OBSERVATIONS 
ON 
VISION. 
BY BENJAMIN F. JOSLIN, M. D. 
PROFESSOR OF MATHEMATICS AND NATURAE PHILOSOPHY IN UNION COLLEGE, 
SCHENECTADY, NEW YORK. 
EXTRACTED FROM THE AMERICAN JOURNAL OF THE MEDICAL SCIENCES, 
FOR MAT, 1831. 
PHILADELPHIA: 
PRINTED BY JOSEPH R. A. SKERRETT. 
1831. OBSERVATIONS ON VISION. 
[Extracted from the American Journal of the Medical Sciences, for May, 1831.] 
& CCOMMOBA TIONof the Eye to different distances.—When the 
pupil of one of my eyes was dilated by stramonium for certain physi- 
ological experiments, (anaccountof which will be published hereafter,) 
some indistinctness of vision was experienced, which was found on 
examination to affect objects at moderate distances only. All objects 
in the room were somewhat obscure, whilst those situated without, 
at the distance of many rods, appeared equally distinct to both eyes. 
By placing in contact with the eye, and directly before the dilated 
pupil, an opaque plate, perforated with a circular aperture of nearly 
the same apparent magnitude* as the other pupil, there was no appre- 
ciable difference in the distinctness of vision with the two eyes at any 
distance. It was, however, found necessary to employ different aper- 
tures for different distances, not only when the object was near the 
distance of distinct vision for minute objects, but at distances of 
many feet. The less the distance, the less was the requisite aper- 
ture j so that by employing artificial diaphragms of different aper- 
tures, the same distinctness of vision was obtained as with the eye 
in its natural state. This, however, was of course only true of direct 
vision; for in oblique vision, the lateral and too oblique rays of the 
pencil would not be thus excluded, and no artificial substitute can in 
that case be provided for the natural interior diaphragm, the iris, so 
happily imitated in the periscopic glasses of Dr. Wollaston. It is 
also probable that even in the case of very distant objects, there was 
a greater spherical aberration in the eye whose pupil was dilated, but 
that this was sensibly compensated by the stronger illumination re- 
sulting from a greater aperture. 
We are warranted in concluding from the foregoing experiments, 
that at least under some circumstances, there is a necessary corres- 
pondence between the virtual magnitude of the pupil, and the accom- 
modation of the eye to distinct vision at different distances—and also 
that the observed effect of the stramonium did not depend chiefly, if 
at all, on the paralysis of any other part concerned in the adaptation. 
Whether any change in the iris generally conduces to, or even at- 
* The incident and emergent rays undergo a similar refraction at the cornea. 4 
tends this accommodation in ordinary vision, is a point Upon which 
physiologists are not agreed. The foregoing experiment seems to fa- 
vour the affirmative. 
Notwithstanding the various unsatisfactory conjectures respecting 
the uses of the ciliary processes, and the no less numerous hypotheses 
respecting the adjustment of the eye to distinct vision, this last has 
perhaps never been attributed to a change of form in the crystalline, 
produced by the simultaneous action of the different parts of the ci- 
liary processes. An action producing lateral displacement has been 
supposed, which can effect no such adjustment, and is less likely to 
take place than a general contraction and dilatation, especially if we 
consider them as belonging to the erectile tissue. 
In the eyes of a horse killed in health, and examined immediately 
after death, I found the attachment of these processes to the capsule 
of the crystalline very strong. It appeared to require more than ten 
times as much force to detach one of them, as to break an equal por- 
tion of the fibres of the crystalline, on whose contraction Dr. Young 
supposed the adjustment to depend. In the human eye, however, 
the attachment is generally considered rather weak. It is perhaps 
strong enough during life to produce some little effect, and it is not 
necessary to suppose the accommodation to depend exclusively on 
any single structure, nor that the different parts of the eye which 
may conspire to produce this effect, exercise the same relative influ- 
ence in different animals. The stronger attachment of the ciliary zone 
In some animals, may indicate, if not a different office, at least, a 
greater share in an office or function performed by the combined ac- 
tion of several parts, and by a different mechanism. I would hardly 
offer the above as a hypothesis, but merely state it for the consider- 
ation of physiologists. 
But whatever hypothesis may be correct, it appears to me that 
the necessity of some adjustment has never been disproved, either 
experimentally or theoretically. It is true, that eminent physio- 
logist, M. Majendie, found the image formed on the posterior part 
of the dead eye of an animal to be distinct, when the object was 
placed at different distances; and this experiment has been consider- 
ed irreconcileable with any theory of adaptation. But unless the an- 
gular magnitude of the object were very considerable, the image 
would be too minute to allow the irregularity to be perceived in such 
an experiment. For in this instance, the image which occasions out 
perception is the image of an image, and has nearly the same ratio to 
the primary image as the latter has to the object; so that it appears 
to me very possible, that the distinctness of the object might be sen- 5 
sibly affected by a change of distance, such as would not sensibly af-1 
feet the regularity of its image formed at the bottom of a dead eye, 
and regarded as an object of vision. 
Theoretical arguments against accommodation are no less incon- 
elusive. They have, so far as I have seen, overlooked the necessity 
of concentrating, upon a single point of the retina, the rays emanat- 
ing from a single point of the object, and the constancy of their an- 
gles necessary for this effect. The difficulty is not removed, as has 
been supposed, by calling the eye a camera obscura; for this instru- 
ment also, when provided with a lens, requires adjustment to dis- 
tance; and the reason why it may seem to “ show objects distinctly 
for many miles round,”* appears to me to be, that at great distances, 
a given difference of distance produces a less difference in the obli- 
quity of the rays, than an equal difference of distance in near objects. 
With objects at small distances, the defect would become instantly 
manifest. But on the other hand, adjust the instrument to near ob- 
jects, and it would be of no avail that “ an infinity of other rays flow 
from a distant object in all directions,”* unless those which flow 
from a single point, make with each other and with the cornea the 
same mean angles as those from a single point of a near object—which 
is impossible. 
Moreover, the removal of a portion of the humours of the eye, or 
of the iris, dilates the image, by preventing the concentration of any 
pencil on a single point of the retina. The same effect is produced 
by any change in the requisite length of the axis. Hence the great 
extent of the vitreous humour is not only of “ use to extend the field 
of vision,”* but is indispensable to the removal of the sentient screen 
to the precise distance at which the image is most perfect. This is a 
point which is not sufficiently appreciated by many physiologists, 
who speak vaguely also of the other humours, as merely “ increas- 
ing the intensity of the light. ”* 
In short, any change in the distance of the object, the refracting 
power of the eye, or the length of its axis, must produce a change in 
distinctness, unless a simultaneous change in two or more of these 
circumstances effects a compensation. 
Now, different single parts, which by various physiologists and 
philosophers have been shown to effect this compensation, have by 
others been respectively either removed or placed in circumstances 
which nullified their influence, and yet the powers of compensation 
remained. These apparently contradictory results are reconciled by, 
* See M, Majendie’s Physiology. 6 
and seem to require a less simple hypothesis. Such an hypothesis 
would be perfectly consistent with the rule of philosophizing, causas 
rerum naturalium non plures admitti debere, qjuam quse et verse sint, 
et earum phsenomenis explicandis sufficiant; for more than one single 
cause has been proved to exist, yet no one of them has been proved 
fully adequate. 
From some of the foregoing considerations, we might be induced 
to suspect that night reading, or any employment which requires the 
examination of minute and near objects with a faint light, and con- 
sequently during the expanded state of the pupil, has a tendency to 
produce myopia, or short-sightedness. For, during the obscurity of 
evening, the dilatation of the pupil necessarily resulting from the in- 
voluntary sympathy between the retina and iris is unfavourable to 
the distinct vision of near objects; but we nevertheless make an ef- 
fort to see distinctly; an effort which may put in requisition to an 
extraordinary degree the voluntary powers of accommodation; and 
those parts in which these powers reside, and those upon which they 
directly operate may become permanently changed in their action 
and conformation, in consequence of a reiterated action in the former 
to a greater extent than is necessary during the light of day, when 
they enjoy in a higher degree the cooperation of the iris. 
Appearance of the Tears on the Cornea.—Of all the proper ob- 
jects of vision, (i. e. those exterior to the eye,) I have for many 
years believed that I had seen one of the nearest which could ever 
be visible to the eye of any animal; that is, the fluid secreted by the 
lachrymal gland, and rendered visible by the light refracted at its 
undulated surface, as it flows down the anterior surface of the cor- 
nea. This fluid contains visible spherical particles, or is mixed 
with some, partly perhaps from the meibomean glands. My experi- 
ments may be repeated in the following manner. 
If any person, with his eyelids nearly closed, looks toward a win- 
dow, or any luminous object nearly stationary, the luminous space 
will appear to be filled with circular bright spots, surrounded by 
dark rings. Some of these circles are much larger than others, and 
are either single, double, or multiple. An undulated appearance 
is frequently presented, each wave being either continuous, or com- 
posed of, or containing a chain of spherules. All these objects move 
slowly downwards in a vertical direction. That this motion depends 
on gravity, I assured myself, by giving to the head various inclina- 
tions, and finding the general direction of the motion still vertical as 
before. I say the general direction, for in whatever position the 7 
head is placed, there are occasional instantaneous motions through 
very short spaces, such as would be produced by slight involuntary 
relative motions of the eyelid and eyeball, impressing on that part 
of the fluid contiguous to the tarsus, a motion which would be ne- 
cessarily transmitted to the whole visible lamina, and produce in it 
a simultaneous displacement. That these slight motions are of this 
nature is verified by observation, for at any one time they agree in 
the instant of their commencement, in their direction, duration and 
extent, for all those objects of which we have been speaking. There 
are, however, other appearances which present themselves in the 
same field, but with which those just described, must not be con- 
founded. They are less distinct points, with rapid and irregular mo- 
tions, and resemble electric sparks. It is also necessary, in order 
to see no motions of the fluid on the cornea, except those which I 
have described, to guard the eye against voluntary motions, by direct- 
ing the optic axis as steadily as possible toward some fixed point in 
space. With these precautions in making the observation, each vi- 
sible portion of fluid will be seen finally to pass the inferior limit of 
the field of vision. This constant relation to the line of gravity in 
every position of the head, proves these objects to be some gravi- 
tating matter. The perpetuity of the phenomenon in the erect pos- 
ture, forbids the supposition of its being in the humours; and the 
luminous centre and dark annulus of the circular spots, and the lu- 
minous middle and dark parallel sides of the linear ones, as well as 
the varieties in the appearance and number of the rings, (for more 
than one is frequently visible,) present the same appearances as per- 
fectly transparent fluid spherules and waves on the surface of the 
cornea might produce, agreeably to the laws which regulate the 
transmission of light through thin plates of variable thickness.* Fi- 
nally, these appearances are more obvious, when from weakness of 
the eye or any other cause, the lachrymal secretion is more abundant. 
Hemiopsis.—I have twice experienced a partial insensibility of the 
retina, attended by some peculiarities not recorded in Dr. Wollas- 
ton’s cases. At each time, objects and parts of objects, situated on 
the left side of the visual axis, were, for about half an hour, either 
indistinctly seen, or totally invisible; for instance, the left side of a 
printed page, of a line, and even of a single word, was not perceived 
whilst the eyes were directed to the middle. There was conse- 
* From neglect of this principle, have not anatomists been sometimes de- 
ceived as to the structure of globules ? 8 
quently an insensibility of the right half of each retina. In both 
cases, a glimmering zig-zag, or broken line, appeared to be situated 
before the eyes above and to the left of the point toward which their 
axh were at any time directed, and as near as could be determined, 
in that part of the field of vision which was most completely ob- 
scured. In the first case, this line had an angular appearance, being 
composed of two lines, (see figure,) meeting at an angle of about 60°. 
It moved very slowly upwards, and to the left, 
and disappeared about the same time that per- 
fect vision was restored. In the second case, 
the line resembled one of the branches of the 
former, was, like it, composed of numerous 
short lines, but its general course was nearly 
straight and horizontal. Its left extremity be- 
came gradually more and more elevated, and in this oblique position 
the imaginary object finally'disappeared. The whole space occupied 
by these objects, subtended a visual angle of about 20°. Their co- 
existence with the hemiopsis may be worth recording, as simi- 
lar observations may possibly throw some light on the pathology 
of both. It is well known that Dr. Wollaston has inferred from the 
latter a semi-decussation of the optic nerves. 
Schenectady, Dec. 1830.