  is, that the  membrane  is set  in motion
by the vibrations of the air in the box  A in separate
parts, since it is divided into many vibrating portions,
and forms the so-called nodal lines,  which divide the
vibrating  portions from each  other.   With tones, then,
of different  pitch,  differently  shaped  portions  of  the
membrane will vibrate.  Moreover, the metal plate, P,
influences the  action by  loading the  membrane.  It
deadens every vibration peculiar to the membrane, and,
of course, every subsequent vibration, though  it is light
enough to be set  in sympathetic vibration by a tone when
it is loudly sung.   The  consequence of this is that the
membrane accommodates itself perfectly to a great variety
of tones.   We are quite justified in comparing it to the
tympanic membrane, and  the little  metallic  plate  to
the ear-bones.
    Another ingenious contrivance, which also resembles
the action of the auditory apparatus, is the Flame Mano-
meter of Konig.
    On the  side  of a box, K  (fig. 68), a membrane of
india-rubber, M, is fitted, over which a funnel-shaped tube
is  fixed, which is left out  in  the figure.  Coal gas is
passed into the box, and burns above in the  flame, /
The membrane, which at first is quite  loose, bulges  out
slightly from the pressure of the gas;  so that it is  not
tightly stretched and has a very low and scarcely percep-
tible fundamental tone.  It vibrates,  therefore,  with a
tolerably large number of tones which can be  sung into
the tube, and the gas, which is  thereby set in  vibration,
causes the flame to vibrate also.  This takes place  with 
202          THE  FIVE SENSES  OF MAN.
such rapidity that it is imperceptible to ordinary obser-
vation.  However, a cube with four reflecting surfaces set
in rapid rotation makes it perceptible.
   Now when the flame burns steadily and we do not
sing into the tube, a bright straight line is  seen on the
reflector.  As soon,  however, as a tone is sounded,  a
number of distinct points of flame are seen, while the
flame itself flickers up and down, each flicker producing
                       Fig. 68.

a momentary image.  The higher the tone, the closer are
the images ; the lower the tone, the further will  they  be
separated from each other; since the number of images
of the flame depend upon the number of vibrations of
the tone.
    Here, also, we have a membrane which resembles the
tympanic  membrane in its properties, for it is able to
take up the vibrations of tones of different  pitch, which
is explained by its having  a very low fundamental tone.
Moreover, the gas here takes part in the action, by loading 
THE SENSE OF HEARING.
203
the membrane with its  pressure, and, at the same  time,
deadening its fundamental vibration.
   Now it is quite certain that the tympanic membrane
possesses similar properties, such as would enable it to
take up the  vibrations of such a large scale of tones.
   This property is communicated to the  tympanic
membrane in two ways ; first, by its funnel-shaped form,
whereby it  is  unequally and only slightly stretched;
secondly,  by its being loaded with the chain of ear-
bones.
    It has been proved by Helmholtz that  a membrane
of  such a  funnel-shaped  form possesses  remarkable
properties.
    If  we stretch a piece of sheet  india-rubber over a
wide tube, and press it with a rod in the centre  perpen-
dicularly inwards, it then  forms a funnel-shaped surface.
Moreover, we notice that its surface is curved from within
outwards.  Now this membrane is in a different state of
tension at different  distances from  the centre, and its
tension increases towards the centre.  This is recognised
from the membrane being more strongly compressed and
being thinner at the middle, and because when the tension
becomes excessive the centre gives way first.
    Such a membrane, however, has no fundamental tone
since its tension is not equal.  The tympanic membrane
also has, in principle, the same form, since from the centre
it radiates from within outwards in a convex form.  It
follows, therefore, that  the  tympanic membrane is not
very extensible, and its tension is just sufficient to draw
it slightly inwards from the  centre, without its being able
to produce any audible  fundamental tone.  Now such a
membrane is extremely well adapted to transmit vibra- 
204          THE FIVE SENSES OF MAN.

tions of different  rapidity, which has been experimentally
proved by  Helmholtz  as  follows :—In fig. 69, upon a
board, A A, is fixed horizontally a tube, R, over which a
funnel-shaped membrane  of bladder, M,  is fastened to a
wooden peg, St, and unites the centre of the membrane with
the stretched  string, s.  If a violin bow is drawn across
the string it gives a very full tone,  since  the membrane
transmits  the vibration very readily to the air.  If we
remove the peg, however, the string produces a soft, weak
                       Fig. 69.

tone, since its vibrations are transmitted to the air with
difficulty.   Moreover, if we hold before the tube, R, a
tuning-fork, the tone of which  corresponds with that of
the string, then the string is thrown into rapid sympa-
thetic vibration, which can readily be recognised, because
small pieces  of paper  placed  upon  the  string are dis-
lodged.  Since this experiment succeeds  equally well
with different tuning-forks, as long as we make the length
of the string correspond with  the  pitch of the tone, it
follows that the funnel-shaped membrane has the power
of transmitting a large series of tones with equal facility.
The tympanic membrane, however, which possesses this 
THE SENSE  OF HEARING.           205
power in the highest degree, likewise derives it  chiefly
from its funnel-shaped form.
   Whilst, in the experiment described, we compare the
tympanic membrane with the curved membrane, we can
compare the peg and the string with the ear-bones, which
serve to convey the sound  still further.   They, however,
play another part, in loading the vibrating membrane as
an inert mass, and thus  deprive  it of every trace of its
fundamental  tone, so  that  it can accommodate  itself
equally  well to vibrations of every degree of rapidity.
Another advantage consists in the fact  that the loading
of the membrane removes every subsequent vibration of
the membrane.   It would be exceedingly troublesome to
the ear if  the membrane continued vibrating, like a drum
after it has been struck.   This evil is removed chiefly by
the ear-bones acting as  dampers, like  the dampers in a
pianoforte, which fall on the wires after every note.
   Another important effect which is due to the  funnel-
shaped form of the tympanic membrane must be men-
tioned.  We have said above that the apex of the funnel
bulges  inwards  like a sail, and consequently the point
where  the vibrations are greatest cannot be the  apex
of the funnel, but must  be situated somewhere between
the apex and the edge.  But the force  of all  these vi-
brations passes  from all sides  towards the centre, and
produces here, at this point, vibrations  of greater inten-
sity.  This action may be compared to the wave-motion
which we can produce on a loosely stretched  horizontal
wire.  If we strike its lower side upwards from below, a
wave then passes along it to the  end, where it can spend
its strength.  In a similar manner the waves of the mem-
brane pass to the  centre, and since their force  is em- 
206
THE FIVE SENSES OF  MAN.
ployed in setting the chain of ear-bones in motion, the
excursions decrease in height.  This diminution  of the
excursions is of great importance ;  for it is evident that
the membrane of the fenestra ovalis cannot execute such
large vibrations as the tympanic  membrane, since it  is
twenty times less.  On these grounds  the vibrations  of
the tympanic membrane decrease in the membrane itself,
during their passage towards the centre and the point  of
the hammer-handle.   At  the same time  this causes an
increase in the force of the sound-waves in the funnel  of
the tympanic membrane, just as the rapidity of a stream
of liquid increases as  it passes down a funnel.
    In this manner the sound-waves are transferred from
the tympanic membrane to a membrane twenty times less;
by the ear-bones the entire force of the vibration  of the
tympanic membrane is concentrated upon a much smaller
surface, and its intensity thereby increased twenty times.
Moreover, the chain of the ear-bones acts as a lever ; and
when both arms of the lever are fully extended, we then
find that the arm attached to the tympanic membrane  is
half as long again as the arm attached to the membrane
of the fenestra ovalis, and, therefore, the force of the
sound-waves at the fenestra ovalis is increased thirty-fold.
In a consideration of the wonderful delicacy with  which
the apparatus of the tympanic cavity is constructed, we
must not omit the  action  of two  small muscles,  which
are attached to  the ear-bones.  One of them is united by
a long thin fibre to the handle of  the hammer, near the
neck (fig. 64), so that it pulls perpendicularly to the tym-
panic membrane.  There can be no doubt that the con-
traction of this  muscle is intended to draw the tympanic 
THE SENSE OF HEARING.
207
membrane more or less inwards, and thus to increase its
tension.  The question arises, therefore, what function is
to be attributed to these muscles ?
   A greater or less expansion  of the  tympanic mem-
brane must evidently have an influence upon its vibra-
tion.  Since its tension is by no means great, an increase
in its tension  by means of the tensor  tympani muscle
must have the power of producing considerable changes.
It has, therefore, been  imagined that the function of this
muscle is to adjust the tympanic membrane for tones of
different pitch, so that  the ear possesses a power of ad-
justment similar to that of the eye.  It might, however,
be argued in opposition to this view that it would be im-
possible for the tension of the muscle to alter with suffi-
cient rapidity to enable us to perceive the great number
of consecutive tones in music, which it  is really in  our
power to do.  For the contraction of the muscle would,
in a  shake, for instance,  evidently lag  behind the  tone
heard, and soon  fall exactly upon the wrong tone.   It is
improbable, therefore, that this muscle can  act continu-
ously, but we may well imagine it to  be set in  action
when the ear is listening attentively to a certain tone of
long  duration.   The  greater its  contraction,  and  the
greater the tension of the tympanic membrane, the better
shall we perceive high tones, while the  deep tones will
be less intense, so that  in this  manner an adjustment of
the ear can be accomplished.
   The function of a damper has also  been ascribed  to
this  muscle.  When  the contraction  of  the muscle
stretches the tympanic  membrane strongly  inwards, it
might be supposed to damp the vibrations of the mem- 
208
THE FIVE  SENSES OF MAN.
brane, just as a vibrating wire is damped when pressed
by the finger.  It is, therefore,  quite possible that this
muscle  may be  set in action when a very deafening
sound reaches the ear, since it diminishes the amplitude
of the excursions of the tympanic membrane.  At the
same time it will press the stirrup more strongly into the
fenestra ovalis, and will prevent it from executing vibra-
tions of an  excessive amplitude.  The damping  action
will be more active for deep than for high tones, since an
increase fn the tension of  the  tympanic  membrane in-
creases the pitch  of  its fundamental tone.
   Very violent concussions, the firing of a cannon, for
instance, are known to have the power of bursting the
tympanic membrane.   This  muscle  cannot afford any
protection against such a violent and sudden action, since
it cannot contract with sufficient rapidity, although it  is
possible that in such cases it may modify the after-vibra-
tions of the tympanic  membrane, which are not inconsi-
derable.
   The  function of the second muscle, the stapedius,
which is  attached to the stirrup, is, as yet, unexplained.
It arises  from the posterior wall of the tympanic cavity,
and is inserted at right angles into the head of the stirrup
near its articulation  with the  anvil-process.   It has been
supposed  that it is intended to act as a damper  to the
sound-vibrations, since  it  places the foot-plate  of the
stirrup obliquely against the fenestra ovalis.   This  action
might also consist in its pulling at right-angles to the
movement of the stirrup, and  thereby diminishing  its
excursions.   For  instance, if to a vibrating spring we
attach a thread, which pulls at right angles to the plane
of the vibrations, the latter will be checked. 
THE SENSE  OF  HEARING.          20g
   It is a very curious fact that a nerve passes through
the tympanic cavity, called by  anatomists  the chorda
tympani, which has nothing to do with the hearing at
all.  It passes out again  through a crevice in the bone
towards the tongue. 
210
THE FIVE  SENSES OF  MAN.
                    CHAPTER V.

The Labyrinth—The Organs of Corti—The Transmission of Sound in the
     Labyrinth—Presence of Sympathetic Vibratory Apparatus.

As we pass to the  Labyrinth we penetrate  into the
deepest and most secret part of the auditory organ, in
which the physiological action of  hearing  takes  place.
In order to  follow correctly the passages of  this compli-
cated structure, we will first consider it from the exterior.
It is represented, in fig.  70, from different sides, after a
drawing by Helmholtz.   In fig. A we are looking at the
Labyrinth from  the left.  The oval  aperture (fenestra
ovalis), Fv, leads into  the broad intermediate space which
is called  the  vestibule.  On both sides  of  it  there are
curiously coiled formations ; on the one  side the cochlea,
S, and  on the other the semicircular canals,  C.  The
shape of the  cochlea is exactly similar to that  of  an
ordinary snail-shell,  and consists of two coils and  a half.
The vestibule has a  second opening, the circular aper-
ture  (fenestra  rotunda), fc, which is situated at the
commencement of the  cochlea.    Both apertures are
covered with a membrane,  and open,  as already men-
tioned,  into the  tympanic  cavity.   The plate of  the
stirrup  presses against the  fenestra  ovalis,  but  the
membrane of  the circular aperture  is free.   The outer 
THE SENSE OF  HEARING.
211
wall of the labyrinth consists of bone, and the whole is
imbedded in the solid structure  of  the  skull.   Within
the osseous walls of the cochlea there is a hollow  pas-
sage, which winds up  to the summit.  This  passage is
divided into two canals by  a partition-wall of bone,
while the  partition-wall passes round the axis from  the
  Fo                             Fc
Fig. 7o, a.                       Fig. 70, b.

                    0
Fig. 70, c.
base to the apex like a spiral staircase.   It is  called the
osseous lamina spiralis.   However, it does not extend
quite to the  outer  wall  of the coils of  the cochlea,
but  is united  to  it  by a  membrane, which, from  its
coiled  shape,  is called the spiral membrane.    Of the 
212
THE FIVE SENSES OF MAN.
two passages of the cochlea  which are formed by the
lamina  spiralis,  the one which faces the  base of the
cochlea is called the scala tympani, since it has its  com-
mencement at the circular aperture, which leads to the
tympanic cavity.   The other passage,  which  faces to-
wards the apex, is called the scala vestibuli, and  opens
directly into the vestibule.
   Fig- 71 gives us a section of the cochlea from top to
bottom, and also a section of  the windings, as well as the
partition-wall, which divides them into two passages, the
so-called scalae.
Fig. 71. 1
   The whole of the interior of  the  bony  labyrinth  is
covered with a membrane, which  is attached to its bony
walls,  and which is, therefore,  called  the membranous
labyrinth.   The whole of the interior is filled with the
fluid of the labyrinth,  which is intended to transmit the
sound-waves.   The walls of the vestibule and of the
semicircular  canals are also covered with this mem-
brane,  the whole of which forms  two closed sacs, whose
sides rest upon the  vestibule.
                  Kolliker, < Gewebelehre.' 
THE SENSE OF  HEARING.           213
   The semicircular canals have a very regular relative
position, their planes being at right angles to one another.
We distinguish, therefore, one horizontal and two vertical
canals, each of which has its origin in an oval dilatation
called an ampulla.
   We find upon the membrane of the  membranous
labyrinth expansions  of the filaments of the  auditory-
nerve, which pierces the bony labyrinth, and  is lost in
fine  terminations ;  these are distributed in the cochlea,
vestibule, and the ampullae.
   Near the wall  of  the membranous labyrinth  are
found very small particles called ear-stones  (otoliths),
consisting of carbonate of lime,  which have  long been
known.   Max Schultze has  lately discovered  the exis-
tence  of  terminal  nerve-organs  in  the membranous
wall, to which the otoliths  appear to be attached.   The
wall is covered   with  epithelial  cells,1 which  have fine
hairs upon their surface, and are  connected with the wall
by fine  nerve-fibres.  We may, therefore, assume  that
when the otoliths are thrown into motion they bend the
little hairs backwards and forwards, and thus mechani-
cally excite the nerve-fibres.
   The  lining membrane in the  ampullae (fig.  72) forms
a  thick projecting  ridge,  called the crista  acustica,
which is particularly  prominent  in fishes.   Upon it are
found tolerably long, stiff, hair-like processes,  which we
may also consider as terminal nerve-organs; for numerous
nerve-fibres penetrate to the ridge, and are spread out
there.   These hair-like  processes are probably thrown
into vibration by the waves of  the  fluid of  the  laby-
rinth, and, thereby, cause a sensation of sound.
           1 Cells which cover the surface of the skin. 
214          THE FIVE  SENSES  OF MAN.
   Still more remarkable and complicated are  the ter-
minal nerve-organs found in  the  cochlea.   The cochlea
nerve,  which  is a branch of the common auditory nerve,
pierces  the axis of the cochlea and spreads  out into
Fig. 72.
fibres, which, by degrees, pass through the osseous lamina
spiralis, and at length reach the spiral membrane.
    Fig. 73 gives a section through a coil of the cochlea.
Upon the right is the outer wall of the coil, and upon the
left the axis of the  cochlea.   The osseous lamina spi-
ralis,  Ls,  divides the  canal into two passages, the scala
vestibuli,  Sv,  and  the scala  tympani,  St.   The spiral
membrane, b,  joins this partition-wall and shuts off the
two scalae entirely from  each other, and is attached to
the outer wall of the cochlea.   A membrane, v, further
divides the scala vestibuli into two longitudinal passages. 
THE SENSE OF HEARING.           215
The nerve fibres enter the osseous lamina from the axis,
pass to its lower edge, where they unite with the spiral
membrane.  Now this membrane is composed of several
                       Fig. 73-
formations", some of which are represented by the dotted
lines in the figure, and which follow all the  spiral wind-
ings of the cochlea to the summit.  We here have be-
fore us the most wonderful formations,  the organs which
were discovered by Corti, and which bear his name.
         A                               B
                       Fig. 74-'
    The  organs  of Corti consist  of S-shaped  fibres,
which may be seen in fig. 74, A.   Two fibres are dis-
tinguished, an inner i, and an outer e, which together form
                  1 After Helmholtz. 
2l6
THE FIVE SENSES OF MAN.
a bow.   The inner fibre  rises  almost immediately from
the commencement of the membrane, beginning with a
broad foot; the outer one terminates in the same manner
upon the membrane,  which has  on that account been
called  the  basilar membrane.   The fibres are joined
                       Fig. 75-'
above in a peculiar  manner, t, and are  prolonged  in a
short horizontal fibre, 11.
    Now these fibres  of Corti stand in great numbers
close together upon the basilar membrane.    About
3,000 of the bows have been counted, ranged together in
rows in a very small space.  Fig. B shows the manner in
which the fibres are placed upon the basilar membrane
which is represented by b. The further discovery has
lately  been made  that the membrane  is composed of
fibres arranged in a transverse direction, so that  every
fibre of Corti must rest upon, and pass into one, or a pair
of these fibres.  This structure is apparently of impor-
tance, for it is very probable that the transverse fibres of
the basilar membrane are intended to vibrate in common
with the fibres of Corti, which rest upon them.
   The remaining space  between the  scalae is chiefly
filled with cells of a different  form.   The greater  num-
ber are small oblong cells, provided with a hair-like pro-
                   1 After Kolliker. 
THE SENSE OF  HEARING.
217
longation, which are particularly numerous in the neigh-
bourhood of the fibres and the basilar  membrane.   In
fig. 75 we see these cells, i i,  the prolongations of which
pass to the basilar membrane,  and which  are  arranged
longitudinally in  three rows.  The bow of Corti, a d e,
is here also represented vertical, standing upon the  basi-
lar membrane, 0.  The walls of the latter are immedi-
ately covered with rounded epithelial cells,  n.
   Viewed from above, the organs of Corti and the ac-
companying  formations  present a  peculiar  structure,
                       Fig. 76.'
which recalls the interior of a piano.  In  fig. j6, we see
in b a c, the inner fibres of  Corti, in e the outer, and  in i
                    ' After Kolliker. 
218
THE FIVE SENSES OF  MAN.
their prolongations to the basilar membrane.   At / is
the highest point of the bow formed by the fibres, where
the horizontal terminal fibre is found.  The cells of the
interior are attached to this horizontal fibre like a mem-
brane,  and show a very regular formation, which may be
seen in fig. 76, where the fibres of  Corti have been re-
moved.  We here find a threefold  row of holes, m n 0,
and between them regularly placed connections, amongst
which we discover an  inner /, and an outer /, and finally
square terminal formations, p q.
    The whole strongly resembles an instrument construc-
ted with great delicacy and  regularity,  in  which the
smallest components are  fitted with the greatest exact-
              j —i—______________^______
                       Fig- 77-
ness.   In reality we find that the fibres of Corti are not
perfectly uniform in size and shape, but that they increase
in length and diminish in height from the base to the
summit of the cochlea; i.e. the short fibres rise more abrupt-
ly from the base, as may be  seen in the accompanying
figure (fig. 7J) b, while at the summit s, they are more elon-
gated and, therefore, not so high, and form a wider arch.
The transition from one form to the other is very gradual
in the greater number of the fibres. We, therefore, cannot
help forming the conclusion that the great number of the
different fibres of Corti was intended to render tones of 
THE SENSE OF  HEARING.          219
different pitch accessible to the sense of hearing.  More-
over, the width of the basilar membrane is not the same
throughout, but increases considerably from the base to
the summit of  the cochlea, and in a higher degree as the
arch of the fibres of Corti increases.
    Before we enter upon the functions of the organs of
Corti, we must return once more to the  transmission of
sound-waves  to the labyrinth.   We  left them at  the
fenestra ovalis, which they had reached through the vi-
bration of  the stirrup.    The membrane of the fenestra
ovalis, therefore,  performs  transverse  vibrations, which
set the fluid of the  labyrinth in motion.  Now it is clear
that if the fluid of  the labyrinth is  here forced inwards,
it  must give way  in another part, for a fluid is not com-
pressible.  The  circular aperture  serves  this purpose.
As soon as the stirrup  vibrates inwards, the  membrane
of the  circular aperture bulges  outwards and thus re
lieves the pressure upon the  fluid of the labyrinth.  If
there were no  circular  aperture  the stirrup  could  only
perform very small vibrations.   The  circular aperture,
therefore, acts  as a counter opening to the oval aperture.
    Now the shock of the stirrup produces a wave in  the
fluid of the labyrinth, which  can be transmitted to all
parts of the labyrinth.   The  wave  originates  at  the
oval aperture, and since  this leads into the vestibule, the
wave first  spreads  out  into the  vestibule.   From  here
it  passes on into  the cochlea, and we must assume that
it  throws the membranous partition with  its organs of
Corti into vibration also.
    Helmholtz  has made some very interesting observa-
tions  upon this  circumstance.   Since the vestibule is
divided by the  two  membranous sacs, which  it contains, 
220          THE FIVE SENSES OF  MAN.
into two portions, each  containing fluid, one of which is
entered by the oval, the other by the circular aperture,
it  is impossible that the fluid in the vestibule, by a shock
of the  stirrup, can uninterruptedly  be driven  from the
oval aperture  to the circular aperture.   Now the space
filled with liquid extends  from the oval aperture with-
out  interruption to the scala  vestibuli  of  the cochlea,
whilst it is prevented from entering the scala tympani by
a membrane.  The wave will, therefore, chiefly penetrate
the scala vestibuli, and flow rapidly up to its head.   In
this manner it will shake the yielding membranous par-
tition which must give  way  inwards towards the scala
tympani, and  thus  throw the fluid against the aperture,
which is situated opposite to the scala tympani.
   We  know from the physiological  observations upon
sympathetic vibrations,  that such  a  condition of things
may easily take place, when the fundamental tone of the
body performing  sympathetic  vibrations exactly coin-
cides with that of the sound.   Moreover, bodies capable
of performing sympathetic  vibrations seem really to
exist in the organs  of Corti.  The basilar  membrane,
which is composed of transverse fibres, we may, after
Helmholtz, regard as a series of stretched strings, sepa-
rate portions of which may be thrown into  sympathetic
vibration  independently of the whole.  If we assume,
further,  that the fibres  of  Corti, which rest upon  the
basilar membrane,  have an influence upon the fundamen-
tal vibration of the membrane, and alter the fundamental
tone,  according to their height and  span, then we  can
easily understand how each  tone  will correspond  to a
certain portion of the basilar membrane at different alti-
tudes between the base  and  summit  of the cochlea. 
THE SENSE OF  HEARING.           221
According to this view, therefore, the whole of the basi-
lar membrane never vibrates to a single tone, but merely
a certain portion of it,  which  renders  our power of
distinguishing such a large series of tones comprehen-
sible.  We  are still,  however,  entirely ignorant of the
manner in which  the vibrations of the basilar membrane
take place, with  the fibres of Corti attached to it, which
are situated in a  soft mass of cells.  The inner fibres,
which spring from the inner edge of  the basilar mem-
brane, can,  from  their position, be but  slightly set in
motion, while the outer fibres which rest upon the mem-
brane  near  its centre, will be  very strongly agitated.
Still we do not know what part they play in this  action ;
indeed, less  importance has lately been ascribed to them
than they were formerly  supposed  to  possess, for it has
been discovered that in the  cochlea of birds the fibres of
Corti are absent, and, therefore, more  weight has been
laid upon the sympathetic vibrations of the basilar mem-
brane.
    It is, however, quite certain that, in the cochlea, we
have to do with  a series  of apparatus  adapted for per-
forming sympathetic vibrations  with  wonderful exact-
ness.   'We  have  here  before  us a musical instrument
which is designed, not to create  musical  sounds, but to
render them perceptible, and which is similar in construc-
tion to artificial  musical instruments, but which far sur-
passes them in the delicacy, as well as the simplicity of
its execution.   For, while in a piano  every string must
have  a  separate  hammer by means of which  it   is
sounded, the ear  possesses a single  hammer of an inge-
nious form in its ear-bones, which can make every string
of the organ of Corti sound separately. 
222
THE FIVE SENSES OF  MAN.
    The basilar membrane, as we have already remarked,
is not of the same breadth in all  its parts, but widens
towards its summit.  If we suppose it to  be rolled out
from its spiral form upon a plane, it will have the appear-
ance of a wedge, as the accompanying figure,  78, shows,
in which the transverse fibres are represented by lines.
Let us suppose the fibres of Corti to be standing upon
                the  transverse fibres, the span of  the
u'<~^~^      ,d' fibres of Corti increasing from a to a',
  \    ■   ---J   then the first portion  of the basilar
   V—  ■   —j   membrane will  gradually increase  in
   \   :  =^|   breadth from b d  to b' df.  It seems
    \    \    j   remarkable that the  summit  of  the
    \    :  ^j    cochlea should contain the broad, and
    \   i    | #   the  base the  narrower portion of  the
     \   '   I    basilar  membrane,  for  we  should
    J^v  1^   imagine the opposite to be more  prac-
     Fi  »g      tical in the formation of a shell.   But
                all  the  several  parts of this  organ
are fitted into each other with such  exactness, that this
position of the membrane is, doubtless, not without its
significance.
    From the varying  breadth of the  membrane and the
varying form of the  fibres of Corti  the  conclusion has
been drawn, with great probability, that the recognition
of high  tones takes place at the base of the cochlea and
of low tones at the summit;  for at the summit the longer
fibres will be thrown into vibration by low tones, and at
the base the shorter fibres by  high tones.  To this the
objection might be made that the  transverse fibres could
not be  set in sympathetic vibrations by low  tones, be-
cause they are so exceedingly short.  The membrane 
THE SENSE OF  HEARING.           223
indeed is but -j of a millimetre (^ of an inch) broad at its
point, and hardly "-^ of a millimetre (^of an inch) at
its base,1  and if  we could  imagine a  violin string  as
short as this, it would,  even with moderate tension, give
very high  tones;  but  we must take into consideration
the fact, that the transverse fibres are not freely stretched
strings, but that they are weighted by the surrounding
parts, and  that the fibres of Corti, which are connected
with them, offer an opposition  to the vibrations by their
tension.   Every  such encumbrance  and check would,
however, lower the  fundamental  tone  of a  stretched
string and, therefore, it is perfectly comprehensible that
the broad  portion of the basilar membrane itself might
possess deep fundamental tones.
    The basilar membrane  is not only weighted by the
fibres of Corti, but by the entire mass of the cells, which
are found upon the fibres and the membrane, and prin-
cipally by  the fluid of the labyrinth ; the latter offers a
resistance to the vibrations by  friction and pressure, and
thus it follows that certain  fibres of the membrane, in
spite of their shortness, are able  to vibrate in the same
period as the waves of the fluid of the labyrinth itself.
    All these observations lead to the conclusion that the
cochlea is only designed for the perception  of musical
sounds.  This organ not only enables us to determine
the  pitch of a tone, but also to recognise tones  which
follow each other in  rapid succession,  as when they are
arranged so as to produce a melody; and, finally, by the
aid of this organ we can even receive the sound pro-
duced  by a great  number of simultaneous tones, so as to
enjoy their harmony.
   1 After  the measurement of Hensen upon the auditory organ of an
infant.
              11 
224
THE FIVE  SENSES OF MAN.
   The relation of the  number of  the fibres of Corti to
our power of distinguishing difference in  tone has been
calculated.   We possess 3,000  fibres of Corti, 2,860 of
which, according to an  estimation  by Helmholtz, may
be distributed among the 7 octaves which contain the
musical  notes  in  general use.  Therefore, about 400
fibres of Corti  will fall  to an octave, and 33^ to every
semitone.  Thus a considerable number of  transverse
fibres and fibres of Corti correspond to the interval of a
semitone, the middle ones of which will be most strongly
agitated, and those on either side in a gradually diminish-
ing degree.   However,  practised musicians can perceive
a much finer difference between two tones, even to -gijth
of a tone,  according  to the assertion of E. H. Weber.
Two  approximate fibres would  exactly  coincide  with
two such tones, and this difference may, therefore, very
well reach the limits of  our perceptive power.   In short,
we see how richly the cochlea  is provided with sympa-
thetic vibratory apparatus for the perception of musical
sensations.
    Microscopic Anatomy has lately been engaged with
the  subject  of the connection  between the  auditory
nerve and  the organs of  Corti.   It had already  been
known for some time, that the auditory nerve penetrates
the axis of the cochlea, and that from the axis it gradually
distributes   its  fibres  outwards  through  the osseous
lamina.  In the latter,  the fibres have been  traced  to
the  edge where  the basilar  membrane  commences;
but,  after this  point,  the  course of  the  fibres can no
longer be satisfactorily observed, and great difficulties are
placed in the way  of further investigation.   It is found to
be very probable,  from  the recent very careful observa- 
THE SENSE OF  HEARING.           225
tions  of some  investigators,  that the  nerve-fibres are
connected with those cells with hair-like prolongations
which  have  been  mentioned  above.   At  least, these
cells are found in great  numbers in the cochlea of birds,
in which the fibres of Corti are altogether wanting ; and
the entrance of fine nerve-fibres into these cells has been
observed with some degree of certainty.
    It is extremely  probable that the extremities of the
nerve-fibres are excited by the vibratory apparatus of the
cochlea in  a mechanical manner, the vibrations  of this
organ causing them to  be stretched or crushed.  Every
such excitement of an ordinary nerve of  the skin would
produce a sensation of pain, but the excitement of the
auditory nerve calls forth its own specific sensation, the
sensation of sound, which is  only produced when the
nerve has  communicated its  excitement to  the brain.
We may assume that each organ of Corti  contains, at
least, one nerve-fibre, so that the brain is made conscious
of tones of different pitch by different nerve-fibres, which
accounts for our power  of recognising  such  delicate
variations in pitch.  We have still to answer the question,
as to what significance is to be attached to  the otoliths
and the hairs in the vestibule and  the  ampullae.   It is
evident that here we have not to deal with various kinds
of definite apparatus which can assist us in the sensation
of  musical sound.   It is, therefore, very probable that
these organs are only  designed for  the recognition of
noises, by receiving a shock from the irregular unperiodi-
cal vibrations of the fluid of the labyrinth, which excites
the free terminations of the nerves.   In the ampullae
the fluid flows in and out through narrow openings, and
here, according to Helmholtz, eddies are probably pro- 
226
THE FIVE SENSES OF MAN.
duced which throw the hair-like processes into vibration.
We must not, however, assume  that we are able to recog-
nise a certain  tone by  means  of these organs, because
they do not appear to be adapted for different tones ; and
if  they  were  to possess  this  power  of distinguishing
different tones,  it would  only  be  in a  very imperfect
degree.   We can, on the contrary, easily imagine that a
noise might agitate these organs sufficiently for them to
excite the terminations of the nerves in connection with
them.
   The  sympathetic vibratory apparatus  in  the  la-
byrinth, and particularly in the cochlea,  further possess
the important property, that they  do not continue the
vibrations  after  the  sound  has  ceased, a  property
which has  already been mentioned in  connection  with
the tympanic membrane and the  ear-bones.   It would,
however, be useless in the latter if the organs of Corti
did not also possess it.  Were they to retain the sound
it  would be impossible  to recognise the shake distinctly,
which, however, we can  really do to the extent of eight to
ten tones a second.  Thus after £ or -^ of a second the
tone will have ceased to sound  in the ear so completely,
that the following tone  will not be confused with it.   In
the piano this property can, we know, only  be attained
by means of a damper, which  falls upon the wire after
every blow of the hammer.   The portions  of the ear,
however, which   are designed  to  perform sympathetic
vibrations,   require no  such damper,  for they possess
from  their combination, which, as  yet, we do not suffi-
ciently understand, the  property of quickly returning to
a state of rest when the cause of motion has ceased to
act. 
THE SENSE OF  HEARING.          227
   Helmholtz has, nevertheless, observed that the  ear
does perceptibly retain  the  sound  of  very  low tones.
For instance, it is very striking, as  pianists  know well,
that a  shake sounds  more and more imperfect  in  the
notes below A, because the tones begin to be confused
with each other.  Now it might be supposed  that this is
to be accounted for by  the imperfection of the dampers
of the instrument, and not by any peculiarity of the ear.
But in other instruments, the violoncello for instance, in
which the  finger producing the shake plays the part of
an instantaneous damper, and even in a physharmonica,
in which the sound ceases the moment the finger is raised,
Helmholtz has  recognised the same phenomenon. From
this we learn that the damping action in the ear is not so
perfect for low as for medium and high tones.
    This circumstance, moreover, clearly%>roves, as Helm-
holtz remarks,  the existence of sympathetic vibratory
apparatus  in the ear, which  are  adapted for tones of
various pitch.   For if there were only a single sympa-
thetic vibratory apparatus in  the labyrinth, which would
answer to all  tones, as  is the case with the tympanic
membrane and the  ear-bones, we   should find that it
would retain the sound  in a  perfectly different manner.
 It would then  possess a fundamental tone, and as soon
as the sound-wave had ceased to act, it would sound, not
with the tone of the sound-wave, but, in accordance with
 certain physical  laws, with its own fundamental tone.
 If, for  instance,  we  move an  elastic spring  at a  rate
which does not coincide with its fundamental tone, it
will immediately assume the  latter when we let it go.
 Were  this the  case with the ear, the low tones of
 the  shake would not be confused with  each other,  but 
228
THE FIVE  SENSES  OF MAN.
would be accompanied by a third tone, the fundamental
tone of the ear.  This, however, is certainly not the case,
for no.  fundamental tone can  be recognised in the ear,
except that of  the auditory canal ; and, therefore, the
retention of the sound of low tones can only be caused
by certain organs which correspond to those tones, the
existence  of which is  most distinctly betrayed by this
peculiarity.
   Finally, very interesting observations have been made
by  Hensen upon  the auditory organs of  small  crabs,
which show the sympathetic vibrations of certain organs.
These  animals  possess  small sacs which  contain ear-
stones, and  are covered  internally  with small hair-like
processes.   Similar hair-like  processes  are  also  found
upon the  surface  of the body, upon the antennae and
upon the tail, wmch are open  to direct observation with
the microscope.  The tone of a horn was conducted to
the vessel containing the crabs,  which  was filled with
water,  and Hensen distinctly  observed the sympathetic
vibrations of some of the hairs, and found  that they
corresponded with tones of different pitch.  Hence, it is
almost proved to a certainty that the apparatus for pro-
ducing sympathetic vibrations in the human ear must have
a similar action ; only we  must remember that, in the
latter, the relations are much more  complicated, but also
that the sensation  of musical  sound must be much more
delicate and perfect than in such crustaceae. 
                   CHAPTER VI.

The Sensation of Music -The Monochord—The Octave and its Divisions-
  Perception of Grave and Acute Tones—The Irritation of the Auditory
  Nerve.

A tone is produced as soon as an elastic body performs,
with certain rapidity, regular periodical vibrations which
are conveyed  to the ear by a conductor of sound.  We
distinguish certain peculiarities in this sensation ;  in the
first place, its strength or  intensity; secondly, the pitch
of the tone ; and thirdly, its colour or quality (timbre).
    The intensity of a tone  depends  entirely  upon the
extent or amplitude of the sound-wave.  If we compare
a sound-wave with the waves upon the surface of  water,
we shall find that, in the latter, the extent is an index  of
the height, since they increase in power and  mechanical
action in  proportion  to  their height.  And with the
amplitude of the vibration of a stretched string, we shall
find that the intensity of a tone increases,  although the
number of vibrations in  a second remains the same; and,
therefore, the further we remove a string from the position
it occupies when  at rest, the greater will be the intensity
of the tone, although its pitch remains unaltered.
    The vibrations of elastic bodies, therefore, obey the
same rules as  the vibrations of a pendulum.   A  pendu- 
230          THE FIVE  SENSES  OF MAN.
lum  of a certain length  will always make the  same
number  of oscillations  in a  second,  independently  of
their extent, within certain limits.   The velocity of the
motion  increases  with  the  extent of the oscillations;
and the action of a vibrating string is  exactly similar.
   The vibrations of sound in the  air are longitudinal;
they consist of alternate expansions and  condensations,
and the greater the condensation the louder will be the
sound  produced.  The particles  of  the air  execute a
motion to and fro, and the  greater  the extent of this
motion the more will they alternately approach and recede
from each other;  in other words, the greater will be the
condensation  and expansion which they will  produce.
The sound-wave itself always retains the  same length in
the air as was imparted to it by  the  source of sound.
During its propagation  through the air the intensity  of
the tone decreases inversely as the square of the  distance,
as is the case with light.   The reason  of  this is, that the
force of  the vibration from a distance I to a distance  II
will be spread over a surface of four times the  size.   In
every point of the latter, therefore, the intensity will not
be in the ratio of  ^ but of (i)2=j.
    The  pitch of a tone depends upon the length  of  its
sound-waves.   The longer the  sound-wave, the deeper
will be the tone,  and the  shorter  the wave, the  higher
the tone.
    To return to the vibrating string, we shall find that the
length of the string represents the  length of the sonorous
vibrations in it. A string of a certain length will, therefore,
produce vibrations of half the length  of  those produced
by a string twice as long.  Moreover,  we know  that the •
first string makes twice as many vibrations in  a second 
THE SENSE OF HEARING.
23I
as the latter, and, at the same time, we perceive that the
tones produced vary in pitch, one of  them being said  to
be an octave  above the other.  We can, therefore, say
that the pitch of atone increases with the number of vibra-
tions performed in a given time.
   Let us consider, further, the sound-waves of different
tones in the air, supposing a definite  number, which we
will call n, of the sound-waves of a tone, to be produced
in a  certain space round the source of sound.  If we now
sound the  octave of this tone,  twice  as many,  or 211
waves will be produced in the same time.  Now all sound-
waves, whether small  or great,  are  transmitted  with
nearly the same velocity, therefore 2  n waves will accom-
plish the given distance in the same length of time as n
waves.  In other words,  the 2 n waves will  now  be
contained within this space,  each wave being only half
the length of the waves of the tone of  n vibrations.
    When, therefore, we hear two tones, one of which  is
an octave above the other, the upper tone conveys to the
tympanic membrane twice  as many sound-waves  in a
second as the  lower one,  but of only  half  the  length ;
and the  tympanic  membrane and all the  sympathetic
vibratory apparatus connected with it in  the ear, repeat
the vibrations  in  the same  length of  time.  It is  very
remarkable that our organ of hearing is naturally gifted
with a perception  of the interval of an octave.   Two
tones whose rate of vibration is in the ratio 1 : 2 produce
upon the ear a sensation very similar to each other, which
is evidently caused  by  their vibrations  standing  in  such
a very simple  relation to each other.   In spite of this,  we
are not in the  least degree informed by our ear of the
existence of the vibrations,  and  still less of the definite 
232          THE FIVE  SENSES  OF MAN.

number which  are produced  in  a certain  time,  which
physical research has first revealed to us.  But our sense
of hearing is extraordinarily delicate in its power  of dis-
tinguishing between the pitch  of different tones ;  that is
to say, of distinguishing the relation between the number
of the vibrations.   We do not, however, recognise them
as such, but simply as specific sensations of sound,  for
it is only through the study of physics that we have
learnt to  transfer the sensations of  tone  into relative
vibrations.
    The great philosopher and mathematician, Pythagoras,
who lived in lower Italy 500 years B.C.,  and by his celebra-
ted teaching laid the foundation of the science of mathe-
matics, was also the discoverer  of the law of vibrating
strings.  He  constructed the  monochord, which,  in  fig.
79, is represented in the form now used by Physicists.
It is said  that  Pythagoras was  led  to the  discovery of
this law  by watching  a forge,  and  listening to   the
different pitch of the sounds produced  by large and small
hammers.
    The monochord consists  of a wooden  box so con-
structed that strings can be stretched  across it.  The two
bridges a and b fix the length of the string to be  set in
vibration. The strings are either stretched by a peg, or dif-
ferent degrees of tension  may be given to them by weights,
the string from the bridge, a,  being led over a roller and
the weights, P, hung on to it  by a hook.  By this means
we are enabled to calculate the amount of  tension from
the weight, and to discover the influence  of the tension
of the string upon the pitch of the tone.
    If we  now strike a string of constant tension, or draw
the bow across it, a tone of a continuous pitch is heard 
              THE SENSE OF HEARING.           233

which we will suppose to consist of n  vibrations.   The
string, as in  fig.  79, consists  of sixty  divisions, and if
we push the bridge up' to thirty, then the half-string will
produce a tone which will be an octave above the other.
If we take \ of  the string,  we shall have  the second
octave ; \,  the third ; y1^-, the  fourth  octave, and so on.
Thus we see  that the sensation of  tone in our ear stands
                        Fig. 79.
in a very remarkable mathematical relation to the length
of the string, very simply expressed numerically by the
ratio 1 : i : i : i : re, etc.  .
   The power of discriminating  tones  which the ear
possesses is very perfect, a very slight deviation from the
octave being recognised as out of tune.  We can measure
the half of the string with much more certainty by the
ear than by the eye, for if we try  to touch the half by
the eye, we shall find that we generally produce a wrong
tone.  The violin player, who by pressure alters the length
of the strings and, therefore, the pitch of the tone, never
calculates the distances with  his  eye but by  means  of
tactual and  muscular sensation,  which,  with  practice,
forms a much more certain guide than the eye. 
234          THE FIVE  SENSES  OF MAN.
   There can be no doubt that this sensation of the octave
is a direct gift and innate, without  any  acquirement on
our part.   For if an air is sung in a low key to a child,
who, as yet, knows nothing of the scale, but has a musical
ear, he will afterwards sing the  air,  in accordance with
the capabilities of his voice,  an octave  higher.   Again,
we often remark that the uneducated  singer,  in singing
any air, will  spring from one octave to another, as it suits
his voice.
   Two tones, therefore,  separated from each  other by
the interval of one or more octaves, appear, amongst all
the other tones, to  resemble each other most  closely in
quality.   Now the  science of Physics has shown that the
number of vibrations varies in a fixed proportion to  the
length of the  vibrating  portion of the  string.  If one
string makes n vibrations in a second, then another string
of half the length, will  in the same time make 2 n vibra-
tions ; and thus we learn  that the octave is produced by
the conveyance of  a double number of vibrations to our
ear.   It  is this very simple ratio of  I .* 2  between  the
vibrations which makes it possible for  our ear to become
conscious of the sensation produced by two very  similar
tones, and, therefore, we are forced to  assume that a pre-
established harmony  exists  in the  ear, by means of
its internal organisation, which enables it to discover this
similarity.
   The question, however, with which we are now dealing
is not yet exhausted ; on the contrary, the puzzle,  which
Pythagoras proposed 2,500 years ago,  was first satisfac-
torily  solved by  the researches  of  Helmholtz.  The
pre-established harmony which exists  between the orga-
nisation of our ear  and the relative vibrations of tones, 
THE SENSE OF HEARING.
235
depends entirely upon the  existence  of  sympathetic
vibratory organs in the ear, the  vibrations of which are
regulated by the same  laws as those of all elastic bodies.
Thus, when we hear two tones at the interval of an octave,
two organs (the organs of Corti) vibrate in the labyrinth,
the number of whose vibrations  coincides with those of
the tones, and, therefore, stand in the ratio of I  '. 2.
    Two such tones  appear to us  so exceedingly alike
because the second is directly contained in the first and
can very easily be produced from it.  Let us consider a
vibrating string: we can easily divide it, whilst vibrating,
into two parts, each of which will vibrate independently,
and, on account of their  length being half that of the
entire string, will perform  twice as many vibrations. We

          grc'"^--------~  ->c-~-_------___--"-'--'"
                         ___«•____

                        Fig. 80.

shall speak, hereafter, more particularly of this division of
the vibrating  body, when  discussing the  production  of
harmonics, and here will only remark that the octave, so
produced, is called  the first  harmonic, or partial tone
of the given fundamental  tone.  Fig.  80  shows how  a
vibrating  string may be divided  into two (b) or three (c)
parts,  each of which  will vibrate independently.  In
short, starting with the fact that every tone physically
contains its octave, it is clear that when we hear  a tone
we also hear the  octave sound more or less loudly; and,
therefore,  in the labyrinth of the ear, the organs which
vibrate in sympathy with this octave will vibrate  gently 
236          THE FIVE  SENSES  OF MAN.
also.  This secondary sound is evidently the cause of the
similarity between two tones at the interval of an octave.
   The division, therefore, of the whole series of appre-
ciable musical sounds, into  octaves, is one evidently
demanded by nature, with both a physical and  physio-
logical foundation.   The  reason  of this will be clearer
when we  have considered the existence of consonance
and dissonance. For the present  I will only say that the
octave is the most perfect consonance, which arises from
the number of its vibrations being in the ratio 1 '. 2.  We
may, therefore, also say that the octave is the interval of
the most perfect consonance.
   Now the series  of sounds within  an octave have,  in
music, been divided into seven intervals, which first gave
rise  to  the  term  octave,  it  being* always composed  of  \
these eight sounds.   The  English  signs  for  them are :
C D E  F G A B C.  In this scale the interval between
E F and B  C  is about half  as great as  that between
the other notes.  The intervals between the latter are,
therefore, called whole tones, and those between E F and
B C semitones.
   Physical science has discovered the ratio  of the dif-
rerent number of vibrations for each  note in the scale
with great exactness, by  means  of the  siren and other
instruments.  The numbers of vibrations stand in the ratio
             C : D : E : F  : G :  A  :  B  :  a
          of I * S. ' 5 •  ±  • A *  5_  • \_$ • o
       or of  8 : 9 :  10 : io§: 12 : 13^ :  15  : 16
that  is to say, while the tone  C makes one vibration,  D
will make £,  E f, F f, G f, Af,B  ^,C 2;  or,  while C
makes eight vibrations, D will make 9, E 10, F io-|, G 12,
A 13},  B 15,  C  16.  This shows  us that the  interval 
              THE SENSE OF HEARING.           237

between F and E, which =  iof : 10, is smaller than that
between E and D, which =  10:9; also, that the interval
between C and B is smaller than that between the other
whole tones (e.g. B : A = 15 : 13^).  Further, the interval
                        Fig. 81.
between the first and the third tone, as that between C
and E, is called a third, that between the first and  the
fourth tone, as from C to F, a fourth, that  between five
tones, from C to G, a fifth, and so on to the octave from
C to C.  We can easily determine by the aid of a siren,
which is capable of producing a chord, the ratio  of  the
vibrations given above.   Such a siren, which is  repre-
sented in its simplest form in  figs. 58 and  81, has four
rows of holes, which,  reckoning from the interior, con-
tain  12, 16, 18, 24 holes each.  These  numbers  are in
the same  ratio  as the  numbers of  the vibrations of
C I E : G : C, namely  1  : -f  : a : 2.   If, now, we blow
upon the rows of holes in succession, the velocity remain-
ing the same, we shall obtain the well-known succession
of the tones named.   The rows of holes  may, of course,
equally well only consist of 8, 10, 12,  16 holes, and  the
same effect be obtained.  The siren, represented  in  fig.
59, could also be made to produce a chord  by giving it 
238          THE FIVE  SENSES  OF MAN.

more rows of holes, which could be opened  and shut at
will by a valve.   Another apparatus which determines
the same ratios is Savart's wheel.   It consists of one, or
more, toothed wheels,  which are made to  revolve with
great rapidity.   If, now,  a card is held against the teeth,
and the wheel be turned  with sufficient velocity, a tone
will be produced, the pitch of which depends upon the
number  of  raps given  by the teeth.   Let  us now
suppose several toothed wheels to revolve upon the same
axis, and that the number of their  teeth is  in the  ratio
of 8 I 10  I 12 I  16, we shall then likewise hear the well-
known succession of tones C E G C.  By means of the
siren and  of Savart's  wheel the exact  number of the
vibrations of tones generally used in music can be found,
either  by direct  calculation, or by  reading off the num-
ber of revolutions in  a  second by means of clockwork.
   Seven octaves are all that are required in music,  the
lowest of which  is represented by c/ (contra octave), the
others by C, C, c', c", C7",  c"".   It has been  decided
in Germany to fix the  number of vibrations for the tone
af at 440 vibrations in  a second.  The relative numbers
of all the vibrations are shown in the following table, given
by Helmholtz.
Notes	Contra Octave C,-B,	Great Octave C—B	Small Octave c-b	Once marked Octave c'-b'	Twice marked Octave c"-b"	Thrice marked Octave c"'-b'"	Fourth marked Octave c»«_b»«
C D E F G A B	33 37-125 4f25 44 49*5 55 61-875	66 74-25 82-5 88 99 no 12375	132 148-5 165 176 198 220 247-5	264 297 330 352 396 44O 495	528 594 660 704 792 880 990	I056 1188 1320 I408 1584 1760 1980	2112 2376 264O 28l6 3168 3520 3960
 
              THE SENSE OF HEARING.           239

   We notice in this table that the numbers  within an
octave always maintain the  same ratio, and that the
corresponding numbers in the following octave are always
the double of those in the preceding one.
   Our pianos generally commence  with the C, of 33
vibrations, or with a still lower, A//=27'S vibrations, and
extend to a"" = 3520 vibrations.
    It is of the greatest interest to discover the smallest
number of vibrations which is capable of producing the
sensation of a tone.  An experiment of this kind  has
already been made by Savart, by means of the following
apparatus.   A large wheel is turned by a handle  and
causes a smaller wheel  r (fig. 82) to  revolve by friction
on the periphery.  A bar, a b, is fixed on the axis of the
smaller wheel, and passes through the slit,  c d, in  the
board, f g.   The consequent pulses produce  a  tone
                  1 Muller-Pouillet, 'Physik.' 
240          THE FIVE SENSES OF MAN.
of considerable strength, and Savart affirms that he has
even  perceived  a tone  of  constant strength with 7-8
pulses.  But the musical character of these deep tones
up to about 30 vibrations in a  second is  so extremely
imperfect that they are of no use in music.  Helmholtz
is of the opinion that the deep tones, which Savart heard
in his experiments, must  have been  harmonics of the
fundamental tone of 7-8 vibrations.   From  his  experi-
ment with closed organ pipes, which have only very weak
harmonics,  and with large tuning-forks,  the musical
character only  begins  with about  28-30  vibrations.
Deeper tones only create a buzzing and  groaning sound
in the ear.
   The combination of the separate vibrations into one
tone, is a process which goes on in the nervous apparatus
of the ear.  We may compare this process to the con-
stant contraction of a muscle, which it is possible to pro-
duce by a  number  of excitements following each other
with great rapidity. If, for instance, 8 to 10 electric shocks
are directed upon a muscle in a second, the  muscle  will
not  relax to its full extent after each shock, and if the
rapidity of the shocks is slightly  increased,  the muscle
will remain contracted, as long as the excitement lasts.
Now, in the organ of hearing, the auditory nerve is not
continuously irritated by the apparatus for performing
sympathetic vibrations, but  the  excitement  takes place
at short intervals.   Our sense of hearing,  however, the
seat  of which must be sought in the brain, combines these
rapid irritations of the nerve in the same manner as the
muscle, and by this combination the sensation of tone is
produced.
   An interesting question, also, is to  determine what is 
THE SENSE OF  HEARING.
241
the highest perceptible tone.  To settle  this question
Savart has arranged an experiment also by means of a
wheel.  The toothed wheel  B (fig.  83) is  made to  re-
volve with great velocity by means of a strap, D, over a
large wheel A, so that the teeth strike a  card, which is
pushed forward upon the plate  E.  The  toothed wheel
which Savart used had 720  teeth,  and  could produce
24,000 raps in a second.  The tone  thus produced was
very weak and acute, but still distinctly  audible.  Des-
                       Fig. 83.'

pretz, moreover, has produced with small tuning-forks a
tone of 38,016 vibrations ; but tones as acute as this are
said to be unpleasant and even painful to the ear.   We
see from these experiments that  no fixed  limit can be
assigned to the perceptibility of acute  sounds according
to their pitch.  This, however, is  certain, that above the
seventh octave, above the fifth marked c, the tones  lose
their pleasant musical character, and that the determina-
tion of their pitch becomes more  uncertain.   Wfc. may,
                 1 Muller-Pouillet, 'Physik.' 
242          THE FIVE  SENSES  OF MAN.
therefore, conclude with some certainty that the organ
of  the  ear designed  for  musical  perception is only
adapted  for  seven  octaves, and it is this adaptation
which  we  meet  with in  the  organs  of  Corti  in the
cochlea.
    From what has already been said it appears that the
sensation of tone depends upon the repetition of a motion
at regular intervals with a  certain velocity.  A single
shock can never produce a tone, so that if only one tooth
were fixed on Savart's wheel it would be quite impossible
to produce a tone, by the wheel  striking the card during
its  revolution ; it has, however,  been observed that two
teeth placed  close together are sufficient to  produce a
tone, and, indeed, the same tone which would be produced
by the full number, only with this difference—that it is
interrupted by intervals.  The reason is clear,  for in this
case also the interval has remained the same though only
created by two shocks.   It  is,  also, an interesting fact
that even  two irritations of the auditory nerve, if they
follow each  other  in  rapid succession, are  sufficient to
produce a  sensation of tone, and that  the  pitch of the
tones entirely depends upon the  rapidity with which they
follow each other.
    In order to understand  how it is  possible for the
auditory nerve to perceive so great a number of different
tones, we must once more glance at  the organs of Corti,
and the distribution  of  the nerve-fibres there.   The
nerve, which pierces the axis of the  cochlea, spreads out
into the finest  fibres, each fibre  coming in contact with
a sympathetic vibratory  apparatus,  by which it  can
be irritated.   When a certain tone is sounded,  the whole
of the auditory nerve is by no means irritated, but only 
THE SENSE OF HEARING.
243
a certain number of fibres which proceed  to  the organ
which is set in vibration.
    Our  perception, therefore, of tones of different pitch
is produced entirely by an irritation of different fibres
of the auditory nerve.  Through the auditory nerve the
brain receives  from  different  fibres an  intimation of
tones of different  pitch, which intimation  enables it to
distinguish  the sympathetic  vibratory  organs which
have answered to the tone.   The  act of recognition is,
indeed, quite unconscious, as  are  many of the faculties
which  we have  acquired by practice ;  just  as in  the
movements of our limbs we select the nerves and mus-
cles whose action will perform the purpose  which we
have in view, without being conscious of the existence of
these organs.
    The  communication of  different sensations of tone
through the medium of different fibres of the auditory
nerve,  is a process perfectly analogous  to  others in  the
region Qf the sensory organs.   It is due  to a property
called  the  specific  energy of  the  sensory  nerves,  a
property which-we have  already closely  examined in
connection with  the optic nerve.   Not only  does  this
nerve answer to  every description of irritation, whether
caused mechanically, by electricity, or by light, with the
sensation of sight; but,  as we are forced to  assume, it
consists of at least three  kinds of fibres,  each  one of
which is designed  for the  reception of one of the three
primary colours,  red, green, or  violet.  In the auditory
nerve this  principle of division of  labour  is still  further
developed.   In this  case  each nerve-fibre is  connected
with one tone of a perfectly  distinct pitch and  can never
render any other tone audible. 
244          THE FIVE SENSES OF  MAN. _

   We  may, therefore, assume that if the separate fibres
of the cochlea were to be irritated in any other manner
than by sound, as, for instance, by mechanical or electric
irritation, their several appropriate tones would be heard.
In fact,  tones and  noises are  heard  when  an electric
current  is-conducted through  the head,'which are pro-
duced by the irritation of the entire  auditory nerve.  It
has been observed, further, that persons suffering from an
affection of the  ear often  experience a constant subjec-
tive sensation of  a  certain tone in  the  ear, which has
been explained as arising from one of the nerve-fibres in
the organs of Corti being irritated from some cause due
to disease.   Indeed, in some diseases,  a deafness to a
certain  series of  tones has been  noticed which, in such
cases, is due to the destruction of certain of the  organs of
Corti. 
CHAPTER VII.
Notes or Compound Tones—The production of Harmonics—The percep-
  tion of the latter by means of Resonators—Graphical representation of
  Quality or Colour.

It is a fact well known from experience, that the tones of
different instruments  and of the human voice are distin-
guished from each other by their note or quality.   If we
sing a tone of  a certain pitch, for  instance a', strike the
same note upon the piano, or  if it is  sounded upon a
violin, a flute, trumpet, or organ, we shall always obtain
a tone of the same  pitch, making 440 vibrations in a
second, and yet these tones differ very widely from each
other in their quality.
    Into the cause of this we have now to enquire.   We
must  return  to the process of vibration to  find the
principle of this fact.   Now, since the tones in the above-
mentioned instruments, when they possess  the same
pitch, invariably possess the same number of vibrations,
it is impossible for the number of vibrations to decide
the  quality of the  tone, and  we are forced to seek the
cause of  the difference in quality in some other property
of the several vibrations.
    Now  it is a fact which has long been known to musi-
cians, that a certain number of higher  tones are produced 
246          THE FIVE SENSES OF MAN.
with every tone which is sounded upon our instruments.
In  acoustics these higher tones have been  called the
Harmonics  of  the given fundamental  tone.   If, for
instance, the note c  is struck upon the piano,  an ear
musically trained will recognise in this note the c above
it.  This may be still better accomplished by means of
apparatus, of which we shall presently speak more parti-
cularly.
    The production of these  harmonics, which has  been
investigated  by Helmholtz in a very ingenious manner,
may be most easily understood  from a vibrating string.
Let us suppose  a stretched string to be thrown into
vibration, we shall observe that.the essential part of the
action consists in the string bending first in one direction
and then in the other.  The action, however, is in reality
not quite so simple as this, but other movements take part
in it also.  A vibrating string has always a strong  incli-
nation to divide into  two parts, each of which will per-
form its own vibrations, as shown in fig. 80 b.  Thus we
can imagine that, whilst the whole string is vibrating,
each half is at  the same time vibrating independently  ;
and, therefore, in addition to the  fundamental tone,  a
softer  tone, the first harmonic,  is produced.  This must,
of course, be the  octave of the fundamental tone, since it
is produced by  the vibrations of the half of the string, the
number of which must have been exactly double that of
the fundamental note.  Therefore, in the same time that
the whole string completes one vibration, the two halves
will have completed two.
    This  action is even still further complicated.  Not
only  is the  string divided  into two halves, but it is
at the same time  further divided into three equal  parts 
THE SENSE OF  HEARING.          247
though in a less degree, as shown  in fig. 80 c.  Each of
these three parts, again, vibrates independently, with three
times the velocity of the whole string, and by this vibra-
tion the second  harmonic  is produced.  If we now im-
agine these three  different vibrations to be combined,
we  shall have,  for every  point of the string,  a very
complicated  motion, which, however, we  can compose
from the separate motions.
   But the action has not  even reached its  limits with
the division into three parts, being still  further divided
into four, five, six, and more parts, and  as each of these
several parts \, \, \, etc., of the entire length, performs its
own vibrations, an entire series of harmonics is produced
in accordance with a  fixed law.  They become weaker
and less perceptible as their pitch  increases, so that the
fundamental tone predominates; but  still  they give to
the fundamental tone a peculiar  character : its  quality
or colour.
   We may easily convince ourselves  of the existence of
the harmonics in a string, and also determine their pitch,
by means of a few  experiments.   For  this purpose we
may very well make use of the string of a monochord,
but the  string of a  piano  or of a stringed instrument
will serve equally well. In the first place we must measure
the exact half of the string, which we will suppose to
give the tone,  c.  This we shall do most easily by pressing
the finger upon the string,  and moving  it about  till the
half of the string, when struck, gives the octave, of.  Now
hold the left forefinger a little distance above the centre,
draw the string aside to some distance forcibly with the
right hand, and  immediately touch the*string lightly with
the  left forefinger.   By  this  means  the  fundamental
            12 
248          THE FIVE SENSES OF MAN.
tone, and also a series of harmonics, will be damped, and
the first harmonic alone will sound  distinctly, because
the vibrations-of the two halves have  not been disturbed.
This tone, however, must have existed before  the  string
was touched.
    In the same manner we may make the higher har-
monics sound distinctly, by laying the finger which acts
as the damper upon ^, £, etc., of the string.  The higher
they are, the weaker they become.
    It is the fundamental tone which determines the pitch
of all harmonics.  For whilst the fundamental tone makes
one vibration, the first harmonic makes two, the second
three, the third four, the fourth five, and so on.  Thus if
the fundamental  tone is c,  the series  of harmonics  will
be '. c' g7 c" e" g" b9" c"'',  for while c makes one  vibra-
tion, c' performs two,  g' three, c" four, e"  five, g'"  six,
W seven, c'" eight.
    That it is possible for a string to  vibrate  in such a
manner that the two halves, or each third and fourth, will
vibrate independently, may be demonstrated by a singular
experiment,  which  every violin  player  knows from
practice.  If the finger is laid lightly upon the centre of
the string, without  pressing  it, and  the string is then
struck, the octave will be  heard, with  a  very delicate
quality, called flageolet.   If a third  part  of the  string
is measured  off by the touching finger,  the fifth  of the
octave is heard.  In this case, not only does  the third
part of the string vibrate  when struck,  but each other
third vibrates independently, as may  be seen  in  fig.
80 c.  Now  between each  of these two thirds  there  is a
point which is at re#st; this may easily be  proved in  the
following manner.   If a small piece  of  paper is placed 
              THE SENSE OF HEARING.           249

across the string,  it will  easily be  thrown  off by the
vibrations caused by a  stroke of  the  bow.  But  if the
rider is placed on the intermediate points it will remain
undisturbed during  the vibrations.   This point,  which
remains at rest, is called a nodal point.
    If the finger is placed upon the fourth of the string,
the second octave is heard when the string is struck.
The vibrating  string is  divided  into four equal  parts
which are separated by  three  nodal points; and in this
manner flageolet-tones  of a still  higher  pitch are pro-
duced.
    We now know that the series of these flageolet tones
exactly correspond with that of the harmonics, since they
are produced by dividing the string into parts which corre-
spond with the harmonic  numbers. Expressed in notes,
the first seven harmonics  can  be written in the following
manner, taking c as the fundamental tone :

      c    c'         g'    c"   e"   g"   \P"    o!"


      12         345678

    The lower figures show the relative numbers of the
vibrations.  The intervals between each two consecutive
harmonics are  as follows:  1—Octave,  2—Fifth,  3—
Fourth, 4—Major  Third,  5—Minor Third,  6—Aug-
mented Second, etc.
    We see how the intervals  between these notes grad-
ually decrease in size.   The higher the pitch of the har-
monics, the more  indistinct they  become, and the more
difficult it is to distinguish them from  each other. 
250          THE FIVE SENSES OF  MAN.
   It is difficult for an unpractised ear to perceive the
harmonics included in the note.  The  ear is so accus-
tomed to  receive  the note  as  a  whole, and  the  funda-
mental tone predominates so strongly, that it is difficult
for us to withdraw our attention from  it.  A very im-
portant and convenient expedient has, therefore, been
invented by Helmholtz, which prevents any uncertainty
or deception in the observation of harmonics.  He em-
ployed for this purpose so-called resonators, which  are
hollow globes, with two openings, as represented in  fig.
84.   The pointed end of the resonator is applied to  the
Fig. 84.
ear, and when a note is sounded which accords with the
fundamental tone of the globe, it is strengthened.  The
fundamental tone of each resonator is distinctly defined,
like that produced by the wind-chest of every mouth-
pipe,  or flute, and it  may  be  found by blowing into
the globe from  the large  opening.   It  is higher if the
globe is  small,  deeper if  it is  large.  When, therefore,
this tone is sounded in its neighbourhood, the re&mator
is  thrown into sympathetic vibration, and the  tone  is
strengthened.  All other tones, on the contrary, are sub-
dued, a fact which may easily  be proved by closing the 
THE SENSE OF  HEARING.          251
other ear and  shutting off every other entrance to the
waves of sound, except that through the resonator.
   To  recognise harmonics in this  manner,  a whole
series of globes of different sizes must be employed.  A
fundamental tone is then sounded upon an instrument,
and  those  resonators, which respond to the harmonics
of the tone sounded, placed near the ear.   They will  be
rendered audible according as they are  more or less
strengthened by the resonators.
   By  this means  the tones  of  different  instruments,
and  also of the human voice, have been investigated  by
Helmholtz.   The  result  of these investigations may  be
summed up in a few words as follows:—
   (1) There is scarcely any musical instrument whose
fundamental tone is not accompanied by harmonics.
   (2) The number and intensity  of the  harmonics is
different in different instruments, which is the  cause of
the characteristic quality of each instrument.
   These harmonics are present to a greater or less de-
gree in  the sounds  produced by  all instruments used in
music.  We have,  at  present, only considered  the pro-
duction of harmonics in vibrating strings, but every other
elastic, vibrating body, shows a greater or less inclination
to produce harmonics by  the division of its vibrations.
This inclination is least displayed by a confined volume
of air; when, for instance, we blow into a bottle or  a  re-
sonator.  The vibrations of a tuning-fork  approach the
simplest form of vibration, since their fundamental tone
is only^  accompanied by very weak  harmonics.   Accord-
ing to Helmholtz it is  possible to  produce artificially a
tone quite  free from harmonics, by fixing a tuning-fork
upon a  box, which rests upon india-rubber to prevent the 
252          THE FIVE SENSES OF MAN.
tone being transmitted to the ground, and then placing
before the tuning-fork the opening of a resonator, whose
fundamental tone coincides with that of the tuning-fork.
    Fig. 85 represents such an apparatus as constructed
by   Helmholtz.   The tuning-fork,  a,  can  be thrown
into a  constant  vibration  by means of  the electro-
                        Fig. 85.

magnet, b b.   The resonator, i, is  furnished with a flap,
m, and upon the removal of this flap  the tone of the
tuning-fork  is  distinctly heard.   We  shall presently
consider the use of this apparatus more  closely.
   If we  notice the tone of a tuning-fork, or that pro-
duced  by  blowing into a resonator, we  shall  observe
141154 
THE SENSE OF  HEARING.           253
that it is very pure, but has a hollow, empty sound.  The
tone seems powerless and unsatisfying, and might be said
to be devoid of all character.   This want evidently pro-
ceeds from the absence, or the weakness, of its harmonics.
Now it is to the presence of harmonics that  the quality
of a note is due.
    A distinction has, therefore, been made in  acoustics
between simple and  compound tones, or notes.  A simple
tone consists  of simple vibrations which do not contain
vibrations of harmonics, and whose form  coincides with
the vibrations of a pendulum.  A  compound tone is a
note which, in addition  to  the  fundamental tone, con-
tains a series of harmonics, and which is, therefore,  com-
posed of vibrations of a more or less complicated form.
    We may obtain a correct idea  of the different forms
assumed  by the vibrations of notes, by using the graphic
                        Fig. 86.

method of representing  vibrations.   If we imagine the
vibrations of a tuning-fork to be depicted upon a rotating
cylinder, as already described, we shall then have a re-
presentation of the vibration.  The horizontal line in fig.
86, called the abscissa, gives the time in which a vibration
is completed.  The curved line represents, in the form of
a wave, the vibration of the tuning-fork ; the  height of
the wave showing the amplitude or extent of the  vibra- 
254          THE FIVE SENSES OF MAN.
 tion, and the length, the duration of a vibration.  For it
 is clear that  the wave will  increase in height  as  the
 vibrations of the fork  increase in amplitude ; and if  the
 vibrations are slower the waves will become longer and
 more extended.  The  tuning-fork completes an entire
 vibration o, ax a2 a3 I,  between the points o and I ; one
 period  of the vibration has been  accomplished at this
 point, and now a second portion of the motion, identical
 with the first, follows from I to 2, in which  the direction
 of the  motion is exactly  the same.   That part  of  the
 curve which lies above the abscissa represents the motion
 in one direction, that which lies below, the motion in  the
 other.   Had we been speaking of the sound-waves of  the
 air, we  should have termed the upper portion, or the ele-
 vation  of the  wave, a condensed wave,  and  the  lower
 portion, or  the  depression of  the wave,  an expanded
 wave.   Both together  form an  entire wave, a complete
 period of vibration.
   A curve of this  kind, which is  quite symrr. etrical in
 form, that is to say, rises and falls with an eqcal velocity,
 and  in  its  form  represents, according to mathematical
 calculation, the law of the vibrations of a pendulum, gives
 us a representation of the vibrations  of  a  simple tone.
 In an almost similar manner the vibrations of a tuning-
 fork or  a confined volume of air are produced, which, on
 this account, make such a peculiar impression on our ear.
   Let us now consider what changes such a vibration
 undergoes when  accompanied by  an  harmonic.   Let O
 bx bz  represent the  first harmonic  of the  fundamental
tone.   It will make two vibrations, while the fundamental
tone makes  one, and, therefore, two periods are accom-
plished  in the  space of time  represented by o  I.   In 
               THE SENSE OF HEARING.           255

 order to combine the  two tones, we must assume that a
 point of the vibrating body, as, for instance, the prong of
 a tuning-fork, is forced to imitate  simultaneously the
 motions of both vibrations.   Let us, therefore, suppose
 that this point has completed the motion as far as av then
 it will have been removed a  certain distance from the
 point of rest.  This distance  is  not, however, that from
 o to  au for it must be remembered that, during the pro-
 duction of the curve, the cylinder, upon which it is being
 depicted, has completed a rotation by the time that the
 prong of  the tuning-fork reaches ax.    The  distance,
 therefore, of ax from the point  of rest  is represented  by
 the perpendicular line  ax which, in geometry, is called an
 ordinate.
    Now at the moment when the vibration of the funda-
 mental tones  reaches a, the vibration of the harmonic
 reaches its greatest amplitude.  The ordinate, bx, gives us
 the height of this amplitude,  and, in order to represent
 the combination of the  two vibrations, we have only to
 add together the two ordinates, ax  and bu which will give
 the position of the vibrating point of the combined tone.
 In this manner we obtain a new form of vibration.   The
 ordinate, cx, is here the sum of ax  and bx; c3 on the con-
 trary  is the difference between the corresponding heights
 a3 and b3, because a3 is above the  abscissa, and b3 below
 it. If, therefore, we add together all those ordinates which
 are situated  upon the same side of the  abscissa,  and
 subtract those on opposite sides, and place the sums and
 differences at their  corresponding distances  from  the
 abscissa, we shall obtain the dotted curve, which differs
considerably in form  from the original  one.  It  rises
more  abruptly at the commencement  and attains  its 
25.6          THE FIVE SENSES OF MAN.
maximum sooner, and then, towards the end, descends in
a longer and more gradual slope.  The depression of the
wave must naturally  be  symmetrical in form with  the
elevation.
   Thus a perfectly new form of vibration is produced,
which, again,  makes  a  particular impression  upon  our
ear.   However, the change has not affected the pitch of
this combined tone.  For it is readily seen that in the
time from  o to I, a whole period of the new vibration
must have  been completed.   The period of a vibration,
and, therefore, the number of vibrations of both tones, is
exactly the same, so that the  pitch  must also have re-
mained the same.   But the two notes are different.
   We may, therefore, lay down the  following law :  that
the quality of a note depends upon the form of the sound-
wave.
   We may imagine other forms of the sound-wave be-
sides that of c, c3, which have exactly the same periods of
vibration.   Let us suppose that, in addition to the  first
harmonic  of  the fundamental  tone,  the  second were
present also,  performing three vibrations in the space
between O and I.  The form of the new vibration would
then evidently be more irregular than before.  It might
be obtained by combining the ordinates of the vibrations
of the  third harmonic with the vibrations, c, c3.   We see
that the sums  and differences would necessarily always
be repeated after each period. . The new vibration will,
therefore, consist  of the same periods and have the same
duration.   The pitch remains unaltered but the note is
again different.
   The variety of forms, which a sound-wave  can thus
assume, is, as  we may readily imagine,  immense.   For 
THE SENSE OF  HEARING.          257
not only is it possible for a still greater number of har-
monics to be present, but one or other of the harmonics
may be more or less strongly, or only weakly, represented,
and lastly this or that harmonic may be altogether want-
ing, so that groups of harmonics of different kinds are
associated with the fundamental tone.  These variations
always give the  same pitch, but all give different  notes,
which are distinctly perceptible  to  our  ear.  Thus we
may imagine the fundamental tone to be associated with
the second and fourth, or the third and, fifth harmonics,
etc., and in each case the note will be different.
   The  difference  between the  notes  of our  musical
instruments may, therefore, be satisfactorily explained by
the fact, that the accompanying  harmonics give a pecu-
liar form to  the original vibration of the tone,  which
form  is  different with each instrument.  This form of
vibration is only recognised by the ear as a  whole, and
produces in it the sensation of a note.  Science has, how-
ever, proved  the analysis of this whole into the funda-
mental tone and the harmonics. 
258          THE FIVE SENSES OF MAN.
                  CHAPTER  VIII.
Analysis of Notes after the Law of Fourier—Helmholtz's Theory of the
    Perception of Notes — Formation of Notes by Electro-Magnetic
    Tuning-Forks.
THE observations in the last  chapter force the question
upon us, whether we are justified in imagining that all
periodical sonorous vibrations, which produce a sensation
of tone, are formed in such a manner that they may be
separated into a fundamental tone and a  series of har-
monics of different strength.
    Let us suppose  that we have to do with a very com-
plicated form of sound-wave, for example,  that  repre-
sented in the accompanying fig. 87, which  may be drawn
quite at will without knowing at all what  kind of sound
will be produced in our ear.   The figure only represents
one period of vibration from o to 1  ; the elevation of the
wave is quite symmetrical with  the depression.   It rises
rapidly, and, after reaching its maximum, descends some-
what quickly, then for  some  distance sinks very slowly,
falls rapidly  once more, and  finally, with  a  gradual in-
clination, passes into the depression, which is formed in
the reverse direction in exactly  the same manner as  the
elevation.
    If the wave under consideration is a sound-wave in
the air, we may conclude from this form, that the conden-
sation  of the air quickly attains  its maximum, then 
THE SENSE OF HEARING.           259
decreases  first rapidly, then  slowly, then  again rapidly,
and finally gradually ; and that the expansion of the air
increases first gradually, then quickly, then again gradu-
ally, and, finally, rapidly, to pass once more  with  great
rapidity into  the  condensation of the second period  of
vibration.   We have to enquire whether it be possible,
that such a vibration as this can be composed  of a funda-
mental tone, whose vibrations exactly resemble that  of
a pendulum, as shown in fig.  86, and of a series of har-
0—L—---------—x______________,----1
                        Fig. 87.

monies, each of which, again, has a form of vibration like
that of the pendulum, in which the elevations and depres-
sions, in accordance with a fixed  law, rise and fall with
the same rapidity.
    The problem which we have here to solve is evidently
a purely mathematical one.  In the first  place, we must
discover whether a simple curve could be drawn upon the
axis of  the supposed curve of vibration, which would
have the same duration  of vibration from o to 1, and
which would correspond  to the fundamental tone.  We
must then try whether the space remaining is sufficient
to produce simple curves of half or double, or 3, 4 and 5,
etc., times the  number  of vibrations, which will corre-
spond to  the  harmonics.  We  should then, of course, 
260          THE FIVE SENSES OF MAN.
discover  how many  such pendulum-curves of greater
number of vibrations would be necessary for the purpose ;
what number of vibrations they each would have ;  and
what relation they would bear in pitch, first to each other,
and then to the fundamental tone.  Finally,  the most
important point to be observed  is, that not a fraction of
the surface  of the supposed curve of  vibration should
remain over.
   We  here have before us  a difficult mathematical
problem,  which fortunately was thoroughly  solved by
science  long before anything was known of harmonics.
The distinguished French mathematician Fourier (born
1768) declared '.— That a vibration of any form whatever
could  be separated into a number of simple curves, if only
it were repeated in the same period.
   The proof of this proposition  can only be obtained
with the help of higher mathematics.  We  must, there-
fore, be satisfied with the  assurance, that it may be re-
garded as an incontestable fact,  and proceed to consider
more  closely its  application  to the theory of notes, for
which we have to thank Helmholtz.
   Fourier's proposition proves  a remarkable accordance
in the production of harmonics.   He shows, namely, that
when the number of vibrations of the original vibration is
one, the number of the first simple vibration is also one,
the second two, the third three,  the fourth four, and so
on.  In short, we see that the first simple vibration is the
fundamental tone, and that the others represent the series
of harmonics. Whilst, therefore, we have been able above
to construct a complicated sonorous vibration from  a
fundamental tone and its harmonics, we can now, by the
help of Fourier's  proposition, reverse the method, and 
THE SENSE OF HEARING.          261
separate the  periodical sonorous  vibration into a funda-
mental tone with a series of harmonics.
   It is most remarkable that nature  is able practically
to solve the problem which we have mentioned.
   If, when sounding a tone, we place before  the ear a
resonator, which corresponds to one of the harmonics of
the tone, we shall hear the harmonic much more distinctly.
An  analysis of a  peculiar kind has  here taken place
before the  sound-wave penetrates the ear, by which a
certain  simple  vibration  has  been  extracted from  the
original compound vibration, so that  it is conveyed to
the ear apart from the rest of the sound.  The resonator,
therefore,   is  an  analysing  apparatus which   works
entirely in accordance  with the principles laid down by
Fourier.                                         ♦
   We  must first consider the  action of the internal
organs of  the ear when a compound note is transmitted
to them.
   If the tone  a  of 440  vibrations is sounded upon  a
violin and upon a flute, the apparatus of the ear which
vibrates in sympathy with tones which perform 440 vibra-
tions in a second, will in both cases be set in motion. Now
the question is, whether this sympathetic apparatus first
imitates exactly the vibrations of the violin, and then the
vibrations of the flute, which must  differ from each other ;
and  whether  we become acquainted in this manner with
the difference in the quality of the notes.
   We may assert with confidence that this cannot  be
the true explanation.
   Let us assume the sympathetic apparatus to be in
connection with a certain nerve-fibre, which  is  excited
by it.  Now we know  from what  we have  learnt of the 
262           THE  FIVE SENSES OF  MAN.
action of the nerves, that a nerve will only produce one
effect, however it may be excited.   If a muscular nerve
is excited by chemical, electrical, or mechanical means,
the result  will  always  be  muscular contraction.  The
nerve-fibre of the  sympathetic apparatus is, therefore,
excited twice, without a possibility of the irritation pro-
duced being different.   It follows, therefore,  that the
sensation of tone could not be different in the two cases.
We are even forced  to  assume  that  if a  nerve-fibre  is
excited either chemically, or by electricity, the sensation
of tone will be the same. This is further proved by the
fact  that,  sometimes, in cases  of an affection of the
internal ear, so-called subjective tones are heard, although
no sonorous vibrations whatever have taken place.   We
must  assume that in such cases some of the  terminal
organs  of the nerves have  been irritated  by abnormal
excitation.
   The sympathetic vibration of a single  organ in the
ear provided with one,  or  a certain  number of nerve-
fibres, can  give us  no information  of the  difference
between the note of a violin and a flute.  A tone of 440
vibrations  will,  indeed, throw the  same  organ  into
sympathetic vibration in both cases;  but according to
the laws of acoustics, in addition  to this  organ, others
must  be set  in  vibration also, and it  is  these organs
which differ with the harmonics of the tone sounded.
   Let us suppose a number of persons, each of whom
has provided  his ear with a  resonator, of which the
first corresponds with the fundamental tone of the note
sounded on the violin, the second with the first harmonic,
the third with the second, and so on, as many resonators
as there are perceptible harmonics.  Then, if every other 
THE SENSE OF  HEARING.           263
access to the ear is excluded, no one wrll perceive  the
note of  a violin.   Each person only perceives  a simple
tone,  which sounds something like  that  produced  by
blowing into a resonator, for the tone of the  violin is
analysed by the  resonator into  its  elements,  each  of
which is perceived independently.  Now  let us further
imagine these  separate  sensations to be combined into
one, and we shall have  a representation  of the process
which takes place in the perception of a note.
   The  cochlea  of the ear resembles a  series of dif-
ferently toned resonators, an analysing apparatus, which
works in  strict  accordance with Fourier's  law,  and
practically  solves the problem in this simple  manner,
The  organ of 440  vibrations selects  the  fundamental
tone, that of 440 x 2 the first harmonic, that of 440 x 3
the second, and so on.   Further, each sympathetic appa-
ratus vibrates with  an  intensity corresponding with the
intensity of the harmonics of the note produced ; and
in this manner the-whole note is analysed into a number
of simple tones, in  a manner as perfect as any we can
conceive.
   After this process of analysis, however, there follows
a  process of combination,  which takes place in other
organs.   The fundamental tone and each of the har-
monics severally irritate a distinct nerve-fibre, and each
nerve-fibre transmits its irritation separately to the brain.
Here, however, where the mysterious process of sensation
takes place, the  several sensations are,  in  an equally
incomprehensible manner, combined into a general one,
which gives rise to the sensation of a compound tone.
   In the  representation just  given we  have  entirely
followed the theory of Helmholtz, which gives the clearest 
264         THE FIVE  SENSES OF MAN.
explanation of all the facts.   We must not forget, how-
ever, that the sympathetic vibration of separate apparatus
in the cochlea, which corresponds to the different tones,
has not, hitherto, been  observed  directly.  And yet this
is the only  supposition which  is  necessary  for  the
foundation of the above  theory ; but in examining the
formation of the organs of Corti the supposition appears
to be so extremely probable, that  the  theory  may be
regarded as well grounded.  We will not, however, omit
another experiment by which Helmholtz has considerably
strengthened  his theory.
    If a fundamental tone and an appropriate harmonic
produce together a certain compound tone, the combina-
tion may take place in different ways.  In observing the
curves of vibration of  both  tones,  we might assume
that both  the fundamental tone  and the harmonic rise
simultaneously at the  commencement of an elevation of
a wave.  After the completion of one period of vibration
the same action is repeated, that is to say,  the elevation
of the second wave is commenced simultaneously by both
tones,  because  the  harmonic has always  exactly corn-
                       Fig. 68.

pleted a certain number of vibrations when the" funda-
mental tone has completed one.  We will now alter the 
THE SENSE OF HEARING.           265
phases, that is, the elevations  and depressions  of the
wave of  the harmonic, with  respect  to those of the
fundamental tone, so that the elevation of the wave of
the harmonic shall always commence  a little after that
of the fundamental tone.  We shall now be able to see
whether the notes produced are different.
    In order to make the matter clearer we will employ
Helmholtz's figure  (fig. 88).   As already observed,  a
note is produced by the fundamental  tone and the first
harmonic (fig. 86).   We will now remove the harmonic
such  a distance to the right, that its commencement
shall exactly coincide with the ordinate, a1.   From  the
sum of the  ordinates we  shall then obtain a curve of
vibration, similar to that  in  fig.  87, in which the ele-
vations are much more abrupt, and the depressions, on
the  contrary, are broad and flat.  The  dotted  curve
represents the fundamental tone.
    We might produce  a great number of curves which
show transitions from  one to  the other, in  a precisely
similar manner to that in which we  have formed two
different  curves from the fundamental tone and the first
harmonic,  by merely  displacing  the harmonic a little
further still.  It is  quite clear that the waves so produced
must differ in form, and the question we have proposed
to ourselves is, whether the note is also different.
    In order  to determine this question  by experiment,
 Helmholtz employed simple tones, which were produced
by electro-magnetic tuning-forks, as shown in  fig.  85.
Let us imagine two such  apparatus, one of which gives
a fundamental tone, the other a harmonic.  If, now, we
choose the fundamental tone, B, and the  octave, b, as the 
266          THE  FIVE SENSES OF MAN.
harmonic, we shall obtain a note, which is distinctly heard
as a deep oo.
    By means of the following apparatus  the  phases of
the harmonic may be still further removed from those of
the fundamental  tone.  The tuning-fork, B,  is kept in
constant  motion by another vibrating tuning-fork, which
                        Fig. 89.

latter is represented  in  fig. 89.  We here see a tuning-
fork, a, the ends of which are placed between the poles
of a horse-shoe magnet, b b.   Upon one or both ends
of the tuning-fork there is a hook  of platinum, c, which
dips into a cup of mercury, d.
   An electric current is now passed from d to e, through
the fork,  and from thence through the coils of the horse-
shoe magnet. When the prong is raised by the attraction 
THE SENSE OF HEARING.          267
of the magnet, contact is broken at c, since the platinum
wire is no longer in contact with the mercury.  The fork
then returns to its first position, the current is renewed,
the fork is again attracted, and is thus kept in  constant
motion.
    The current from the  vibrating fork goes from it
through the electro-magnet of  the tuning-fork, B, and
throws the fork into unisonant vibration; the current also
passes through the magnet of the tuning-fork, b, which is
also thrown into unisonant vibration, because it receives
a magnetic impulse after every two vibrations.   The two
tuning-forks, B  and b, therefore  give a common note 00,
as soon as the flaps of their resonators are opened.
    By such an arrangement the elevations of the waves
of  the fundamental  tone,  B,  and of the  harmonic b,
commence simultaneously,  for  b  receives  a  magnetic
impulse in the same direction and at the same  moment
with B.   If, however, the opening of the resonator for b
is a little diminished  in size,  then, as  Helmholtz  has
shown by calculation, the commencement of the  phase
 is retarded, and the harmonic is removed from the funda-
 mental tone.  Nevertheless, in these experiments the note
 produced by the two tones remains the same, although
 the elevations and depressions of their waves are  not
formed simultaneously.
    From this important fact we may conclude  that  the
ear does not perceive the different forms of the sound-
waves as a whole, but separates them  into  their simple
 components.
    This fact can  only  be  explained  by  assuming that
the fundamental tone throws one organ into sympathetic
vibration, and the harmonic another, so that in  this way 
268
THE FIVE  SENSES  OF MAN.
the sonorous vibration is analysed  into  its components.
This analysis is always performed in the same manner,
however differently the phases of the two tones  may be
situated. The result must, therefore, consist in the excite-
ment of two nerve-fibres, which in all cases must remain
the same, and will, therefore,  always produce the  same
sensation in the ear.
    Thus the sensation of a note is caused by the irritation
of a certain fibre of the nerve of the cochlea, which is pro-
duced by the fundamental tone, and also by the more or less
feeble irritation of certain other nerve-fibres, the terminal
organs of which have been irritated by the harmonics. 
CHAPTER IX.
Harmony of Notes—Consonance and Dissonance—Production of  Beats
   —Dissonance to be attributed to the existence of Beats—Influence of
   Harmonics upon Harmony—Resultant Tones—Chords—Noises.

IN music we  not only enjoy the impression produced by
a succession  of melodious tones, but also experience
great pleasure from an agreeable combination of several
tones.  We  are  best  able to  judge  of the  sensation
produced if only  two notes are sounded together.   The
pleasant combinations of tones are termed' consonance,'
and the unpleasant ' dissonance.'
   An octave, c, and c', will be consonant.   Also a fifth,
c and^-, and the third, c and e, is also called consonant. If,
however, we strike two approximate notes  on the piano,
as c and  d, or  two  which are  still nearer, as c and  c
sharp, they produce a distinct  feeling of an unpleasant
sound, which we, therefore, call dissonant.  Although
there are other intervals  which are  dissonant besides
that  between two approximate notes, we will first  pro-
ceed to examine the approximate notes more closely.
   If two tones are dissonant which are separated by the
interval of a  tone or a semitone, we must  suppose  that
the sensation of dissonance would be increased if the 
270
THE FIVE  SENSES  OF MAN.
 difference between the pitch of the two notes were still
 further diminished.  This" is, in fact, the case.  By means
 of two tuning-forks,  which differ but  little from  each
 other, we can produce a combination of tones, which will
 be in the highest  degree dissonant.  Now, disregarding
 the aesthetic impression, we make  the  acoustic observa-
 tion that the note is produced  by separate shocks, which
 are less frequent the nearer the  tones are together.  These
 shocks are called beats.
    Two  approximate tones may  be observed  to  beat
 with one  another  in  all musical instruments, but most
 clearly in those in which the pitch of  the tone can be
 easily altered.   If we sound the same tone in two open
 organ pipes, the tone of one may be slightly lowered, by
 covering part of the upper opening with the hand.   The
 tones will immediately begin to beat, at first slowly, but
 increasing in velocity as the hand  is advanced.   If the
 beats  become still  quicker, a buzzing, whirring sound
 is produced.  This may be observed  upon  the piano
 when two deep tones at  the interval  of a semitone are
 struck together.
    These beats are no subjective phenomena, but really
 interfere with, or weaken the tone.  They are caused by
 interference of the sound-waves.
    By interference of the waves is meant the coincidence
 of the elevations of one  series  of waves with the  depres-
 sions of the other.  Whenever  this happens  the  motion
 of each is arrested, for every depression  is filled up  by
 an elevation, and the elevations levelled  by the  depres-
 sions.  These interferences may be  observed in a  tuning-
 fork, by holding it close to the  ear and turning it round
on its longitudinal axis.  The tone  is most intense when 
THE SENSE OF  HEARING.           271
it is held upright, with the two  prongs in a straight line
with the  ear.   As soon  as  the  tuning-fork  is turned
through 450 the tone becomes weaker, but, upon being
turned further,  again increases in strength.  Thus in one
revolution  the  tone is weakened in four symmetrical
positions.   A series of waves  proceeds from each prong
which to a certain extent interfere with each other.  The
prongs vibrate in such a manner,  that  they  both move
simultaneously  either outwards or inwards.  They, there-
fore, impart to the surrounding air a motion in opposite
directions, for when they both vibrate inwards they throw
the particles of air, with which they come  in contact,
against each other, and when they vibrate  outwards the
particles of air  move in the opposite direction.
   There  is, therefore,  one position  of the two  prongs
with regard to the ear,  in  which an  interference of the
two series of waves is distinctly  perceptible.
   If, instead of two perfectly similar series of waves, we
have  two  with unequal periods of vibration, but still
very nearly equal, interference will again take place.
Let us suppose the first two elevations of a wave to begin
at the same time, then  the  second  elevation of  the tone,
the vibrations of which  are slower, will commence a little
later, the third twice, and the fourth three times as late,
and so on.   At length  the  lower tone will be the length
of a whole elevation behind the other, and we shall now
have reached a  moment when the elevation of  one wave
will coincide with  the depression of the other, and  an
interference is the result.   But this condition is not con-
stant ; the waves of the  higher tone  hasten onwards, so
that  another moment  is  soon reached when  the eleva-
tions of both waves coincide, and the tone is strengthened.
            13 
272          THE FIVE SENSES OF MAN.
The result of this is, the tone is alternately strengthened
and weakened.
    The nearer the tones are together, the slower will be
the beats, because it will take longer for the higher tone
to get an entire elevation in advance of the lower.   The
greater the interval the more rapid will be the beats, and
when the tones are removed a certain distance from each
other, the beats will be imperceptible to our ear.
    Helmholtz has attributed the cause of dissonance to
the  existence of  beats.    The  rapid changes in the
strength   of the  tone  produced by beats make an
unpleasant impression upon the ear, just as the flickering
of a light is so extremely troublesome to the eye. When
the two tones are very near together,  we are conscious of
the cause of the unpleasant sensation, because we perceive
the beats separately.  But at the  interval of a semitone
or of a whole tone, when we can  no  longer perceive the
beats separately, their presence is only recognised by a
feeling of dissonance.   Helmholtz shows that  the note
possesses a certain amount of roughness which is produced
by the beats.
    The unpleasantness in this  sensation may  be attri-
buted  to  a perception common  to  all the regions of
nervous activity.  Every intermittent excitement  of  a
sensitive  nerve-fibre is more tiring than a continuous one.
The  flickering  of  a  light  is  so unpleasant  because,
between  every two   luminous  excitements, the retina
increases  in  sensitiveness,  and will  be more  strongly
affected by each excitement.  If, on the  contrary, the
light is continuous, the sensitiveness of the retina  becomes
gradually  blunted,  and will be  less excited   with its
duration.   In coming out of the dark into a bright light, 
THE SENSE OF  HEARING.
273
we are for a moment quite blinded, because  the  excite-
ment is here  very strong ; by the gradual  decrease of
the  excitement  the eye  becomes  accustomed  to the
light.  Flickering may, therefore, be  compared  to  a
rapidly repeated  transition from darkness  to light,  and
therefore powerfully excites the eye.
   The  case seems to be the same with  the  ear, or
rather with the auditory nerve.  Dissonance is an inter-
rupted  note,  which  excites  the ear to  an unpleasant
degree.  A refined musical ear will be extremely sensi-
tive  to this excitement, just as the flickering of a light is
much more unpleasant to  the eye of a  sensitive, educa-
ted man, than it is to the uneducated.
    From the comparison of colours and musical  sounds
it has become general  in common conversation to speak
of a harmony in colours, and, therefore, a discordant
arrangement of colours has been compared to dissonance.
From the stand-point  of natural science this comparison
is very imperfect. Two discordant colours, such as green
and  blue,  are unpleasant to us  because they weaken
rather than strengthen each other by both strongly irri-
tating, and therefore fatiguing, the same kind of nerve
fibres, whilst yellow and blue appear harmonious because
the  fibres sensitive to yellow remain at rest while we are
looking at the blue colour, and vice versd.  The common
physiological point of comparison between discordant
musical sounds  and  colours  therefore consists in the
fatiguing of  the  sensory nerve,  which,  however, is ac-
complished   in different ways, through an  intermittent
irritation in the case of the auditory nerve, and in the case
of the optic nerve by the irritation being concentrated on
a certain kind of nerve. 
274          THE FIVE  SENSES  OF MAN.
    The number of beats produced by two tones may be
 determined from the number of their vibrations.   If the
 higher tone makes only one vibration in a second  more
 than the  deeper  tone, then,  in  this time, it will only
 happen once that the elevation of one wave will coincide
 with the depression of another,  and  if the difference  is
 two vibrations, then it will happen twice, and  so on.  In
 short, the number of beats in a second equals the difference
 between the number of vibrations.
    The sensation  of  dissonance reaches  its  maximum
 with a certain number of beats.  Slow beats between two
 tones which are very near to each other, since we perceive
 them as separate pulses, are not so unpleasant to us as
 the  more  rapid beats between two tones at the interval
 of a second or a minor second.   If the note  b'  of 495
 vibrations and the  following  minor  second,  c"  of 528
 vibrations, are struck together, we shall perceive 528 —
 495 = 33 beats, and the dissonance will  be very marked
 and  will reach its maximum in this portion of the diatonic
 scale.   If, however, we strike the deep  tones,  B  and C,
 which  make 62  and 66 vibrations, we shall distinctly
 perceive four beats in a second,  and  the  sensation of
 dissonance is much less  marked.  Therefore,  the disso-
 nances are not so prominent in  the lower as in the  middle
 tones.   With the higher tones  again the  sensation of
 dissonance, with the same interval, decreases, so that the
 maximum of dissonance is  attained when about 30 beats
 are produced in a second.
   We have, as yet, confined ourselves to attributing the
dissonance between two approximate   tones to beats.
The  interval of a second, however, c—d, and  still more
that  of a semitone, c—c sharp, are decidedly dissonant; 
THE SENSE OF HEARING.           275
but the sound is no longer unpleasant to our ear when the
minor third, c—e flat, is struck, which thus represents the
transition to consonant intervals.
   We must  observe, further, that a  dissonance  may
exist between two tones  which are  situated at  a great
distance apart. The seventh, c—b, is a decided dissonance,
almost  as  decided as the minor second, b—cf.   The
cause of this marked dissonance between c and b cannot
lie in the number of beats, for they amount to 115  in a
second.  We must, therefore, seek another cause for the
phenomenon.
   Thus, when  we  reflect that a series  of harmonics
always belong to the fundamental tone  of a note, we
may be led to suppose, that the harmonics would  also
be able to produce beats, which would necessarily have
an influence upon the note produced  by the combination
of the  two tones.  In fact, in the case of the seventh,
c—b, the influence of the first harmonic of c,  the upper
octave cf is perceptible, for between b and  cf the  rough-
ness of the beats reaches its maximum.   Therefore, when
two tones are sounded together, we must take  into  con-
sideration the relation which the harmonics of  the lower
tone bear to the upper  tone,  and further, the relation
between the harmonics of the lower tone and those of the
upper tone.
   If the harmonics  of both  tones  coincide we shall
have a perfect consonance.  This is  exactly the  case
with the octave, as the following example will show.

          Fundt. tone.  |______Harmonics.____________
                    j  c' g' c" e" g" W" c'"
                      d \ c"   gf    c" 
276           THE  FIVE SENSES OF  MAN.
    On  the left of the perpendicular line are the two
fundamental tones c and  d',  and  their first harmonics
follow.  We see that every harmonic of d coincides with
a harmonic of c, so that there is  here  nothing to cause
beats..  The octave is the most perfect consonance.
    The case is similar  with the  twelfth,  c g*, which is
also called a perfect consonance,  as the  following  table
shows:
              c | c' g' c" e" g" bk" c"'  d"'________
                   g7 I    g"        <*"''
    The first harmonic of g' coincides with the fifth Of c,
and the second with the eighth.  The higher  the pitch
of the harmonics,  the less important is their discordance,
because they diminish in strength with  the  height of
their pitch.   When, however, the first and second har-
monics do not coincide, everything depends upon whether
or not they together produce a  dissonance.   For it is
this dissonance which they impart to the whole note.
    If, therefore, we proceed to analyse the fifth, c—g,  we
shall  perceive that the  consonance  is  not  nearly so
perfect as that of the octave.  For  the table shows us the
following  series of harmonics :
                    c 1 c' j c" e" g"
                    g|   g7 d"  g'
    We here observe that the first and  third  harmonics
of g  coincide with the corresponding  harmonics of c;
but the  second d" falls between c" and e", and  must
therefore  produce a dissonance.  As,  however, the dis-
sonance falls between  the second and third harmonics,
which are not at all powerful, the whole note  retains  its
consonant character, though with a trace  of roughness,
which gives to the fifth its peculiar  character. 
THE SENSE OF  HEARING.           277
   The harmonics of the fourth, c—fi show the following
relation to each other:

                    c  | c' g' c" e" g"
                    f |  f  c" f" a"

   Here we see that the first harmonic f of f does not
coincide with any  harmonic of c, but must produce a
dissonance with gJ'.  The harmonics d', however, coincide:
the others which follow do not.  The fourth,  therefore,
is not so consonant as the fifth, but still seems  pleasant
to the ear.
   The major third, also, c—e, seems to the ear to be a
consonance, although the degree of consonance  is less
than  that  of the  fourth, as the following table will
show :
                   c |  c' g* c" e" . .  .
                   e I   e'   b' e" . .  .

   The third harmonic of e is the first which  coincides
with a harmonic—the fourth of c.  The second b' produces
a very marked dissonance with c".   The first  harmonic
cf  does not  form any  very decided  dissonance with d
and^7.
   The major sixth, c—a, is similar in form to the major
third, as we  may easily prove for ourselves.  It possesses,
also, about  the same degree of consonance as the major
third.
   The minor third already shows the  transition  to the
dissonances, because between the fundamental tones, for
example, between c and  e  flat,  the  number  of beats
already begins to be perceptible to the ear.  We may,
however, designate it as an imperfect consonance, because 
278          THE FIVE SENSES  OF MAN.
the degree of roughness it produces is still not unpleasant
to the ear.
   The minor sixth, like the minor third, is also an im-
perfect consonance.
   It is, moreover, impossible  to  draw a  well-defined,
that is to say, an absolute boundary between dissonances
and consonances.   It is rather a matter of musical taste,
which is subject to variation, as to  where we  place the
boundary.  Thus the ancient Greeks and  Romans con-
sidered  the third a dissonance and avoided  it in singing,
probably because,  as Helmholtz supposes, their ear  was
more sensitive to beats than ours,  and because  in the
deep voices of the  men the  third would more  nearly
reach its  maximum of roughness,  than in the middle
tones of our instruments.  It was  not till  the develop-
ment  of concerted instrumental music in  the  middle
ages that  the third was admitted  to be an  imperfect
consonance.  The  seventh, for example, c—b, as already
explained  above,  is a marked  dissonance because the
first harmonic of c  only differs  from b by  a  semitone.
The first harmonic always has great  influence  upon the
whole note.
   Helmholtz has constructed a  figure  in which the
degree of  roughness is given  for each interval,  thirty-
three beats per second being taken as the maximum of
roughness.   The  horizontal line  c'—d',  in fig. 90,  re-
presents the diatonic scale between the once and twice
marked  c; and let us suppose each  note to  be sounded
with the fundamental tone d'.   The roughnesses  which
arise between  these tones and their harmonics are  re-
presented by elevations ;  so that the elevations give the
degree of  dissonance, whilst the consonances  lie  in the 
THE SENSE OF HEARING.
279
depressions.  The figures represent  the  harmonics be-
tween which the roughnesses take place, the fundamental
tone being marked  as  1, and  the harmonics with the
o
following numbers.  The octave  c" lies in the  deepest
depression, which shows it to be the most perfect  con-
sonance,  entirely devoid  of  roughness.    If  the  fifth 
28o           THE  FIVE SENSES OF  MAN.
g/ is sounded with c, the roughness again is very slight.
The depression is not so deep for the fourth f, and  still
less  for the third d', and the minor third ef flat.  The
maxima of the dissonances  lie in the proximity  of d
and c".
    It has been observed  further,  that  when  two tones
are sounded together, tones of an entirely new nature are
produced, which have been called resultant tones.  The
lowest resultant tone has long been known ; the number
of its vibrations equals the difference of the  two tones,
on which account it is  called the difference tone.  The
difference tone for the  fifth is the lower octave ; so that
when, for example, d' —g' is sounded, the lower c is added
as a difference tone; for the vibrations of c '. g are in the
ratio of 2 I 3, and the  difference 3 — 2=1 is the number
of vibrations for c.
    Besides the difference tone, Helmholtz has  pointed
out a much weaker summational tone,  the vibrations of
which  are  produced by  the  sum of those  of the  two
tones.   With  the assistance of the resonators, the  two
tones may readily be distinguished,a proof that they are
not, as was formerly supposed, first  formed in the  ear,
but  are present in the  air.   The  production of these
tones has been referred by Helmholtz,  with the aid of
mathematical  analysis, to the fact that the vibrations of
sonorous bodies and the air  only partially follow the
laws of the pendulum, their deviation from them increas-
ing with the amplitude of the vibrations.  Intense  and
constant notes, like those of an organ, therefore produce
intense resultant tones.
   Now the resultant tones, also, can give rise to beats,
and chiefly because the first harmonic has the power of 
THE SENSE OF HEARING.
28l
assisting in the creation of resultant tones.   This,  how-
ever,  is  only  the  case  when,  as often happens, the
deviation of the intervals  from  their true number of
vibrations is at a minimum.  If, for example, we take a
fifth, the numbers of vibrations of which are in the ratio
200 :  300, the first  harmonics will make 400 and 600
vibrations.   The fifth itself will give a difference tone of
300—200= 100 vibrations ; the  harmonic of 400 vibra-
tions will, with the tone of 300 vibrations also give  a
difference tone of 100 vibrations. The two difference
tones coincide, and therefore give no occasion for beats.
If, however,  the fifth is a little out of tune  so that
the numbers, of vibrations are, for example, in the ratio
200 : 301, the difference tone would have 101 vibrations.
One of the harmonics of 400 vibrations would now form,
with the tone of 301, a difference tone of 99 vibrations,
and  the two difference tones of 99 and 101  vibrations
will immediately produce beats.
   Altogether, however, the beats caused  by resultant
tones are less important for consonance and dissonance
than the harmonics, because they possess a small amount
of intensity.
   In concerted vocal and instrumental  music we  have
not only to consider the  effect  produced  by two, but
also that produced by several tones, which is  commonly
called a chord.   In this  case, also, everything  depends
upon the tones not producing beats together; for if every
two tones are consonant, the chord  containing  all the
tones will be consonant  also.  Thus,  for example, the
well-known triad, C E G, is a pleasing chord,  as are also
a number  of  other chords, with which we have become
familiar through musical experience. 
282
THE FIVE SENSES OF MAN.
   A distinction  has long been made in music between
a'major and a  minor  chord.   The triad,  C  E  G,
forms  the  primary  chord  of the former;  the  triad,
C E flat  G, that of  the latter.   They  sound  very
different to  our  ear, although  their  intervals  are  the
same, being  changed only in position.  For in C E G
the minor  third,  E  G, follows the major third,  C  E,
and  in  C  E  flat  G  the  major  third, E  flat  G,
follows the minor third, C E flat.  This difference in the
note is very difficult to describe, but is felt distinctly by
every ear, and may perhaps be  best represented in the
following manner : the major  chord conveys an impres-
sion of clearness, decision, finish, and thus gives rise to
a feeling of  satisfaction ;  whilst the minor chord has an
undecided, mysterious character, which makes it pecu-
liarly adapted to  express sorrowful emotions.   The
acoustic  cause of  this difference, as Helmholtz has shown,
lies in the relation held by the resultant  tones to  each
other.    In the  major  chord the resultant  tones are
consonant, whilst  in the  minor  chord the  resultant
tones produce a dissonance, and thus impart to the chord
a strange contradictory sound, which seems to give  it its
peculiar, undecided character.
   The sensation which we call noise  is quite different
in character from  the musical sensation of sound.  Noise
is produced by irregular, unperiodical movements of those
bodies which convey sound.  In  rubbing two bodies
together we obtain a characteristic  noise, which arises
from shocks  experienced  by  the two surfaces, which,
however, do not follow each other at regular intervals of
time, and throw the air  into irregular vibrations.   The
shocks will  be  stronger  or  weaker  according  as the 
              THE SENSE OF HEARING.           283

surfaces are rougher or smoother.   The  sympathetically
vibrating apparatus of the ear, the tympanic membrane,
the ear-bones, and the fluid of the labyrinth  repeat these
irregular vibrations and  convey  them  to  the auditory
nerve.  But, in the transition of this movement to the au-
ditory  nerve, it is very improbable that the cochlea with
its  many  delicate sympathetically vibrating apparatus,
which  seem  to  be  adapted  to tones of definite  pitch,
should have the task  of taking up irregular  vibrations
of sound.   For  an elastic  body  adapted  to  a certain
tone can only be thrown into  sympathetic  vibration by
tones which approach its fundamental tone.   Therefore,
even if the labyrinthine fluid  in the cochlea  were  to be
thrown into irregular movement, the sensitive  organs  of
the cochlea would be, in proportion, but slightly disturbed
from their  state  of rest.  In  the vestibule,  on  the con-
trary, there are formations to which we might  very well
ascribe an irregular sympathetic motion,  namely, the
otoliths, or ear-stones, which  are  here situated close  to
the  wall  of the membranous  labyrinth.   T/hese little
crystals, which are  very  easily  set into  motion, cannot
possibly act as elastic bodies, cannot possess any vibra-
tion proper,  and will,  therefore,  readily  perform   a
sympathetic vibration with a  motion of the fluid, how-
ever irregular.   The nerves in the labyrinth, which are
irritated by the  otoliths, probably constitute the  means
by which we are made conscious of noises.
    Although we may be justified in assuming that tones
and noises are received by different nerves, the problem
is still unsolved as  to how the nerves are endowed with
such different properties.   We must here recall to mind
similar phenomena in the region of the  sensory nerves ; 
2S4           THE  FIVE SENSES OF  MAN.
 the  specific  energy of the optic nerve of producing a
 sensation of light alone ;  that of the auditory nerve of
 only calling forth a  sensation of sound ; and the con-
 nection of this with cases in which, within the sphere of
 a sensory sensation, we discover  several qualities of the
 same sensation.
    In conclusion  we  will   remark, that  from  daily
 experience we learn to distinguish one noise from another.
 We  therefore call them grating, creaking, hissing noises.
 Their character depends partly upon the strength of the
 separate shocks, partly upon the rapidity with which they
 follow each other, but partly also upon  the  presence of
 real  tones of various pitch which  are intermingled with
 the noise.  Low tones are often connected with a grating
 noise, and with a hissing noise generally very high tones,
which impart to  them their peculiar  character.  It is
often by means of this intermixture of tones that  we
are able to distinguish the cause of the noise. 
PART   IV.
            SMELL  AND   TASTE.


                  CHAPTER  I.
                   The Sense of Smell.

Certain substances, in  a gaseous form,  when  inhaled
by the nostrils together with the air, create  a sensation
of smell.  The substance smelt here comes  into direct
contact with the sensory organ, as is the case with taste,
and excites it directly in a certain manner, the result of
which is  an entirely specific sensation of the  senses.   It
would be quite impossible to include the  description of
the sense of smell with that of the other senses, merely
as a species of touch  or  taste, for  it differs from  them
quite  as much as sight  and hearing  differ from each
other, and is a sensation of quite a peculiar kind.
   The action of the organ of smell is, therefore, due to
a special nerve, the  olfactory nerve, which differs from
the others both in origin, position, and extension.   It has
its origin in  the  anterior portion  of the cranium in  a
bulbous swelling, the olfactory ganglion, which is strongly
developed  in  the lower  animals.   Its  fibres   spread 
286
THE FIVE  SENSES  OF MAN.
themselves out in the base of the skull, and force their
way through the cribriform plate of the cribriform bone,
which lies between the sockets  of the eyes,  by a great
number of small apertures into the upper portion of the
nose.  This part of the nostril is itself divided into three
mussel-shaped passages, which  are covered by a mucous
membrane.
    The inferior, and, partly, the middle passage of the
nostril  serve  principally for  inhaling  and  exhaling
the air, and are,  therefore, called the respiratory  region
(j'egio respiratorid).   Like the other  air-passages in the
wind-pipes and lungs, it is covered with cylindrical cells
(epithelial cells)   packed  closely together, and  at their
free extremity provided with fine hairs,  which, by a sort
of waving motion, propel outwards all mucous  secretion
and dust.
    The  upper and,  partly, the lower  passages of the
nostril are occupied by the sensory organ for the sensation
of smell  and have  therefore been called the  olfactory
region  (regio olfactoria).  It is distinguished  from the
respiratory region by its  yellow colour, caused by pig-
ments, and, unlike the latter, is not  covered with hairy
epithelial cells, but presents a different organisation upon
its surface.
    The manner in which the olfactory nerve terminates
in the mucous membrane of the nostril  has  only  lately
been discovered by Max Schultze.   The analogy with
other sensory  organs  would  lead  us  to suppose that
these nerve-fibres are furnished with peculiar terminal
organs, capable of receiving  sensory impressions.
   The olfactory mucous membrane  is also covered with
cylindrical epithelial cells, as  represented in fig. 91 e. 
                 SMELL  AND TASTE.              287

They present their broad end to the surface (o),but become
attenuated when traced inwards towards the  underlying
network.   Between them we  find long rod-like filaments
(s) which in their lower part swell out into  0
a nut-shaped  expansion,  and then are pro-
longed into a fine fibre  towards the interior.  *
Now, as  this fibre has a great resemblance    /
to the finest  nerve-fibres, and  loses itself   /((
near the  terminations of the  finest fibres of  (@)
the  olfactory  nerve,  it  very probably  has   y
some connection  with the olfactory nerve.
These  formations  have  been  termed  by
Schultze olfactory  cells.   Very  small fine
hairs have been observed  upon the ends of   '
the  small  rods in some animals,  when  the
specimens have been perfectly fresh.
    The odorous %substances, which, together    Fig. 91.
with the air inhaled, are brought into contact  with the ol-
factory mucous membrane, and  thus evidently act upon
the terminal apparatus of the olfactory nerve, are not, how-
ever, directly  received  by the nerve-fibres, but by the
olfactory cells, the fibres themselves very probably  not
being  excited in the  slightest  degree by the greater
number  of odorous substances, certainly not  by those in
a gaseous form.  We remember that neither are the optic
nerve-fibres affected by the waves of  light, nor the audi-
tory nerve-fibres by the  waves  of  sound,  and shall
here find ourselves justified in coming to a  similar con-
clusion.
    We must, therefore, suppose that the olfactory nerve
would not be conscious of a  pleasant  odour,  if we were
to place its ramifications, at the  point where they pass
 
288          THE FIVE SENSES OF  MAN.
 through the cribriform bone, in an atmosphere of Eau de
 Cologne,  nor would  it  be sensible  of  an  unpleasant
 odour if we exposed it to an atmosphere of sulphuretted
 hydrogen.   The  action of odorous substances consists
 rather in their producing a  change of condition  in the
 terminal apparatus in the mucous membrane, the termi-
 nal  apparatus  then   exciting  the  nerve-fibres  whose
 terminations are situated in close proximity to  them.
 The terminal apparatus alone, and not the  nerve-fibres,
 can receive impressions from an  odorous substance, and,
 as  is the  case  throughout the body,  the  nerve-fibres
 serve only to convey the message to  the  brain that  an
 irritation has taken place.
    The substances which we perceive  through the organ
 of smell must be  conveyed  to the  olfactory mucous
 membrane, together with the inspired air, in a gaseous
 form ; and since all gases and vapours are more or less
 soluble  in  water, they  penetrate the moisture  of the
 mucous membrane  in  small particles.  Their influence
 upon  the terminal apparatus of the olfactory nerve with
 which  they  come  in  contact,  is  very  probably  of a
 chemical nature, for the smell of substances  differs  with
 their chemical composition.  There is, however, a mechani-
 cal condition without which we could have no perception
 of smell—namely, a continual current  of air through the
 nostrils, which is maintained by the process of breathing.
 If we hold our breath  all sensation of  smell ceases, even
 if we  are in an atmosphere  very strongly impregnated
with  odorous  substances.   Again,   the  sensation  is
strongest at the moment when we snuff up the air,  and
we,  therefore, repeat it quickly several times in succession
when  we wish to  perceive a delicate odour.   The cause 
                  SMELL AND TASTE.              289

of this is that our nerves are chiefly excited by sudden
changes in their condition, not by a permanent condition,
as, e.g., by interruptions in an electric current and not by
a continuous electric current.  This will account for the
constant renewal of air in the nostrils being so favourable
to the perception of odours, and for the diminution of
sensibility when the supply of air diminishes.   It there-
fore  follows  that a greater number of substances are
brought in contact with the olfactory mucous membrane
by the current of air.
   The  amount of a substance which we are enabled to
recognise by the organ of smell is extraordinarily small.
The  merest trace, in a gaseous form, of a drop of Oil of
Roses is all that is necessary to produce in  our  nostrils
the  impression  of  a pleasant odour.   The  smallest
particle of musk is sufficient to impart its characteristic
smell to our clothes for  years, the  strongest current of
air being insufficient to drive it away ; and Valentin has
calculated that we are able to perceive about the 3  one
hundred millionth of a grain of musk.   The delicacy of
our sense of smell thus far surpasses that  of the  other
senses.  The minute particles of a  substance which we
perceive by smell, would be  quite imperceptible to our
taste, and if they were in a solid form we should  never
be able to feel them, nor to see them, even if illuminated
by the  strongest sunlight.   No chemical reaction can
detect such minute particles of substances as those which
we perceive by our sense  of smell,  and even spectrum
analysis, which can recognise 15 millionths of a  grain,
is far surpassed  in delicacy by our organ of smell.
   The development of the sense of smell is even  more
astonishing in  animals  than it is in man,  and plays a 
290          THE FIVE SENSES OF MAN.
very important part in their organisation.  Hounds will
recognise by smell the  trace  of an animal perfectly
imperceptible to sight.  But the acuteness of their sense
of smell is far surpassed by that of the animal itself,
which is able, when the wind is in a favourable direction,
to scent the huntsman at  a distance of several miles.
The number, therefore, of those volatile substances which
are perceived by animals at  such  great  distances  must
be inconceivable.   Their minuteness defies estimation.
    It is not  as yet definitely proved whether animals
living in water possess a  sense  of smell.  If we  form
our conclusion from  the development of the organs of
smell, it must certainly be decided that they  do.   For
in fishes we find  a very strongly developed  olfactory
nerve, which has its origin  in the  anterior lobe of the
skull, the olfactory plate, and which spreads  out in the
mucous membrane of the so-called nasal sacs  which open
out upon the skin  of the head.   The terminal apparatus
of the olfactory nerve  in these animals are only affected
by substances in a liquid, not in a  gaseous, form.  Per-
haps the process is somewhat similar to that which takes
place in the organ of taste, which also can  only be
affected by liquids.  At any rate, the organ of smell in
aquatic  animals cannot exactly  resemble that of air-
breathing animals.   It is unlikely that a  man would be
able to smell under water, even if he were able to draw
a stream of water  through his nostrils without danger to
himself.  So, at least, an interesting  experiment by E.
H. Weber would lead us to  suppose.  The experiment
consisted in his filling  his  nostrils  with  water strongly
impregnated  with eau de Cologne.   This may be done
with perfect ease if the body is placed  in a horizontal 
SMELL AND TASTE.
29I
position and the head allowed  to  hang back in a per-
pendicular position, so that  the nostrils point  upwards.
The velum palati shuts off the  cavity of the nose from
that of the mouth, so that no water can escape.   As long
as the water remained in the nose, Weber  experienced
no trace of a sensation of smell, though he  had done so
while the water  was being poured in.  He remarked,
further, that every sensation of smell  was suspended for
several minutes after the water had been allowed to flow
out again, which was also the case after the application
of  pure  water.   It  seems,  therefore, as if water, with
man, were not a proper  medium for the organ of smell.
    In all the other sensory organs we have been able to
divide  the sensations into distinct kinds.  But in  the
organ of smell these sensations are so numerous, that it
is almost impossible  to classify  them.  As a general rule
we call those odours  which are pleasant to us, agreeable
smells, and those which are  unpleasant,  bad smells.
Agreeable smells are particularly characteristic  of some
kinds of  ethers  and  essential  oils, which are  found  in
many plants, and are, therefore, used in the  preparation
of  such  essences as eau de  Cologne.   Each  of these
pleasant smelling substances, moreover, possesses a per-
fectly distinct smell, which cannot be further  defined.
When  strongly  concentrated  they sometimes  become
pungent or overpowering.
    In  direct opposition to these  substances are those
which have a bad smell, to which some gases and vapours
of simple composition belong.   Some of the represen-
tatives of this group are sulphuretted hydrogen,  phos-
phuretted hydrogen, arseniuretted  hydrogen, bisulphide
of carbon, and a number of volatile  hydrocarbons.   A 
292
THE FIVE SENSES OF MAN.
great number of compound organic bodies also possess
a distinct bad smell. In this group the first place belongs
to all  those substances which  are  formed  during the
decomposition of animal matter.
   It is due to a remarkable harmony in our organisa-
tion  that nearly all substances with a bad smell  have an
injurious effect  upon the body.  The gases with a bad
smell, such  as  sulphuretted  hydrogen  and  others, are
indeed,  powerful poisons, which in large   quantities
have a fatal effect.   Meat, also,  which is in a  state of
decomposition,  is repugnant not only to  our smell,  but
to our taste, and, if eaten, may be the  cause of dangerous
illness.  The organ of smell is, therefore, a very  im-
portant protection to the entire organism, and prevents
the entrance of many injurious bodies.  It is not every
injurious substance,  however, that is betrayed  by the
sense of smell,  as, for instance, carbon protoxide, a
pernicious gas without smell, which, through carelessness,
has been the cause of many accidents.
   Many injurious substances cannot be  said to have a
bad  smell,  but still, when present in large quantities,
their odour is  unpleasant,  as,  for   instance,  chlorine,
bromine, iodine, and ammonia.  But, besides the influence
exerted upon the sense of smell, we  have to take into
consideration a general  excitement  of the entire nasal
mucous membrane, which is richly provided with delicate
nerve-fibres of the  trifacial nerve (N. trigeminus).   For
instance, the sharp stinging sensation caused by ammonia,
is not due to an excitement of the olfactory nerve,  but
to an  excitement   of  the  trigeminus,   although  the
olfactory nerve is strongly irritated at the same  time.
   It has been observed that those  gases are  odorous 
SMELL AND  TASTE.
293
which have a great tendency  to  combine, and to react
rapidly upon  organic tissues.  Sulphuretted  hydrogen
belongs to this class,  turning  blood black, and  decom-
posing  it;  and  chlorine, iodine,  and  bromine, which
rapidly destroy organic substances, as well as ammonia.
In this group we may place also the vapours of alcohol,
ether and chloroform, which all act rapidly upon  organic
tissues.
   Those gases, on the contrary, which are not odorous
have no chemical action upon organic tissues, or  only a
very slow one.   To this group belong nitrogen, which is
quite an indifferent  substance, hydrogen and carbonic
acid, which can cause  no decomposition.  But  carbon
protoxide, which is also devoid of odour, although very
poisonous,  does not destroy the essential components  of
the blood, since  it can  be given  out  again without the
blood having lost its essential  properties.   Neither does
oxygen react rapidly upon animal tissues, and, without
taking into consideration  slow oxidization, is, therefore,
indifferent.  Only in its  active  state, which  is  called
ozone, does it ever possess even at  ordinary  tempera-
tures, a stronger affinity,  and  it is a rather  significant
fact, that  when in this  state it  possesses  a peculiar
odour.
    Besides those  substances  which  have an agreeable
odour, and those  whose odour is  unpleasant  and bad,
there are  a vast number of odorous  substances, which
cannot  be classified  at all.  They may be chemically
classified, if we  wish to  become  acquainted  with their
composition.  But the sensations  of smell  which they
excite are  specifically different and very characteristic of
each substance.   We recognise a  great  number of the 
294
THE FIVE  SENSES OF MAN.
different kinds of meat with perfect ease by their odour,
which seems agreeable to us when we  are hungry,  but
disagreeable, and even revolting, when we have appeased
our appetite.
    Up to the present time science has not  been able to
offer any explanation of the existence of  the  different
kinds of sensations of smell, or what difference there is
between the irritations  produced  by good and bad
smells.   From analogy with  the kinds of sensation of
light, we should  be  led to suppose, that in the olfactory
nerve,  there  must  also be different  kinds of terminal
apparatus, which transmit the different kinds  of smell to
those nerve-fibres which are connected with them.  But
we should have to assume the  existence of an exceed-
ingly large, or even infinite, number of such nerves, when
we cannot even classify distinctly the different sensations
of smell.  We must leave all questions  upon this subject,
which, as yet, is  but little understood, for future research
to answer. 
CHAPTER  II.
                   The Sense of Taste.

ALTHOUGH  the sense of taste is most necessary to the
enjoyment and welfare  of  man, science as yet knows
but little of its nature with  certainty.   Even the extent
of its diffusion in the  cavity of the mouth  has  not yet
been  satisfactorily  determined.   It is, indeed,  certain,
that the tongue is the  principal  seat of the organ of
taste, and that the sensation of taste is most intense at
the back or root of the tongue.  The tip of the tongue
also possesses a sense  of taste,  which every  one must
know from experience.  The opinions of the different
experimenters are greatly at variance as to  the  proper-
ties  of  the remaining  portions of the surface of the
tongue.   According to the  greater number, the under
surface of the tongue  possesses no power of taste,  or a
very dull  one, though in most  cases  the  edges of the
tongue possess this power.   Observations are, however,
very  difficult to  make, and  uncertain  in  their result,
because substances  placed  upon a certain spot of the
tongue,  will  not readily remain  isolated,  but  spread
very rapidly, and since  the slightest  trace is sufficient
to be recognised,  we  are exceedingly liable to  decep- 
296           THE  FIVE SENSES OF  MAN.
    On this account the result of experiments  upon the
delicacy of the power of taste possessed by the palate is
still more doubtful, although it is commonly considered
to have an extremely delicate sensation of taste.   Many
observers assert that  the whole of the soft palate  and
the pillars of the fauces possess a sensation of taste, and
yet it has been observed by other careful  experimenters
that all sensation is wanting when a substance is applied
with a pencil upon an isolated point,  and every move-
ment  of  the root  of the tongue is carefully avoided.  It
is possible that this may be caused  by individual pecu-
liarities,  and  that  the  sensitiveness  is more strongly
developed in  some persons than in others.   It seems, in
short, only a  confirmation of the old  saying, ' There is
no accounting for taste.'
   The anatomical and microscopical investigations of
the organ of taste afford a much safer stand-point.  The
entire surface of the tongue is covered with little eleva-
tions  called gustative papilla, which  are  visible  to  the
naked eye.  Some of them  terminate in  a bundle of"
fibres, and others  are  broad and bushy on  their surface.
At the root of the tongue a semicircle is formed by larger
papillae,  each of which is surrounded by a  circular
mound.   Small depressions  have been  observed  sur-
rounding  these  circumvallate  papillae.   The  papillae
stand  in  the depressions formed by the mounds, and are
filled internally with oblong cells,  which  are connected
by prolongations with nerve-fibres.   Similar organs have
been  observed upon  the  other papillae of the mucous
membrane of the  tongue, and it is probable that in them
we must look for  the  true instruments  of taste.
    It is not so easy to decide whether there be a special 
SMELL AND  TASTE.
297
nerve of taste  as was  the  case  with the other senses.
There is certainly a  nerve, the glosso-pharyngeal nerve,
which must, without doubt, be  regarded  as the most
important  nerve of taste,  but its  gustative fibres  are
connected with innumerable motor  nerves  of the lower
part of the head, whilst the  optic, auditory, and olfactory
nerves  are  entirely  free from any foreign  admixture.
When this nerve has  been severed it has been observed,
that animals after this operation  will devour  food, even
when mixed with the bitterest substances, which an
animal in a normal condition would refuse to  touch.
    Besides  the  nerve  named above, another  sensory
nerve is found in the tongue,  the  lingual  nerve, which
provides it with the sense of touch and with sensitiveness.
It is still uncertain whether it  possesses gustative fibres,
besides  the  ordinary sensory fibres.   At any rate it  can
certainly be excited by sapid substances, when they  are
of a sharp caustic nature,  such as  strong  acids, alkali,
strong roots, etc.
    One of  the qualities of the sensation of taste is, that
sweet and bitter substances  seem to produce a  distinct
sensory impression, apart from any other irritation of the
sensory nerves.  No  concentration,  however strong, of
these sensations will ever amount to pain,  whilst a sour
taste will  produce a  contractive  and painfully  burning
feeling.   They stand  in direct  opposition to each other,
for the sweet taste appears  to  us the pleasant, and  the
bitter taste the unpleasant sensory impression.   It is the
sweetness of the milk which has such a charm for  the
infant,  and  which, when  it is hungry, conduces to its
reception of nourishment.   The bitter and  sour taste
which we allow to a certain extent in our food, would 
298           THE  FIVE SENSES OF MAN.
be distinctly refused, by the much more sensitive gus-
tatory organs of the infant, as an unpleasant sensation.
   The sweet taste is the opposite to the bitter and sour
taste, inasmuch as we are able to mitigate the unpleasant-
ness of the two latter by the former, when, for instance, we
mix sugar  with food which has a bitter or sour taste.
Since we  thus, as  it were, correct  the taste  without
allowing the sugar to react chemically  upon the  bitter
and sour substances, it seems to us  that some  kind  of
interference with  the sensations  must take place for
which we can find no  explanation.  It  is  also possible
for the sweet taste to be combined  with the sour and
bitter tastes, and produce a pleasant one.   But it is well
known, on  the  other hand, that a  saline taste is not
mitigated by the addition of sugar, and  that they  never
combine so as to produce a pleasant sensation.
   The contrast between these  sweet and bitter tastes is
shown by the following phenomena.  After having tasted
any bitter or saline substance, pure water, if taken imme-
diately afterwards, will appear to  be sweet ;  and some-
times a  sour taste will remain in the  mouth even after
we  have eaten  a large amount of sugar.  We cannot
help thinking of the great similarity between these phe-
nomena and that of  the contrast of colours, but we have
as yet no stand-point upon which to  follow out such a
comparison.
   We ought first to enquire what is the  cause of the
differences  in the sensations produced by different gus-
tative irritations, but unfortunately no positive  informa-
tion  can be given upon this point.   We might, indeed,
very  well  assume  the existence of several kinds  of
nerves with different terminal  organs, one producing a 
SMELL AND  TASTE.
299
sweet, another a bitter, and a  third a  sour taste.  But
science has not yet been  able to give sufficient infor-
mation  upon  this question  by any  experiments  or
observations.
    Still  less  are we able to say  why  one  substance
should  taste  sweet,  and  another bitter.   At least the
chemical composition of substances can give no  expla-
nation of the fact,  for many  substances  of an entirely
different  composition,  have  the same taste.  Besides
sugar, which  is  composed  of  carbon, hydrogen  and
oxygen, acetate of lead has a sweet taste  also.  Again,
many substances  have a  bitter  taste,  such as quinine,
sulphate of magnesia, and others, which differ entirely in
their  composition, and have nothing in  common in their
chemical characteristics.
    A better  agreement  between taste  and chemical
properties is shown by the acids  and alkalis.  There are
certain compounds in chemistry,  which  are distinguished
by a  sour taste, and are therefore termed  acids.  They
have  also the property  of altering certain  colouring
matters, e.g. turning blue  litmus  red.   In  opposition  to
these acids stand bases, and the soluble bases, or alkalis,
which are caustic like  the acids,  and turn reddened
litmus blue  again.   In the combination  of acids  and
bases to form  salts, both lose their characteristic peculi-
arities, and even  their peculiar taste,  and  then have a
different  one,  either  a  saline taste,  as  in chloride  of
sodium, or a bitter taste, as in  sulphate of  magnesia,  or
even a sweet one, as in acetate of lead.
    The acid, alkaline, and saline tastes belong, generally
speaking, to three different series of compounds, which
differ from each other by definite chemical characteristics, 
300          THE  FIVE SENSES OF  MAN.
 so that we have some reason  for  comparing their taste
 with their chemical properties.  But we  must  add that
 neither the  chemical nor gustative properties of these
 compounds are separated by sharply defined limits.
    It is  very remarkable that  the acid  and alkaline
 tastes may be artificially induced  by an electric current.
 Let an electric current be passed through  the  tongue,
 the positive pole being placed on  the tip  of the  tongue,
 and the negative pole on the nape,  so that the current
 passes from the tip of the  root  of the tongue ; a sour
 taste will  then be experienced on  the tip  of the  tongue.
 If, however, the negative pole is placed on the tongue,
 the taste will be different, and is generally  described as
 alkaline.  Now we  know that the  electric  current decom-
 poses salts, and that  the acids appear at the positive,
.and the alkalis at the negative pole, and  also that  salts
 are found in the saliva of the mouth, which  might lead
 us to suppose that the salts are decomposed at the poles
 into acid and alkali, both of which we recognise  by the
sense of taste.  But it is not so ; for even when the pole
is not brought into  contact with the tongue, if liquids or
the lips intervene, we still experience the electric taste.
Now it is  possible that the tissue of the tongue  itself is
decomposed, which  causes the taste, but so little is known
of these processes, that no satisfactory explanation has
at present been offered.
    Even weak currents are sufficient to produce the
taste, which may  most easily be induced  by  placing
polished pieces  of zinc and copper upon the  tongue,  so
that the edge of one will touch the tip, the other the under
surface  of the tongue.  If the  outer edges are  now
brought into contact with each other, we experience a 
                 SMELL  AND  TASTE.             30I
             >
distinct taste,  which  is acid when  the  zinc is placed
beneath  the tongue, and  alkaline when the copper is
placed on  the  tip.    The  moisture  of the  tongue here
forms a weak  galvanic element  with the two metals.
   The sensitiveness of our gustative organs for certain
substances is very considerable, but not to be compared
with that of smell.  We can recognise by taste a solution
of one part of sulphuric acid in 1,000 of water.  A drop
placed upon the tongue would contain about -g *0 6- of a
gramme ( = ^%-q of a grain) of sulphuric acid, an  infini-
tesimally  small  quantity, the detection of  which by
chemical analysis would be difficult.
   In the  ordinary  course of life we meet with other
kinds of tastes in addition to those which we have already
mentioned, such as corrupt, rancid, oily,  aromatic, and
similar tastes.  We must remember however, that in such
cases, our judgment is seldom founded upon the sensa-
tion  of taste alone, but is assisted both by'the sensation
of smell and the sensation of  touch.  The former is
caused by  the vapours from the substance which is being
eaten, rising through the fauces into- the cavities of the
nose ;  and the  latter is  produced  by  the form and
cohesion of the particular kind of food.  The'taste or
flavour of wine is principally decided by the smell of
several kinds of ethers.   The potato has no taste of its
own, and yet we imagine it to taste differently in different
forms.  It  is  clear  that in  all  such  cases we ex-
perience the combined sensations of smell,  taste, and
touch. 
 
INDEX.
Adjustment, 62
Aqueous humour, 52
Arborescent figure, 80
Auditory canal, 185
-----resonance of, 186
Auditory nerve, 224
-----irritation of, 243

Beats, 253
Binocular vision,  128
Blind spot, 93

Camera obscura, 55
Chords, 281
Choroid, 49
■— notes, 245
— Helmholtz's theory of,  263
Cochlea of the ear, 211
Colour blindness, 114
Colours, 100
— complementary, 108
■— harmony of, 122
— sensation of, 109
— Young and  Helmholtz's  theory
  of, 113
Colour top, 106
Combination of colours, 106
Common sensations, 4-
Consonance, 269
Cornea, 49
Corti, organs of, 215

Dissonances, 269
Double images, 131

Ear-bones, 188
Entoptic phenomena, 80
Eustachian tube,  189
Experience,  influence  upon  the
  senses, 5, 22, 37
External ear, 180
— world, 5
Eye, the, 48
— defects  of;  short   and  long
  sight ;  unsymmetrical   forma-
  tion, 72
— illumination of, 82, 85
— motion of,  123

Far point of the  eye, 72
Fenestra ovalis and  rotunda, 166,
  210, 219
Flame manometer, 201
Fourier's law, 260

Giddiness,  152

Harmonics, 235, 246
Harmony, 269
Hearing, organ of, 164
— sense of, 164
Hyaloid membrane, 53

Incidental images, 116
-----positive and negative, 118
— colours,  120
Interior of the eye,  perception of,
  78
Intuition,  160
Iris, the, 50,  58
Irradiation of the eye, 77

Labyrinth, the, 210
— semicircular canals of, 213
Larynx, 176
Lenses,  53 #
Light, action of, upon the retina, 97
— irritation of, 95
Long-sightedness. 72
Luminosity, 147 
304                           INDEX.
Membrane, vibration of, 198
Monochord, 232
Mouth-pi^.2, 170
Muscular sense, 40
-----of the muscles of the eye, 129
— Tensor  choroidese, 67

Near point of the eye, 70
Nerves, irritation of,  19
Noise, 282

Octave, 231
Ophthalmoscope, 85
Optical centre of the eye, 57
— illusions, 150
Optometer, 72
Otoliths, 213

Pain, sensation of, 14
Perception of solidity, 130
Phantascope, 117
Pigment of the eye, 50, 83
Primary colours, 109
Pseudoscope, 145
Pupil, 50,  79

Radiating zonule, 67
Reed-pipe, 174
Refraction  of light-rays in the eye,
  55
Resolution of light rays in the eye,
  75
Resonator, 250
Resultant tones, 280
Retina, 51
— identical points of, 130
— structure of, 89, 94
Retinal images, 79  *

Schemer's experiment, 70
Sclerotic coat of the eye, 49
Screen, 58
Sensation,  19
— after amputation,  22
— centre of. 4, 34
Sense of locality,  16
— of pressure, 13, 40
— influence of the memory,  42
 Senses, perception of the,  34, 162
 Sensory circles, 27
 — illusions of touch, 36
 — nerves, 1,  2
 — organs, 1
 Short sight, 72
 Sight, sense of, 48
 Siren, 171
 Skin, structure of,  17
 Smell, sense of, 285
 Sonorous vibrations, 167
 — transverse, 170
 -----of a string, 231
 Sound,  direction of, 182
 — transmission through air, 177
 -----through bones, 188
 Spectacles, 73
 Spectrum, 100
 Stapedius, the, 208
 Stereoscope, 139
 — Brewster's, 140
 — Wheatstone's, 131
 Struggle of the fields of vision, 148

 Tactile corpuscles, 18
 Taste, 295
 Telephon, the, 199
 Telestereoscope,  145
 Temperature,  sense of, 13,  43
 Tensor tympani muscle, 207
 Tone, 167
 — deepest, 239
— highest, 240
Tones, vibration-numbers of, 238
— sensation of, 220
Touch, organ  of,  16
— sense of,  13
Tuning-fork, 168
-----electro-magnetic, 251
Tympanic membrane, 205

Vitreous humour, 52

Wax of the ear,  185

Yellow spot, 89

Zollner's figure,  150 
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W. LAUDER LINDSAY, M. D., F. R. S. E.  Mind in the Lower Animals.
Sir JOHN LUBBOCK, Bart, F. R. S.  On Ants and Bees.
Prof. W. T. THISELTON DYER, B. A.,  B. Sc.  Form and Habit in Flowering
    Plants.
Mr. J. N. LOCKYER, F. R. S.  Spectrum Analysis.
Prof. MICHAEL  FOSTER, M. D.  Protoplasm and the Cell Theory.
H. CHARLTON  BASTIAN, M. D., F. R. S.  The Brain as an Organ of Mind.
Prof. A. C. RAMSAY, LL. D., F. R. S.  Earth Sculpture:  Hills, Valleys, Moun-
    tains, Plains,  Rivers, Lakes ; How they  were Produced, and how they havt
    been Destroyed.

Prof.  RUDOLPH VIRCHOW (Berlin University).   Morbid Physiological Action.
Prof. CLAUDE BERNARD.  History of the Theories of Life.
D. APPLETON & CO.,  Publishers,  549 & 551 Broadway, N. Y. 
THE  INTERNATIONAL  SCIENTIFIC  SERIES.
                   FORTHCOMING  VOLUMES.
riof. H. SAINTE-CLAIRE DEVILLE.  An Introduction to General Chemiitrj.
Prof. WURTZ.  Atoms and the Atomic Theory.
Prof. De QUATREFAGES.  The Human Race.
Prof- LACAZE-DUTHIERS.  Zoology since Cuvier.
Prof. BERTHELOT.  Chemical Synthesis.
Prof. C. A. YOUNG, Ph.' D. (of Dartmouth College).  The Sun.
Prof. OGDEN N. ROOD (Columbia College, N. Y.).  Modern Chromatics anJ it\
    Relations to Art and Industry.
Dr. EUGENE LOMMEL (University of Erlangen).  The Nature of Light.
Prof. J. ROSENTHAL.  General Physiology of Muscles and Nerves.
Prof. JAMES D. DANA, M. A.,  LL. D.  On Cephalization; or, Head~:haracteri
    in the Gradation and Progress of Life.
Prof. S. W. JOHNSON, M. A.   On the Nutrition of Plants.
 Prof. AUSTIN FLINT, Jr., M.D.   The Nervous System, and its Raatiou to tkt
    Bodily Functions.
 Prof. BERNSTEIN (University of Halle).  The Five Senses of Man.
 Prof. FERDINAND COHN  (Breslau University).  Thallophytes (Alga, Licluiu
     Fungi).
 Prof. HERMANN (University ol Zurich). Respiration.
 Prof. LEUCKART (University of Leipsic).  Outlines of Animal Organization.
 Prof. LIEBREICH (University of Berlin).  Outlines of Toxicology.
 Prof. KUNDT (University of Strasburg).  On Sound.
 Prof. REES (University of Erlangen).  On Parasitic Plants.
 Prof. STEINTH AL (University of Berlin).  Outlines of the Science of Language
 P. BERT (Professor of Physiology, Paris).  Forms of Life arid other Cosmical C-r
     ditions.
  E. ALGLAVE (Professor of Constitutional and Administrative Law at Douai, and o
     Political Economy at Lille).   The Primitive Elements of Political Constitute:*.
  P  LORAIN (Professor of Medicine, Paris).  Modern Epidemics.
  Prof.   SCHUTZENBERGER (Director of the  Chemical  Laboratory  nt  the Scs
     bonne).   On Fermentations.
  Mons. FREIDEL.  The Functions of Organic Chemistry.
  Mons. DEBRAY.   Precious Metals.
  Prof. CORFIELD, M. A, M. D. (Oxon.).  Air in its Relation to Health.
  Prof. A GIARD.  General Embryology.
     D. APPLETON & CO.,  Publishers,  549  & 551 Bmadvav.  N. V- 
Opinions of the Press on  the  "International Scientific Series.™
                                    I.

        TyndaH's  Forms  of  Water.

I vol.,  i2mo.   Cloth.   Illustrated........Price, $1.50.
   " In the volume now published, Professor Tyndall has presented a noble illustration
of the acuteness and subtlety of his intellectual  powers, the scope  and insight of his
scientific vision, his  singular command of the appropriate language of exposition, and
the peculiar vivacity and grace with which he unfolds the results of intricate scientific
research."—N. Y. Tribune.
   " The ' Forms of Water,' by Professor Tyndall, is an interesting and instructive
little volume, admirably printed and illustrated.   Prepared expressly for this series, it
is in some measure a guarantee of the excellence of the volumes that will follow, and an
indication that the publishers will spare no pains  to include in the series the freshest in-
vestigations of the best scientific minds."—Boston Journal.
   " This series is  admirably commenced  by this little volume from the pen of Prof.
Tyndall.  A perfect master of his subject, he presents in a style easy and attractive hia
methods of investigation, and the results obtained, and gives to the reader a clear con-
ception of all the wondrous transformations to which water is subjected."—Churchman,
                                    II.

     Bagehot's   Physics  and   Politics.

                       I vol., 121110.   Price,  $1.50.

   " If the ' International Scientific Series ' proceeds as it has begun, it will more than
ftilfil the promise given to the  reading public in its prospectus.  The first volume, by
1'rofessor Tyndall, was a model of lucid and attractive scientific exposition ; and now
we have a second, by Mr. Walter Bagehot, which is not only very lucid and charming^
but also original and suggestive in the highest degree*.  Nowhere since the publication
of Sir Henry Maine's  'Ancient Law,' have we seen so many fruitful thoughts sug-
gested in the course of a couple of hundred pages. .  .  .  To do justice to Mr. Bage-
hot's fertile book, would require a long article.  With the best of intentions, we are
conscious of having given but  a sorry account of it in these brief paragraphs.  But we
hope we have said enough to commend it to the attention of  the thoughtful reader."—
Prof. John Fiske, in the 'A tlantic Monthly.
   " Mr. Bagehot's style is clear and vigorous.   We refrain from giving a fuller ac-
count of these suggestive essays, cnly because we are sure that our readers will find it
worth their while to peruse the book for  themselves; and we sincerely hope that the
forthcoming parts of the  'International Scientific Series' will be as interesting."—
A thenteum.
   " Mr. Bagehot discusses an immense variety of topics connected with the progress
of societies and nations, and the development of their distinctive peculiarities; and hii
book shows an abundance of ingenious  and original thought"—Alfred Russeli
Wallace, in  Nature.
>

     D.  APPLETON &  CO.,  Publishers, 549 & 551 Broadway, N. Y. 
Opinions  of the  Press on the " Lnternational Scientific Series."
ill.
Foods.
                     By  Dr.  EDWARD  SMITH.


I vol.,  i2mo.  Cloth.   Illustrated........Price, $1.75.

   In making up The International Scientific Series, Dr. Edward Smith was se-

lected as the ablest man in England to treat the important subject of Foods.  His services
were secured for the undertaking, and the little treatise he has produced shows that the

choice of a writer on this subject was most fortunate, as the book is unquestionably the

clearest and best-digested compend of the  Science of Foods that has appeared in our

language.
   " The book contains a series of diagrams, displaying the effects of sleep and meals
on pulsation and respiration, and of various kinds of  food on respiration, which, as the
results of Dr. Smith's own experiments, possess a very high value.  We have not far
to go in this work for occasions of favorable criticism ; they occur throughout, but are
perhaps most apparent in those parts of the  subject with which Dr. Smith's name is es-
pecially linked."—London Examiner.
   " The union of scientific and popular treatment in the composition of this work will
afford an attraction to many readers who would have been indifferent to purely theoreti-
cal details. . .  . Still his work abounds in information, much of which is of great value,
and a part of which could not easily be  obtained from other sources.   Its interest is de.
cidedly enhanced for students who demand both clearness and exactness of statement,
by the profusion of well-executed woodcuts, diagrams, and tables, which accompany th^
volume. ...  The  suggestions of  the author on the use of tea and coffee, and of the va.
rious forms of alcohol, although perhaps not strictly of a novel character, are highly in-
structive, and form an interesting  portion of the volume."—N.  Y. Tribune.
IV.
                    Body  and  Mind.

  THE  THEORIES  OF  THEIR  RELATION.

                  By  ALEXANDER  BAIN,  LL.  D.

1 vol.,  izmo.    Cloth..........' •  •   Price> fr-So-

   Professor Bain is the author of two well-known standard works upon the Science
•f Mind—" The Senses and the Intellect," and " The Emotions and the Will."  He is
one of the highest living authorities in the school which holds that there can be no sound
or valid psychology unless the mind and the body are studied, as they exist, together.
   " It contains a forcible statement of the connection between mind and body, study-
ing their subtile interworkings  by the light of the most recent  physiological "jvest.ga-
tions   The summary in Chapter V.,  of the investigations of Dr. Lionel Beale of the
embodiment of the intellectual functions in the cerebral system  will be found the
fr«hestTnd most interesting part of his book.  Prof. Barn's own theory of the ccnnec
tioffcweend?  menS Ind fiebodfly part in man is stated by himse If to be as follows;
There is ' one  substance, with two sets of properties, two s.des, the physical and the
men al^a double faced unity.' While, in the strongest, manner asserting the union
rTf mindwith brain, he yet denies 'the association of union tn place,  but asserts the
urbn of close succession in time,' holding that 'the »me being is, V akernate fits, ur,
der extended and under unextended consciousness." '—Christian Register.

    D. APPLETON & CO., Publishers,  549 & 551 Broadway, N. Y. 
Opinions of the Press on  the  " International Scientific  Series."
                                     V.


          The   Study   of   Sociology.

                      By  HERBERT  SPENCER.

I vol., i2mo.  Cloth.............Price, $1.50.

   "The philosopher whose distinguished name gives weight and influence to this vol-
ume, has given in its pages some of the finest specimens of reasoning in all its forms
and departments.  There is a fascination in his array of facts, incidents, and opinions,
which draws on the reader to ascertain his conclusions.  The coolness and calmness of
his treatment  of acknowledged difficulties and grave objections to his theories win for
him a close attention  and sustained effort, on the part of the reader, to comprehend, fol-
low, grasp, and appropriate his principles.  This book, independently  of its bearing
upon sociology, is valuable as lucidly showing what those  essential characteristics are
which entitle any arrangement and connection of facts and deductions to be called a
science.''—Episcopalian.
   " This work compels admiration by  the evidence which it gives of immense re-
search, study, and observation, and is, withal, written in a popular and very pleasing
style.  It is a fascinating work, as well as one of deep practical thought."—Bost. Post.
   " Herbert Spencer is unquestionably the foremost living thinker in the psychological
and sociological fields, and this volume is an  important contribution to the science of
which it treats. ... It will prove more popular than any of its author's other creations,
for it is more plainly addressed to the people and has a more practical and less specu-
lative cast.  It will require thought, but it is well worth thinking about."—Albany
Evening Journal.
                                    VI.


             The    New   Chemistry.

                     By JOSIAH  P. COOKE, Jr.,

       Erving Professor of Chemistry and Mineralogy in Harvard University.

I vol.,  l2mo.   Cloth............Price,  $2.00.

   " The book of Prof. Cooke is a model of the modern popular science work.  It has
just the due proportion of fact, philosophy, and true romance, to make it a fascinating
companion, either for the voyage or the study."—Daily Graphic.
   " This admirable monograph, by the distinguished Erving Professor of Chemistry
in Harvard University, is the first American contribution to 'The International Scien-
tific Series,' and a more attractive piece of work in the way of popular exposition upon
a difficult subject has not appeared in a long time.   It not only well sustains the char-
acter of the volumes with which it is associated, but its reproduction in European coun-
tries will be an honor to American science."—New York Tribune.
   " All the chemists in the country will enjoy its perusal, and many will seize upon it
as a thing longed for.  For, to those advanced students who have kept well  abreast ol
the chemical tide, it offers a calm philosophy.  To those others, youngest of the class,
who have emerged from the schools since new methods have prevailed, it presents a
generalization, drawing to its use all the data, the relations of which the newly-fledged
fact-seeker may but dimly perceive without its aid. ...  To the old chemists, Prof.
Cooke's  treatise is  like a message from beyond the mountain.  They have heard 0/
changes in the science; the clash of the battle of old and.new theories has stirred them
from afar.  The tidings, too, had come that the old had given way; and little more than
this they knew. . . . Prof. Cooke's ' New Chemistry' must do wide service in bringing
to close  sight the little known and the longed for. ...  As a philosophy it is elemen-
tary, but, a^ a book of science, ordinary readers will find it sufficiently advanced."-'
Utica Morning Herald.

      D. APPLETON & CO., Publishers, 549 & 551 Broadway, N.  Y. 
Opinions of the Press on  the '■'•International Scientific Series."
                                 VII.


    The  Conservation  of   Energy.


           By BALFOUR STEWART,  LL. D., F. R. S.

With an Appendix treating of the Vital and Mental Applications of the Doctrine.

                 I vol., i2mo.  Cloth.   Price, $1.50.

   " The author has succeeded in presenting the facts in a clear and satisfactory manner,
using simple language and copious illustration  in the presentation of facts and prin-
ciples, confining himself, however, to the physical aspect of the subject.  In the Ap-
pendix the operation of the principles in the spheres of life and mind  is supplied by
the essays of Professors Le Conte and Bain."—Ohio Farmer.
   " Prof. Stewart is one of the best known teachers in Owens College in  Manchester.
   "The volume of The International Scientific Series now before  us is an ex-
cellent illustration of the true method of teaching, and will well compare with Prof.
Tyndall's charming little book in the same series on ' Forms of Water," with illustra-
tions enough to make clear, but not to conceal his thoughts, in a style simple and
brief."—Christian Register, Boston.
   " The writer has wonderful ability to compress much information into a few words.
It is a rich treat to read such a book as this, when there is so much beauty and force
combined with such simplicity.—Eastern Press.
                                 VIII.


             Animal   Locomotion;


  Or, WALKING,  SWIMMING, AND  FLYING.

                    With a Dissertation on Aeronautics.


      By T. BELL  PETTIGREW, M. D., F. R. S., F. R. S. E.,
                            F. R.C. P.E.

              1 vol., l2mo......Price, $1.75.

   " This work is more than a contribution to the stock ofentertaining knowledge,
though, if it only pleased, that would be sufficient excuse for its publication.  But Dr.
Pettigrew has given his time to these investigations with the ultimate purpose of solv-
ing the difficult problem of Aeronautics.  To this  he devotes the last fifty pages pHiii
book.  Dr. Pettigrew is confident that man will yet conquer the domain of the air."^
A'". Y. Journal of Commerce.
   "Most persons claim to know how to walk, but few could explain the mechanical
principles involved in this most ordinary transaction, and will be surprised that the
movements of bipeds and quadrupeds, the darting and rushing motion of fish, and the
erratic flight of the denizens of the air, are not only anologous, but can be reduced to
similar formula  The work is profusely illustrated, and, without reference to the theory
it is designed to expound, will be regarded as a valuable addition to  natural history '
—Omaha Republic.

    D.  APPLETON & CO., Publishers, 549 & 551 Broadway, N. Y. 
Opinions of the Press on the " International Scientific Series.."
                                   IX.


Responsibility  in   Mental   Disease.

                 By HENRY MAUDSLEY,  M.  D.,  .

   Fellow of the Royal College of Physicians;  Professor of Medical Jurisprudence
                       in University College, London.

               I  vol.,  i2mo.  Cloth.   .  .  Price,  $1.50.

   " Having lectured in a medical college on Mental Disease, this book has been a
feast to us.  It handles a great subject in a masterly manner, and, in our judgment, the
positions taken by the author are correct and well sustained."—Pastor and People.
   " The author is at home  in his subject, and presents his views in an almost singu-
larly clear and satisfactory manner. . .  . The volume is a valuable contribution to one
of the  most difficult, and at the same time one of the most important subjects of inves-
tigation at the present day."—N. Y. Observer.
   " It is a work profound and searching, and abounds in wisdom."—Pittsburg Com-
mercial.
   " Handles the important topic with masterly power, and its  suggestions are prac-
tical and of great value."—Providence Press.


                                    X


              The    Science   of  Law.

                     By SHELDON AMOS, M. A.,

Professor of Jurisprudence in University College, London; author of " A Systematic
      View of the Science of Jurisprudence," " An English Code, its Difficulties
                  and the Modes of overcoming them," etc., etc.

            I  vol., i2mo.   Cloth.....Price, $1.75.

   " The valuable series of ' International Scientific' works, prepared by eminent spe-
cialists, with the intention of popularizing information in  their several branches of
knowledge,  has received a good accession  in this compact and thoughtful volume.   It
is a difficult task to give the outlines of a complete theory of law in a portable volume,
which he who runs may read, and probably Professor Amos himself would be the last
to claim that he has perfectly succeeded in doing this.  But he has certainly done much
to clear the  science of law from the technical obscurities which darken it to minds which
have had no legal training, and to make clear to his ' lay' readers in how true and high a
sense it can assert its right to be considered a science, and not a mere practice."—Tht
Christian Register.
   " The works of Bentham and Austin are abstruse and philosophical, and Maine's
require hard study and a certain amount of special training.   The writers also pursue
different lines of investigation, and can only be regarded as comprehensive  in the de-
partments they  confined  themselves to.  It was  left to Amos to gather up the result
and present the  science in its fullness. The unquestionable merits of this, his last book,
are, that it contains a complete treatment of a subject which has hitherto been handled
by specialists, and it opens up that subject to every inquiring mind. . .  . To do justice
to ' The Science of Law' would require a longer review than  we have  space for.   We
have read no more interesting and instructive book for some time.   Its  themes concern
every one who  renders obedience to laws, and who would  have  those laws the best
possible. The tide of legal reform which set in fifty years ago has to sweep yethighei
if the flaws in our jurisprudence are to be removed.  The process of change cannot be
better guided than by a well-informed public  mind, and Prof. Amos has  done greal
service in materially helping to promote this end."—Buffalo Courier.

    D.  APPLETON &  CO., Publishers, 549 & 551 Broadway, N. Y- 
Opinions of the Press on the " International Scientific  Series."
                                  XI.


              Animal   Mechanism,

       A  Treatise on Terrestrial and Aerial Locomotion.

                          By E. J. MAREY,

 Professor at the College of  France, and Member of the Academy of Medicine.

   With 117 Illustrations, drawn and engraved under the direction of the author.

              1 vol., i2mo.  Cloth.....Price, $1.73

   " We hope that, in the short glance which we have taken of some of the most im-
portant points discussed in the work before us, we have succeeded in interesting our
readers sufficiently in  its  contents to  make them curious to learn more of its subject-
matter. We cordially recommend it to their attention.
   "The author of the present work, it is well known, stands at the  head of those
physiologists who have investigated the mechanism of animal dynamics—indeed, we
may almost say that he has made the subject his own.  By the originality of his con-
ceptions, the ingenuity of his constructions, the skill of his analysis, and the persever-
ance of his investigations, he  has surpassed all others in the power of unveiling the
complex and intricate movements of animated beings."—Popular Science Monthly.
                                  XII.


 History   of   the    Conflict   between


             Religion   and   Science.

          By JOHN  WILLIAM  DRAPER, M.D.,  LL.D.,

             Author of " The Intellectual Development of Europe."

      1 vol., i2mo.........Price, $1.75.

   "This little ' History' would have been a valuable contribution to literature at any
<ime, and is, in fact, an admirable  text-book upon a subject that is at present engross-
ing the attention of a large number  of the  most serious-minded people, and it is no
small compliment to the sagacity of its distinguished author that he has so well gauged
the requirements of the times, and so adequately met them by the preparation of this
volume.  It remains to be added that, while the writer has flinched from no responsi-
bility in his statements, and has written with entire  fidelity to the demands of truth
and justice, there is not a word in his book that can give offense to candid and fair-
minded readers."—N. Y. Evening Post.
   " The key-note to this volume  is found in the antagonism between the progressive
tendencies of the human mind and the pretensions of  ecclesiastical authority, as devel-
oped in the history of modern science.   No previous writer has treated  the subject
from this point of view, and the present monograph will be found to possess no less
originality of conception  than vigor of reasoning and wealth of erudition. . . . The
method of Dr. Draper, in his treatment of the various questions  that come up for dis-
cussion, is marked by singular impartiality as well as consummate ability.   Through-
out his work he maintains the position of an historian, not of an advocate.   His tone is
tranquil and serene, as becomes the search after truth, with no trace of the impassioned
ardor  of controversy.  He endeavors so far to identify himself  with the  contending
parties as to gain a clear comprehension  of their motives, but, at the same time, h«
submits their actions to the tests of acool and impartial examination."—N. Y. Tribune.

    D. APPLETON  & CO., Publishers, 549 & 551  Broadway, N. Y. 
Opinions of the Press on the  " International Scientific Series."


                                XIII.

                   THE  DOCTRINE OF


        Descent,    and    Darwinism.

                     By  OSCAR SCHMIDT,

            Professor in the University of Strasburg.

                          With 26 Woodcuts.

     I  vol., i2mo.  Cloth........   .   Price, $1.50.

   " The entire subject is discussed with a freshness, as well as an elaboration of de-
tail, that renders his work  interesting in a more than usual degree.   The facts upon
which the Darwinian theory is based are presented in an effective manner, conclusions
are  ably defended, and the question is treated in more compact and available style
than in  any other work on the same topic that has yet appeared.  It is a valuable ad-
dition to the ' International Scientific Series.' "—Boston Post.
   " The present volume is the thirteenth of the ' International  Scientific Series,' and
is one of the most interesting of all of them.  The subject-matter is handled with a
great deal of skill and earnestness, and the courage of the author in avowing his opin-
ions is  much to his credit.  . .  . This volume certainly merits a careful perusal."—
Hartford Evening Post.
   " The volume which Prof. Schmidt has devoted to this theme is a valuable contri-
bution to the Darwinian literature. Philosophical in method, and eminently candid,
it shows not only the ground which Darwin had in his researches made, and conclu-
sions reached before him to plant his theory upon, but shows,  also, what that theory
really is, a point upon which many good people who talk very earnestly about the
matter are very imperfectly informed."-w-Detroit Free Press.


                                 XIV.


     The   Chemistry   of  Light   and


                     Photography;

In  its Application  to Art,   Science,  and   Industry.

                   By Dr. HERMANN VOGEL,

             Professor in the Royal Industrial Academy of Beilin.

                       With 100  Illustrations.

     izmo..............Price, $2.00.

   "Out of Photography has sprung a new science—the Chemistry of Light—and, in
giving a popular view to the one, Dr. Vogel has presented an analysis of the principles
and processes of the other.   His treatise  is as entertaining as it is instructive, pleas-
antly combining a history of the progress and practice of photography—from the first
rough experiments  of Wedgwood and Davy  with sensitized paper, in 1802, down to
the latest improvements of the art—with technical illustrations of the scientific theories
on which the art is  based.  It is the first attempt in any manual of photography to set
forth adequately the just claims  of the invention, both from an artistic and a scientific
point of view, and it must be  conceded that the effort has been ably conducted."—
Chicago Tribune.

    D. APPLETON & CO., Publishers, 549 & 551 Broadway,  N. Y. 
Opinions of the Press on  the '•'•International Scientific Series."
                                   XV.

                              Fungi;

   THEIR  NATURE,  INFLUENCE, AND  USES.

                 By M.  C.  COOKE,  M. A., LL. D.

         Edited by Rev. M. J. BERKELEY,  M. A., F. L. S.

                 With 109 Illustrations.   Price, $1.50.

   "Even if the name of the author of this work were not deservedly eminent, that of
the editor, who has long stood at the head of the  British fungologists, would be a suf-
ficient voucher for the accuracy of one of the  best botanical monographs ever issued
from the press. . . .  The structure,  germination, and growth of all these widely-dif-
fused organisms, their habitats and  influences for good and  evil, are systematically
described."—New York World.
   " Dr. Cooke's  book contains an admirable rdsume' of what is known on the struct-
ure, growth, and reproduction of fungi, together with ample bibliographical references
to original sources of information."—London Athenceum.
   "The production of a work like  the one now  under review represents  a large
amount of laborious, difficult, and critical work, and one in which a serious slip or fatal
error would be one of the easiest matters possible, but, as far as we are able to judge,
the new hand-book seems in every way well suited to the requirements of all beginners
in the difficult and involved study of fungology."—The Gardener's Chronicle (I.on-
doti).                   _______________________________
                                  XVI.


The Life  and Growth of  Language:

    AN OUTLINE  OF  LINGUISTIC  SCIENCE.

              By WILLIAM DWIGHT WHITNEY,
         Professor of  Sanskrit and Comparative Philology in Yale College.

                  I  vol., l2mo.   Cloth.  Price, $1.50.

   "Prof. Whitney is to be commended for giving  to the public the results of his ripe
scholarship and unusually profound researches in  simple language.  He draws illus-
trations and examples of the principles which he wishes to impa*t, from common life
and the words in frequent use.
   " The topics discussed in this volume are,  for the most part, those which have
been already  treated by other writers on philology, and even  by the author himself, in
his volume on ' Language, and the Study of Language,'  published a few years  ago,
and, though many of the  truths here set forth are those with  which students in the
same line of investigation  are generally familiar, all will rejoice to see them restated in
such a fresh and simple way.                                           >(
   "This work, while valuable to scholars, will  be interesting to every one. —The
Churchman.                                 .      ....    ,      .     .-,
   " This work is an important contribution to a science which  has advanced steadily
under conditions that appear constantly to throw an increasing light on difficult ques-
tions, and at  each step clear the way for further discoveries."—Chicago Inter-Ocean.
   " Prof. Whitney is undoubtedly one of the foremost of English-speaking philologists,
and occupies an enviable position in the wider circle of European students of language.
   " His style, clear,  simple, picturesque, abounding  in striking illustrations, and apt
in comparisons, is admirably fitted to l.e the vehicle of a popular treatise like the work
under consideration."—Portland Daily Press.

  D. APPLETON & CO., Publishers,  549 &  551  Broadway, N.  Y. 
Opinions of the Press on  the uInternational Scientific Series."
                                  XVII.


 Money  and  the  Mechanism  of  Ex-


                             change.

           By W. STANLEY JEVONS,  M. A., F. R. S.,
    Professor of Logic and Political Economy in the Owens College, Manchester.

                  i vol.,  i2mo.   Cloth.   Price,  $1.75.
   "He offers us what a clear-sighted, cool-headed, scientific student has to say on the
nature, properties, and natural laws of money, without regard to local interests or na-
tional bias.  His work is popularly written, and every page is replete with solid instruc-
tion of a kind that is just now lamentably needed by multitudes of our people who are
victimized by the grossest fallacies."—Popular Science Monthly.
   " If Professor J evons's book is read as  extensively as it deserves to be, we shall
have sounder views on the use and abuse of money, and more correct ideas on what a
circulating medium really means."—Boston Saturday Evening Gazette.
   " Professor Jevons writes in a sprightly but colorless style, without trace  of either
prejudice or mannerism, and shows no commitment to  any theory. The time is not
vory far distant, we hop?, when  legislators  will cease attempting to  legislate upon
money before they know what money is, and, as a possible help toward such a change,
Professor Jevons deserves the credit of having made a useful contribution to  a depart-
ment of study long too much negle;ted, but of late years, we are gratified to say, be-
coming less so."—The Financier, New York.
                                  XVIII.


             The   Nature  of   Light,

  WITH  A GENERAL ACCOUNT  OF PHYSICAL  OPTICS.

                    By Dr. EUGENE LOMMEL

                          (University of Erlangen).

         I  vol., l2mo.   Cloth.     .     .     .     Price, $2.00.

   "In the present treatise, Professor Lommel has given an admirable outline of the
nature of light and the laws of optics.
   " Unlike most other writers on this subject, the author has, we think, wisely post-
poned all reference to theories of the nature of light, until the laws of  reflection, re-
fraction, and absorption, have been clearly set before the reader.  Then, in the fifteenth
chapter, Professor Lommel discusses Fresnel's famous interference  experiment, and
leads the reader to see that the undulatory theory is the only conclusion that can be
satisfactorily arrived at.  A clear exposition is  now given of Huyghen's theory, after
which follow several chapters on the diffraction and polarization of light-bearing waves.
   " The reader is thus led onward much in the same way as the science itself has un-
folded, an(J this, we think, is the surest and best way of teaching natural knowledge.
   " We have said enough to show that Professor Lommel's treatise is a useful contri-
bution to the ' International Series'—a book that can thoroughly be understood and
enjoyed by any intelligent reader who may not have had any special scientific train-
ing."—Nature.
   " In a style singularly lucid, considering the abstruse nature of the subject treated
Dr. Lommel unfolds the  learning of the scientists on  the  nature and phenomena of
lijht."—Philadelphia Inquirer.
   " As a popular introduction to physical optics, it would be difficult to find a more
satisfactory work than the one by Dr.  Lommel,  which has just appeared  in the  excel-
lent ' International Scientific Series.' "—The English Mechanic.

  D. APPLETON & CO., Publishers,  549 & 551  Broadway, N.  Y. 
JUST   PUBLISHED.
    Money  and  the  Mechanism  of Exchange.

Vol. XVII. of the International Scientific Series.  By W. Stanley Jevons, M. An
     F. R. S., Professor of Logic and Political Economy in the Owens College, Man-
     chester.  1 vol., 12mo.  Cloth.  Price, $1.75.
   " He offers us what a clear-sighted, cool-headed, scientific student has to say on the
nature, properties, and natural laws of money, without regard to local interests or na-
tional bias.  His work is popularly written, and every page is replete with solid instruc-
tion of a kind that is just now lamentably needed by multitudes of  our people who are
victimized by the grossest fallacies."—Popular Science Monthly.
   " If Professor Jevons's book Is read  as extensively as it deserves to be, we shall
have sounder views on the use and abuse  of money, and more correct ideas on what a
circulating medium really means."—Boston Saturday Evening Gazette.
   " Prof. Jevons writes in a sprightly  but colorless style, without trace of either
prejudice or mannerism, and shows no commitment to any theory. The  time is not
very far  distant, we hope, when legislators will  cease  attempting to legislate  upon
money  before they know what money is, and, as a possible help toward such a change,
Prof. Jevons deserves the credit of having made a useful contribution to a depart-
ment of study  long too much neglected, but of late years, we are  gratified to say, be-
coming less so."— The Financier, New York.
 Weights, Measures, and Money, of all  Nations.

Compiled by F.  W. Clabke, S. B., Professor of Physics and Chemistry in the Uni-
     versity of Cincinnati.   Price, $1.50.
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the arrangement is highly convenient for reference, and in a form and classification
never before presented to the public.  It also contains a series of tables, arranged alpha-
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standards. The metric system is used coextensively with the ordinary system, and
is a characteristic feature of the book.
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sive tables, viz.:  I. Measures of Length, in both the English or American  feet or
inches, and in French metres.  II. Road-Meas-ires in English miles and French kilo-
metres.   III. Land-Measures.  IV. Cubic Measures.   V.  Liquid  Measures.  VI.
Dry Measures.  VII. Weights, and finally Money.  This latter table is one of the most
useful and valuable tables probably to' be found, giving as  it does the standards in
dollars, francs, sterling, and marks, and alone is worth the cost of the book."—N. Y.
Commercial and Financial Chronicle.
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wish to acquire information on the  subject of which it treats."—Boston Globe.
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planned and skillfully executed."— Chicago Tribune.

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   " Miss Buckley, the friend of Sir Charles Lyell, and for many years the secretary of
the great geologist, in this  volume has given a continuous, methodical, and complete
sketch of the main discoveries of science from the time of Thales, one of the seven
wise men, B. C. 700, down to the present day.   The work is  unique in its way, being
the first attempt ever made to produce a brief and simple history of science.  The au-
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A Hand-Book of Architectural  Styles.  Translated from the  Ger-
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English language.  This knowledge will be facilitated by the profuse illustrations, of
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and modern."—Evening Mail.

           D.  APPLETON &> CO., 549 &•• 551  Broadway, N. Y. 
 
 
 
 
APR 5   1960 '/ii\ 
NATIONAL LIBRARY OF MEDICINE
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