' . ' ' ' * >' 
. 
r - 
...... 
t, « x - The Relative Position of Rest of 
the Eyes and the Prolonged 
Occlusion Test 
BY 
F. W. MARLOW, M.D., M.R.C.S. Eng., F.A.C.S. 
• t» 
Professor of Ophthalmology in the College of Medicine, Syracuse Univer- 
sity; Member of the American Ophthalmological Society, and 
of the Ophthalmological Society of the United 
Kingdom; Fellow of the Royal Society 
of Medicine, Etc. 
ILLUSTRATED WITH 
ORIGINAL DIAGRAMS AND CHARTS 
PHILADELPHIA 
F. A. DAVIS COMPANY, Publishers 
1924 COPYRIGHT, 1924 
BY 
F. A. DAVIS COMPANY. 
Copyright, Great Britain. All Rights Reserved. 
PRINTED IN U. S. A. 
PRESS OF 
F. A. DAVIS COMPANY 
PHILADELPHIA. PA. PREFACE 
The methods of examination of the ocular muscles, 
their functions and anomalies, have been brought to such 
a high degree of perfection by the work of Von Graefe, 
Landolt, Stevens, Duane, Maddox, Howe and others, that 
it is with much hesitation and some sense of presumption 
that this incursion into the field is made. 
The modification in the method of using the screen 
test with which this essay deals has, however, yielded the 
writer such valuable aid in the elucidation of many other- 
wise-to him-very obscure problems, that he feels justi- 
fied in publishing in a fuller form than an ordinary paper 
to be read at some society meeting would permit, an ac- 
count of the method, the observations that have been made 
with it, and the inferences which it seems should be 
drawn from them. 
The fact that many ophthalmologists have met with so 
many disappointments in their attempts to deal with these 
conditions that they have ceased to pay attention to them, 
is in itself evidence that the methods ordinarily used for 
diagnosis fail to reveal the truth, and that treatment has 
consequently been based on insufficient and often on mis- 
leading data. 
The writer wishes to acknowledge the invaluable as- 
sistance received from his son, Dr. S. B. Marlow, in the 
work of tabulating and classifying the cases. 
F. W. M. 
iii CONTENTS 
CHAPTER I. 
Introductory. 
PAGE 
Comparison of methods of examination in ametropia and heter- 
ophoria 1 
Cycloplegia and the Prolonged Occlusion Test 2 
Does the latter cause an artificial dissociation? 3 
Frequency of vertical deviation 6 
Effect of insufficiency or paralysis of one muscle on innervation of 
the other muscles 8 
Clinical observation leading to adoption of the method 10 
CHAPTER II. 
The Method. 
Ground glass or black patch, etc 12 
The "master", or sighting eye 12 
Dolman's observations on relation of sighting eye to weak muscle .. 12 
Instructions to patient 14 
Importance of perfect continuity of test 15 
Duration of test-repetition of test 16 
CHAPTER III. 
The Relative Position of Rest. 
Bielschowsky's definition-his observations in 289 cases 19 
Periods of occlusion in author's 700 cases 19 
General results of occlusion 20 
CHAPTER IV. 
Orthophoria. 
Definition in text books variable 22 
Definition analogous to that of emmetropia 24 
Hansen Grut on the position of "functional rest" 24 
Analogy with method for establishing the limit of dark adaptation .. 25 
v vi 
CONTENTS. 
CHAPTER V. 
Tabulation and Analysis of Cases and Some Inferences 
to be Drawn. 
PAGE 
Tables showing errors in muscle balance before and after the test. 
Table 10, a Summary 26-44 
Ellett's views on relative importance of errors of refraction and 
muscle balance45 
Change in point of view brought about by test as to prism exercise 
and operation 46 
Objections to the constant use of prisms 50 
New field opened up for prismatic or operative treatment 51 
Causes of ill repute of such treatment in the past 52 
Effect of operation uncertain without postoperative occlusion test ... 54 
Effect of partial tenotomies 54 
CHAPTER VI. ' 
Muscle Balance in Relation to Refraction. 
Table showing incidences of various forms of heterophoria in various 
forms of ametropia 56 
CHAPTER VII. 
Age, Heredity and Evolution. 
Heterophoria may be latent in childhood or in old age. Table show- 
ing incidence in each disease 59 
Heterophoria apparently bears no relation to age. Appears to run in 
families 60 
Exophoria a reversion to an earlier type61 
Treacher Collins' Bowman Lecture-panoramic vision and the evolu- 
tion of binocular vision 61 
Probable vestigial nature of abduction-hyperphoria sursumduction 
deorsumduction 62-63-64 
CHAPTER VIII. 
Abduction. 
Definition-relation to exophoria-its variability 65 
Table showing change in abduction after occlusion 67 
What is the true measure of abduction 66 CONTENTS. 
vii 
CHAPTER IX. 
Indications for the Use of the Test. 
PAGE 
Cases in which correction of refraction aggravates symptoms .... 71 
Cases in which correction of refraction gives no relief 74 
Cases in which correction of refraction and demonstrable faults in 
muscle balance gives no relief 74 
Some cases of monocular pain 74 
Any case in which an ocular explanation of nerve symptoms is sought 
by patient's physician 75 
Cases of persistent accommodative spasm 76 
Cases of progressive myopia 78 
CHAPTER X. 
Effect of Occlusion Symptoms. 
Relief from symptoms, or aggravation 81 
CHAPTER XI. 
Near Point of Convergence. 
Practically unaffected by test 82 
Table showing near point exophoria and P. P. C. before occlusion .. 82 
Table showing near point exophoria and P. P. C. after occlusion 84 
CHAPTER XII. 
Treatment. 
General health. Refraction effected by correction 85 
Exercise. Constant wearing of prisms 86 
Operations 89 
Appendix 90 
Index 95 CHAPTER I. 
INTRODUCTORY. 
Of the two strictly ocular groups of causes of asthen- 
opia, the ametropias and the heterophorias, the former 
presents a field the thorough exploration of which has 
been made possible by a number of subjective and ob- 
jective methods, the efficiency of which has been materi- 
ally raised by the use of cycloplegia, thus eliminating the 
action of the accommodation and permitting the static re- 
fraction to be measured with a high degree of accuracy. 
This somewhat trite statement is made only for the pur- 
pose of comparison of the refractive with the muscle 
balance problem. 
In hypermetropia and to some extent in astigmatism we 
have anomalies the effect of which is offset by an exact 
adjustment of accommodation to maintain clear vision, and 
this accommodative adjustment has to be maintained so 
constantly that the ciliary muscle gets into a state of 
chronic contraction-accommodative spasm-and does not 
completely relax when its action is made unnecessary by 
glasses, for a length of time varying in different cases, 
unless a cycloplegic is used. 
In the heterophorias we have anomalies the effect of 
which is offset by an exact adjustment of the extrinsic 
muscles to maintain single vision, and that adjustment has 
to be maintained so constantly that the extrinsic muscles 
get into a state of chronic contraction-muscular spasm- 
and when their action is made unnecessary by the annul- 
ment of binocular vision they do not completely relax, for 
a length of time varying in different cases, unless some 
1 2 
POSITION OF REST AND OCCLUSION TEST. 
drug analogous to a cycloplegic be found to relax the 
spasm. 
It is unnecessary to say that no such drug has yet been 
found, and consequently the investigation of the muscle 
balance lacks the completeness and precision which charac- 
terize that of the refraction. To accomplish a relaxation 
of the extrinsic muscles, which have become contracted in 
response to the habitual demands of the binocular function, 
we have, therefore, to fall back upon an annulment of that 
function, maintained for a length of time sufficient for the 
purpose. Hitherto, what length of time is sufficient to 
effect a relaxation has been left out of consideration. 
The object of this monograph is to indicate the effect 
of prolonging the cover test sufficiently to allow the de- 
sired relaxation. The term "Prolonged Occlusion" seems 
sufficiently descriptive of this test. 
"When some member of the A. M. A. has discovered 
and made practical the use of a myoplegic that will para- 
lyze the extraocular muscles, as a mydriatic does the intra- 
ocular muscles, we shall have made giant strides towards 
solving the physiology of the extraocular muscles. 
"It would probably upset some of our pet notions of 
today and require us to set up new standards." (Reber: 
Jour. A. M. A., Jan., 1901.) 
Although the prolonged occlusion test for heterophoria, 
and the cycloplegic test for ametropia are not exactly 
analogous, the difference between them is not so great as 
at first thought appears to be the case. 
In each case we have muscle action, resulting from a 
nerve impulse originating in a nerve center. In the one 
case the impulse arises from a reflex, the purpose of which 
is to effect clear vision; in the other, from a reflex whose 
purpose is to effect single vision. In the former case the 
impulse arises and is stopped by the action of the cyclo- INTRODUCTORY. 
3 
plegic at the nerve endings in the muscle. In the latter 
the conditions which start the reflex action are absent and 
consequently no impulse arises in the center; or, to put it 
briefly, in cycloplegia no impulse reaches the muscle because 
it is stopped at the nerve endings : in the "Prolonged Oc- 
clusion Test" no impulse reaches the muscle because none 
originates in the center for fusion. 
It is obvious that of the three different types of tests 
for the relative position of rest, the cover or screen test 
is the only one suitable for a prolongation of the period of 
time for which it is used. Neither the diplopia tests nor 
tests of the Maddox rod type are adapted for pro- 
longed use, and, moreover, they do not entirely abolish the 
tendency to fusion. The cover test on the other hand com- 
pletely annuls binocular vision and makes any attempt to 
fuse impossible. Its mode of action is absolutely passive, 
if such a statement be permissible. It has the further 
advantage of being an objective test, its objective findings 
being also controlled by its subjective phase, the parallax 
test. 
The "Prolonged Occlusion Test" is, in other words, 
nothing but a screen test extended in point of time. Theo- 
retically, and practically when convenient and expedient, 
the test should last until stability in the position of rest is 
attained, that is, until the findings are essentially the same 
from day to day; but as a rule a week's occlusion will fur- 
nish valuable information. 
The question has been raised whether such prolonged 
occlusion does not induce an artificial dissociation,* but 
treatment based on its findings is so much more satis- 
factory than under ordinary conditions, and repeated tests 
are so consistent with one another that it seems to the 
* See Appendix. 4 
POSITION OF REST AND OCCLUSION TEST. 
writer that this question should be answered in the nega- 
tive. Moreover, what artificial condition can occur? 
Muscles are capable only of different degrees of contrac- 
tion, a maximum contraction being the response to a maxi- 
mum innervation, and a minimum contraction (i.e., a 
maximum relaxation) being due to a minimum innerva- 
tion. The muscles in their function of maintaining binocu- 
lar single vision (convergence) are subject to innervation 
from two sources only: First, that due to the fusion power 
itself; and second, that due to the association of accom- 
modation with convergence. In this test all attempts at 
fusion, and consequently all innervation due to attempts 
at fusion, are abolished by occluding one eye, and all ex- 
cessive accommodation by a full correction of refractive 
error in the working eye. Under these conditions it is 
reasonable to expect that muscle spasm, due to long con- 
tinued over-innervation from either of these sources, will 
in time pass away and the muscles resume their normal 
tone or state of maximum relaxation, in a state of mini- 
mum innervation. 
It must be admitted at once that there is no means of 
knowing absolutely when this state of minimum contrac- 
tion (maximum relaxation) has been reached, and, there- 
fore, of knowing precisely what the relative position of 
rest is, but it seems likely that when observations made 
from day to day become stable for several days in suc- 
cession, an approximation to it at least has been reached; 
and when circumstances do not permit the test to be main- 
tained long enough for stability to be attained, that the 
general tendency shown by a curve plotted out in accord- 
ance with daily observations points, at any rate, in the 
direction of the truth, even if the whole truth is not 
revealed. INTRODUCTORY. 
5 
CHART I. 
1920 
MAY 
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POSITION OF REST AND OCCLUSION TEST. 
It is instructive, in this regard, to examine the charts 
here presented, showing the gradual development of the 
error in three members of the same family. It will be 
observed that there is a gradual increase of error in the 
case of R. M., Chart I, with some variation until the 
thirteenth day, after which date the amount of exophoria 
remains practically unchanged. If some artificial dissocia- 
tion has taken place, this must have happened at some 
point between the first and thirteenth day of the test, 
but the chart gives no indication of anything but a gradual 
relaxation until stability is reached. 
Chart II, Mrs. M. F. W. (sister of R. M., Chart I), is 
still more striking, because at an earlier period of life an 
exophoria of 20° had been diagnosed. This had been 
reduced by exercises to 70, without any relief from symp- 
toms. A Prolonged Occlusion Test brought the error grad- 
ually up to 190. 
Chart III, A. M. (brother of two preceding), shows 
also a development of the same kind of error. That the 
test in these cases brought out real and not artificial as- 
sociations was confirmed by the fact that treatment based 
on the findings was beneficial in each. 
In the series of cases here presented I have seen no 
reason for thinking that the test is ever actually mislead- 
ing, although cases occasionally occur which are difficult 
of interpretation at the present time. It seems probable 
that the method, instead of inducing, gets rid of artificial 
conditions and permits the true underlying relations to be 
seen. One of the points which emerge from the mass of 
observations is that vertical deviations are exceedingly 
common, and almost all apparently due to weakness or in- 
sufficiency* of one or sometimes more than one muscle. 
* Possibly paretic but for reasons which will appear later, I am ab- 
staining from the use of this term. INTRODUCTORY. 
7 
CHART II. 
1919 
MARCH 
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12 13 1415 
MAY 
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POSITION OF REST AND OCCLUSION TEST. 
Let us consider the effect of an insufficiency of the R. 
superior rectus upon the binocular function, and the effect 
of occlusion in analyzing it: 
(1) The right superior rectus, being insufficient, re- 
quires excessive innervation to hold the R. eye 
on a level with the L. eye in movements up and 
to the right. 
(2) This excessive innervation is also communicated 
to its associate in upward movements to the R., 
the L. inferior oblique. 
(3) Diplopia due to overaction of the L. inferior 
oblique would result if its direct antagonist, the 
L. superior oblique, did not counteract this 
tendency. 
(4) The excessive innervation of the L. superior 
oblique necessary for this purpose would be com- 
municated to its associate in movements down 
to the R., the R. inferior rectus. 
(5) Diplopia would result if the action of the R. 
inferior rectus were not held in check by its 
direct antagonist, the R. superior rectus, thus 
putting an extra demand upon the insufficient 
muscle. 
A vicious circle in other words is formed. Binocular 
vision is maintained so constantly in such cases that no 
doubt spasm of the extrinsic muscles, strictly analogous to 
accommodative spasm in hypermetropia is induced, which 
obscures the deviating tendencies as effectively as the latter 
renders hypermetropia latent, and the total result of the 
activities described as revealed by the ordinary short test 
may even be orthophoria (functional). 
If now the R. eye be occluded, fusion is made impos- 
sible, and consequently there will be no effort to maintain INTRODUCTORY. 
9 
CHART III. 
14° 
13° 
12° 
11° 
10° 
9° 
8° 
7° 
6° 
5° 
4° 
3° 
2° 
10 
Ortho. Q° 
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POSITION OF REST AND OCCLUSION TEST. 
it; excessive innervation of the R. superior rectus will 
cease, and consequently will not spread to its associate in 
upward movements to the R., to be counterbalanced as 
above described. In fact all abnormal innervation and all 
attempts at fusion cease and the eyes gradually assume 
their relative or anatomical position of rest, experience in 
a large number of cases showing that many days of rest 
are necessary in many cases to permit relaxation to take 
place. 
The adoption of the method was brought about by the 
consideration of certain clinical observations. In the first 
place the well-known facts that a squint is apt to develop 
in children or adults, the subject of any condition such as 
corneal ulceration, annulling binocular vision for any 
length of time, that it develops not infrequently in pa- 
tients who have had one eye bandaged for any length of 
time, and that a divergence or other deviation develops in 
many eyes (but not in all) which have become blind or 
very defective from any cause, would at least suggest that 
the duration of the annulment of the binocular function is 
an important factor in the detection of faults in the muscle 
balance. 
In the second place, the common clinical observation, 
which makes the foregoing inference of practical import- 
ance, that there are a great many patients, the character of 
whose symptoms leaves no doubt that they are the subjects 
of some kind of eye-strain, who fail more or less com- 
pletely to receive benefit from an apparently accurate cor- 
rection of the manifest refraction and muscle errors, or are 
made worse by such correction, can only be explained on 
the theory that some etiological factor has been undetected 
or is not demonstrable. 
A case also occurred in my practice, the facts of which 
strongly suggested the value of Prolonged Occlusion. The INTRODUCTORY. 
11 
patient was suffering from a burn of the eyelids of one eye 
from molten iron, necessitating the bandaging of the eye 
for a week. When the bandage was removed at the end 
of the period he complained of seeing double, and examina- 
tion showed an exotropia of 50 to 6° and a hypertropia 
of i° to 20. Seven days after removing the dressing no 
fault in the muscle balance could be found. A i° prism 
placed in any position destroyed the orthophoric condi- 
tion. This man had been under the care of Dr. George T. 
Stevens some years previously for diplopia. The treat- 
ment, which consisted of prism exercises, had apparently 
succeeded in making the deviation latent. 
The inferences to be drawn from this case seem to be 
that a week's occlusion is sufficient, at any rate in some 
cases, to render a latent deviation manifest, and that while 
exercises may improve the muscle tone or stimulate the 
function of the nerve center they do not cure the deviation. CHAPTER II. 
THE METHOD. 
The test may be carried out practically by substituting 
a ground glass for one of the patient's lenses, or by past- 
ing a piece of dark paper over the back of one lens, or by 
covering one eye with a black patch. Of these three 
methods the ground glass has the great advantage of com- 
parative inconspicuousness, and is quite efficient, but the 
glare caused by light shining upon it is annoying to many 
neurotic patients, some of whom do not tolerate it. In 
such cases the dark paper or black patch can be substituted 
with advantage and success. A full correction of the re- 
fraction is worn over the other eye. 
It is obviously not a matter of indifference which eye 
is covered. If a latent deviation be present and due to a 
paretic muscle the result will vary according to the selec- 
tion made. As it is not always possible to determine this 
before the test, the selection must often be made on other 
grounds. In the series of cases upon which this article 
is based, this was decided in some cases by difference in 
visual acuity, the more defective eye being occluded, and 
in other cases-the great majority-by allowing the pa- 
tient to decide whether he was right-eyed or left-eyed by 
sighting a distant light through a ring held at arm's length, 
both eyes being open, or by some analogous method. If 
he sighted the light with the right eye the left was oc- 
cluded. Incidentally this test showed the right to be the 
"master" eye in a very large majority of the cases. 
Dolman has shown (Amer. Jour, of Ophth., April, 
1920) that in ordinary tests for heterophoria a greater 
12 THE METHOD. 
13 
amount of error is demonstrated in 66 per cent, of the 
cases, when the sighting or master eye is used to fix the 
light, though the difference is quite moderate in degree. 
This is contrary to what one would expect if the devia- 
tion were paretic in nature for it is in accordance with 
common experience that the non-paretic eye is selected 
for fixation, unless its visual acuity is decidedly below that 
of the other, and the primary deviation is always less than 
the secondary. This criticism is not made to raise any 
question as to the essential accuracy of Dolman's observa- 
tions, but to point out that as far as they go, they are not 
in harmony with the paresis theory. 
I was under the impression, until I actually counted my 
cases, that the determination of the sighting eye had some 
relation to the weak muscle, but a correlation of these two 
factors in 109 cases in which both were carefully noted 
fails to reveal any such relation. 
In 55 cases, or 50 per cent., the non-paretic eye was 
selected for sighting a distant object. 
In 37 cases, or 34 per cent., the eye with the insuffici- 
ent muscle was selected for sighting a distant 
object. 
In 17 cases, or 16 per cent., there was an insufficient 
muscle on each side, and of these cases the R. 
eye was the sighting eye in 13 cases. 
Of these 109 cases the R. eye only was affected (i.e., 
had one or more weak muscles) in 39 cases. 
The L. eye only was affected in 54 cases. 
Both eyes were affected in 16 cases. 
In all three groups the right is the sighting eye in a 
large percentage of the cases. 
In cases of R. insufficiency, in 31 out of 39 cases, or 
in 80 per cent. 14 
POSITION OF REST AND OCCLUSION TEST. 
In cases of L. insufficiency, in 50 out of 54 cases, or 
in 92 per cent. 
In cases of R. and L. insufficiency, in 13 out of 16 
cases, or in 81 per cent. 
Evidently the determination of which shall be the sight- 
ing eye depends but little, if at all, upon the presence of 
insufficiency of any muscle or muscles, and this observa- 
tion also is not favorable to the paresis theory. 
Just as most people are right-handed, so most are right- 
eyed, but the two functions do not always go together. 
But it has been necessary not only to occlude one eye 
but also to create in the patient a state of mind which will 
prevent him from nullifying the value of the test in the 
many ways which can easily be imagined. Therefore, 
only those patients have been selected for the purpose 
who, by a combination of intelligence and a sincere desire 
to rid themselves of serious or annoying symptoms, have 
seemed suitable for it. In the next place the object and 
way of working of the test have been thoroughly explained 
to them, and finally they have been warned in detail of the 
precautions to be taken to avoid giving the binocular func- 
tion an opportunity to be active. I have been surprised at 
the willingness with which it has been accepted by patients, 
and at the conscientiousness with which it has almost in- 
variably been carried out. 
To be specific, the directions given to the patient are 
that he shall put the glasses on in the morning before 
opening his eyes, that he shall not look over or under them, 
that if he wishes to remove them for any purpose-as 
washing the face-during the day, he shall close his eyes 
or at least one of them before doing so and replace the 
glasses before re-opening them, and that he shall wear 
them until after he has closed his eyes at night. In other THE METHOD. 
15 
words that one eye must not be allowed to associate with 
the other during the whole period of the test. Strict 
continuity is apparently a very important detail. It was 
my good fortune to come across a case previously in the 
hands of one of my colleagues, in which sufficient em- 
phasis had not been laid upon this point. Through his 
courtesy I am able to give the results of a seven and also 
a fourteen day occlusion test in which the patient did not 
put his glasses on until after dressing. 
Before the first occlusion test, examination showed an 
exophoria of % degree and no hyperphoria. 
After the seven day test the exophoria rose to 
degrees and there was left hyperphoria of 1% 
degrees. 
After the fourteen day test the exophoria was 2 de- 
grees and the hyperphoria again 1% degrees. 
A ten day test without any break in its continuity 
gave exophoria 7 degrees and left hyperphoria 3 
degrees. 
Before beginning the test the patient is also warned in 
detail as to the loss of judgment of distance, and cautioned 
about walking up and down stairs, stepping into and out of 
cars, etc. 
At the end of the period of occlusion, or whenever a 
test of the muscle balance is made during it, a full cor- 
rection of the refraction is placed in a trial frame, with a 
Maddox rod, or whatever other means is selected for the 
test, the patient is directed to close his eyes, and his glasses 
are removed and replaced by the trial frame. He is 
directed to open his eyes, and the test is then made, with- 
out any break in the continuity of occlusion. It is my 
usual practice to use the Maddox rod for the vertical, and 
the phorometer or vertical diplopia test for the lateral 16 
POSITION OF REST AND OCCLUSION TEST. 
deviations, with the screen and parallax test as a confirma- 
tory test. The difference between these tests is not always 
in favor of the screen test, and is negligible compared with 
the change occurring under prolonged occlusion. 
It is impossible to emphasize too strongly the necessity 
of extreme care in carrying out the details of this test. 
If the occlusion be not continuous but broken, if the final 
test be not made with the strictest precaution for avoiding 
binocular fusion, the value of the test is probably much 
diminished, but if all the precautions necessary are taken, 
and the patient does his (usually her) part, as is almost 
invariably the case, it is of high value always, for it does 
one of two most desirable things; either it provides infor- 
mation which makes it possible to give the desired relief 
completely or partially, or it enables one to make the pa- 
tient understand why relief cannot be attained. 
Duration of Test. 
The length of time necessary to effect a complete re- 
laxation of the muscles is in doubt. In the majority of 
cases the final test has been made at the end of a week's 
occlusion, and the result accepted for working purposes; 
but in a considerable number, daily observations have been 
made for the purpose of watching the mode and rapidity 
of the manifestation of the error and the period necessary 
to reach stability. 
Reference may be made here again to Chart I, 
which shows the manifestation of an 8° exophoria in a 
man having perfect orthophoria all over the field at the 
pre-occlusion test. There was no break in the continuity 
of the test, the patient, at first quite reluctant to adopt it, 
finally becoming much interested in it. It will be noted 
that a 24-hour occlusion had no effect whatever in bring- THE METHOD. 
17 
ing about any relaxation of his muscles, and that it was 
not until the thirteenth day that the highest degree of 
error showed. The test was continued five days longer 
without increase in the degree of error. It is also worthy 
of note that there was an occasional return of spasm as 
indicated by the irregularities in the curve. These were 
temporary only and there can be little doubt that the real 
position of rest was indicated by the highest point in the 
curve. 
Light is thrown also on the question of the necessary 
duration by repetition of the test. 
Thus in 72 cases a second test has been made at varying 
intervals after the first. 
Briefly stated: 
In 6 cases no change took place in either the vertical 
or lateral deviations. 
In 14 cases no change took place in the lateral devia- 
tion. 
In 24 cases no change took place in the vertical devia- 
tion. 
In 33 there was an increase in the hyperphoria. 
In 31 there was an increase in the exophoria or 
diminution in the esophoria. 
In 15 there was an increase in the esophoria or 
diminution in the exophoria. 
In 4 there was a diminution in the hyperphoria. 
These changes were for the most part small in degree. 
Thus, there were only 17 cases in which the lateral change 
amounted to more than 30, and only 20 in which the 
vertical change amounted to more than 1 degree. So far 
as the actual changes are concerned they confirm the 
value of the first test as being reliable so far as it goes. 
Thus, in 4 only of 72 cases did this second test diminish 18 
POSITION OF REST AND OCCLUSION TEST. 
the hyperphoria, the amount of diminution not exceeding 
i degree. 
In 45 of the cases, exophoria remained the same or in- 
creased. 
On the whole, the figures give a high value to a week's 
occlusion, but show that in some cases it fails to bring 
about a complete relaxation. CHAPTER HI. 
THE RELATIVE POSITION OF REST. 
In a paper by Bielschowsky, read before the Heidelberg 
Ophthalmological Society and abstracted in the Archives 
of Ophthalmology for January, 1914, the relative position 
of rest is defined as the position the eyes assume when un- 
influenced by any innervation due to the power of fusion. 
The author admits the difficulty of determining it, because 
the influence of the fusion power cannot be sufficiently 
excluded. He, therefore, determined it in 289 cases in 
which binocular vision had been lost by disease of one eye. 
His results are as follows: 
Per cent. 
Practical parallelism in 20 to 25 
Divergence of 2 degrees or more in 60 " 70 
Divergence with hyperphoria in 5 " 10 
Convergence of 2 degrees or more in 10 
The longer binocular vision had been lost the greater 
was the percentage of high degrees of strabismus. Length 
of time of annulment of the binocular function is then an 
important element in determining the position of rest. 
In the 700 cases upon which this monograph is based 
the periods of occlusion were as follows: 
Cases 
1 day 2 
2 days 5 
3 " 7 
4 " 16 
5 " 24 
6 " 33 
7 " 438 
8 " 65 
9 " 44 
10 " 30 
Cases 
11 days 8 
12 " 7 
13 " 2 
14 " 12 
15 " 2 
16 " 1 
17 " 1 
19 " 1 
22 " 1 
27 " 1 
19 20 
POSITION OF REST AND OCCLUSION TEST. 
The relative position of rest at the end of these periods 
was found to be: 
No. p. c. 
Parallelism in ; 24 3.4 
Divergence without hyperphoria 86 12.2 
Divergence with R. hyperphoria 171 24.4 
Divergence with L. hyperphoria 287 41.0 
Total divergence 544 78.0 
No. p. c. 
Convergence without hyperphoria 16 2.2 
Convergence with R. hyperphoria 35 5.0 
Convergence with L. hyperphoria 42 6.0 
Total convergence 93 13.2 
No. P. C. 
Hyperphoria without lateral deviation 39 5.5 
Hyperphoria with lateral deviation 554 79.0 
Total hyperphoria 593 84.0 
In these statistics lateral deviations of less than i°, and 
vertical deviations of less than %°, have been ignored. 
Degrees refer to the angle of the correcting prisms. 
It is true that the two groups of cases are not strictly 
comparable, as in the former the loss or impairment of 
vision of one eye is the only common factor, whereas in 
the latter the grouping is due to the presence of unrelieved 
asthenopia of one type or another in all, and therefore 
some anomaly of the muscle balance is, on the whole, 
probable. 
Moreover, on account of the conditions under which 
Bielschowsky's examinations were made, the fact that in 
those cases in which the measurement was made by obser- 
vation of corneal reflexes, the angle gamma was neglected, 
and the refraction apparently not corrected, the results 
must be regarded as approximate only. In view of the THE RELATIVE POSITION OF REST. 
21 
association of convergence with accommodation it would 
seem that the effect of all refractive errors causing activity 
of the ciliary muscle should also be eliminated and that 
Bielschowsky's definition should be amended accordingly. 
In the cases here recorded the refraction was fully 
corrected. CHAPTER IV. 
ORTHOPHORIA. 
It should be superfluous at this time to discuss just 
what conditions constitute orthophoria, but an examina- 
tion of various text books and monographs as to its defini- 
tion brings out such great differences, that for the purpose 
of discussing it here with clarity, it is necessary to state 
precisely what is meant by it. Conversation with col- 
leagues also suggests much indefiniteness and vagueness 
of thought about this matter. 
I find the following definitions: 
(1) Stevens' original one, "A tendency of the visual 
lines in parallelism," to which was added later, 
"when the minimum impulse is sent along the 
nerves governing the ocular movements." 
(2) "The condition in which muscular equilibrium 
is maintained with the minimum of nervous 
effort." 
(3) "That state of perfect equilibrium in which both 
eyes continue to be accurately directed at the 
object looked at, even when the controlling influ- 
ence of the fusion sense is temporarily with- 
drawn." 
(4) "When the visual lines meet at the object to- 
wards which they are directed, the condition is 
known as orthophoria." 
(5) "Perfect muscle balance." 
(6) "When the co-ordination is so perfect that when 
the eyes are directed towards an object, the visual 
lines correspond, and the images of the object 
22 ORTHOPHORIA. 
23 
fall upon corresponding points in the two retinae, 
the condition is known as orthophoria." 
(7) "Right tending, or muscular balance." 
(8) "If a person looking with both eyes fixates the 
same object with both eyes, under all conditions, 
even for example when the image in one eye is 
artificially made different from that of the other, 
or is suppressed by covering one eye, he is said 
to have orthophoria." 
(9) "Orthophoria is the term applied to a perfect 
balance of the ocular muscles when the head is 
in a primary position and the eyes are looking 
straight forward with practical infinity at a dot 
or small light" and the "same equilibrium is 
maintained in all secondary and convergent 
positions." 
(10) "A condition in which both eyes look straight 
at the same object whether both see it or not. 
There is not even a tendency to deviation." 
(11) "The oculo-muscular state in which the visual 
axes tend to remain parallel when the eyes are 
adjusted for distance, and to converge properly 
upon the point of fixation for other distances." 
(12) "Perfect binocular balance." 
The first two only can be said to be correct. 
The fourth and sixth and perhaps the ninth are defini- 
tions of binocular fixation, and are as true of any case of 
heterophoria as of orthophoria. 
The fifth and seventh, also the twelfth, simply substitute 
other phrases, without definition or explanation. 
The third, eighth, tenth and eleventh are incomplete in 
that they make no reference to the essential condition of a 
minimum innervation of the extrinsic muscles. 24 
POSITION OF REST AND OCCLUSION TEST. 
What emmetropia is to the refraction of the eye, ortho- 
phoria is to its muscle balance. 
Emmetropia exists when parallel rays of light are 
brought to focus on the retina, the accommodation being in 
a state of minimum innervation. 
Orthophoria exists when the visual lines are parallel, 
the extrinsic muscles being in a state of minimum inner- 
vation. 
The day has long gone by when a determination of the 
static refraction without cycloplegia would be considered 
valid, except in old age. 
But definitions of the muscle balance, and its anomalies, 
and of the tests for its determination refer exclusively to 
the functional conditions, without any attempt being made 
to insure or induce a maximum relaxation, that is, a 
minimum innervation of the muscles. 
That this functional position of rest does not neces- 
sarily represent the truth has been long recognized. 
Hansen Grut, in his Bowman Lecture of 1890, says: 
"There are reasons for believing the parallelism found in 
the ordinary manner of making the examination to be 
often a position of functional rest which, in many cases, 
conceals a structurally divergent position of rest." 
Though a divergent position of rest is without doubt 
far more common than a convergent one, the observations 
here recorded show that convergent, and far more com- 
monly vertical deviations, may also be concealed, presum- 
ably by what Grut terms "habit," that is, the habit of 
maintaining parallelism and so inducing a secondary spasm. 
Stevens, of course, recognized fully that manifest find- 
ings might leave more or less error unrevealed, and used 
prisms partly correcting the manifest error, for the pur- 
pose of inducing further muscle relaxation. ORTHOPHORIA. 
25 
Nevertheless, I believe that it is true today that when 
no deviation is found by the screen, diplopia or rod tests, 
the condition is recorded as orthophoria and dealt with 
as such, usually no attempt being made to induce a mini- 
mum innervation of the extrinsic muscles. 
Having unfortunately no drug analogous to the cyclo- 
plegics to eliminate the innervation of the extrinsic 
muscles we must fall back upon physiological principles 
and bring about a minimum innervation by removing all 
stimulus thereto; viz., by making all efforts at fusion im- 
possible by occluding one eye, by fully correcting all re- 
fractive errors in the working eye, and by allowing suf- 
ficient time to elapse for the spasm to become relaxed. 
The cases here recorded have been dealt with in this 
manner, and it seems safe to assume that the results ap- 
proximate the truth more closely than those obtained with- 
out the precautions detailed. 
Although not strictly analogous, this method may be 
compared with that for establishing the limit of complete 
dark adaptation by keeping the eyes in the dark until 
they cease to vary as to thresholds. In the former case 
all stimulation of a center-that for fusion-is withheld, 
in the latter all stimulation of nerve endings. In each 
case the function of the part involved is tested at the end 
of a period of complete rest. CHAPTER V. 
TABULATION AND ANALYSIS OF CASES AND SOME 
INFERENCES TO BE DRAWN THEREFROM. 
Among the 700 cases upon which this report is based 
are 154 in which the pre-occlusion test showed ortho- 
phoria. 
TABLE 1. 
Orthophoria Before. 
3° 
1* 
l 
R. HYP. 2° 
4 
1 
1 
1 
ls° 
1 
1 
1 
3 
1 
1° 
2 
1 
4 
3 
1 
1 
1 
1 
1 
i° 
2 
1 
2 
2 
1 
3 
1 
0° 
1 
1 
1 
1 
1 
13 
3 
3 
1 
2 
3 
4 
2 
1 
1 
1 
i° 
2 
3 
3 
2 
1 
1 
1 
1° 
1 
3 
4 
1 
3 
4* 
1 
1 
1 
1 
L.HYR|e° 
1 
3 
2 
1 
1 
1 
2° 
1 
2 
3 
1 
2 
1 
3 
1 
3° 
1 
1 
1 
1 
3 
4° 
1 
1 
5° 
6° 
1 
7° 
1 
1 
8°7O6O5O4°3O 
E50PH0RIA 
2° 
1° 
0° 
1° 
2° 3° 4° 5° 6° 7° 8° 9° 
EX0PH0RIA 
10° 11° 
12° 13° 14° 16° 
Deviation after Occlusion. 
Table i shows the distribution of these cases among the 
different forms of heterophoria at the post-occlusion test. 
* Figures within the squares show the number of cases having devia- 
tions corresponding to the horizontal and vertical scales. Thus there 
were 4 cases showing Ex. 4° and L. Hyp. 1°, one showing Exoph. 16° 
and Hyp. 0°. etc. 
26 TABULATION AND ANALYSIS OF CASES. 
27 
Stated briefly, of 154 cases of orthophoria before the 
occlusion test, only 13, or 9 per cent., were orthophoric 
after it; of the remaining 141 cases: 
Exophoria, varying from 1° to 16°, 
developed in 21 cases or 13.6 per cent. 
Hyperexophoria, varying from Hyp. 
y2° to 3°, Ex. 1° to 13° de- 
veloped in 97 " " 63.0 " " 
Esophoria, varying from 1° to 8°, 
developed in 5 " " 3.2 " " 
Hyperesophoria, varying from Hyp. 
1° to 3°, Eso. 1° to 8° devel- 
oped in 4 " " 2.6 " " 
Hyperphoria, varying from J4° to 
1%°, developed in 14 " " 9.0 " " 
To use Hansen Grut's expression, it may be said that 
orthophoria as ordinarily determined represents a "habit" 
acquired in the interest of single vision, and may not, and 
often does not, represent the truth as to the relative posi- 
tion of rest. 
In the Ophthalmic Year Book for 1916, in concluding 
a review of Reber's paper on heterophoria, the writer of it 
says: "Patients with asthenopia in the presence of ortho- 
phoria have a general myasthenia and do not need an 
oculist, but an all-wise medical practitioner." This, no 
doubt, has been the common view, but in the light of the 
above observations evidently needs reconsideration. 
What is true of orthophoria at the pre-occlusion test 
is true of all other pre-occlusion findings. They represent 
the degree of success which has been attained by the fusion 
power in establishing a habit of equilibrium, in the pres- 
ence of deviating tendencies, or actual paretic (?) con- 
ditions. 
The following tables show the character of the hetero- 
phoria at the pre-occlusion test and the change in the 28 
POSITION OF REST AND OCCLUSION TEST. 
degree and kind of error at the end of the period. Table 
2 is compiled from the cases, 104 in number which showed 
pure exophoria before the test. Of these cases one only 
remained unchanged and of the remainder all but eighteen 
developed hyperphoria, R. hyp. 29, L. hyp. 56. In 7 
cases the degree of exophoria diminished: 10 in two cases, 
20 in four cases, 30 in one case, and in three cases eso- 
phoria of 2° to 50 developed. These cases of diminished 
exophoria are difficult to explain. It will be noted that 
the diminution is slight or moderate in degree. It will 
also be noted that the charts in which daily observations 
of developing exophoria are shown, show variations from 
day to day apparently due to temporary return of spasm, 
and had the test been stopped during one of these periods 
of spasm the exophoria would have been much less than it 
finally was proved to be. It is possible that in these cases 
of less post-occlusion exophoria, the duration of the oc- 
clusion was not long enough to bring about a complete 
relaxation of the muscles, and hence results which are 
perplexing and possibly misleading. It is also possible 
that just as in hypermetropia the accommodation some- 
times acts to excess and produces a pseudo-myopia, so 
the extrinsic muscles may overcorrect a real esophoria 
causing temporarily an exophoria, though the reverse con- 
dition is far more common. TABULATION AND ANALYSIS OF CASES. 
29 
Exophoria without hyperphoria at pre-occlusion test, 
104 cases. Amount of change in exophoria and of hyper- 
phoria added. 
TABLE 2. 
4° 
1 
1 
3° 
1 
1 
1 
R.HYP. 2° 
1 
1 
Hf 
1 
1 
1 
1 
1° 
1 
2 
-I 
3 
1 
1 
i° 
1 
2 
-t 
1 
1 
3 
NO. HYP. 0° 
1 
2 
2 
I 
4- 
1 
1 
I 
i 
T 
1 
1 
-i 
i 
1 
1 
1 
1 
2 
1 
1° 
1 
3 
-i 
1 
1 
1 
2 
2 
1 
1 
2 
L.HYP. 12° 
1 
1 
1 
2° 
1 
1 
3 
-i 
i 
1 
2 
I 
2 
1 
1 
3° 
1 
1 
1 
1 
1 
4° 
1 
2 
1 
1 
5°4O3O2° 
AMOUNT OF 
CHANGE TO 
E50PH0RIA 
1° 
0° 1° 2° 
NO 
CHANGE 
3°4° 5° 6° 7° 8° 9° IO°JI° I3°I6° 
INCREASE IN EXOPHORIA 
- Decrease in exophorfa. 
Of 104 cases of pure exophoria at the pre-occlusion 
test- 
Exophoria increased 1° to 16° in .. 95 cases or 91.3 per cent. 
No change in 6 " " 5.6 " " 
Decreased in 3 " " 2.9 " " 
Hyperphoria of %° to 4° developed 
in 85 " "81.7 " 
(R. hyp. 29.) 
(L. hyp. 56.) 
In pure exophoria the final exophoria is almost always in 
excess of the primary measurement, and hyperphoria is 
present in 81.7 per cent, of the cases, L. being nearly twice 
as common as R. hyperphoria. 
Summary of Table 2. 30 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 3. 
Cases with exophoria and R. hyperphoria before. 64 
cases. Amount of increase in exophoria and changes in 
hyperphoria. 
5° 
1 
DECREASE OR 
INCREASE IN 
R. HYP. 
4° 
3° 
1 
1 
1 
? 
1 
-2 
3 
1 
l 
Ii° 
-1 
1° 
-2 
1 
1 
1 
2 
>2 
1- 
1 
1 
i* 
2 
-2 
1- 
1 
I 
1 
1 
1- 
I- 
NO. CHANGE 
2 
1 
-1 
1 
3 
2 
1 
1 
xo 
2 
AMOUNT OF 
1° 
1 
1 
CHANGE TO 
lz° 
1 
2 
1 
I. HYP. 
2° 
1 
2 
2 
3° 
1 
4° 
1 
5° 
1 
1 
0° 
1° 
2° 3° 4° 5° 
6 
7° 
8° 
9° 10° 11° 12° 13° 14° 15° 16° 17° 18° 19° 
Change in amount of exophoria. 
- Decrease in hyperphoria. TABULATION AND ANALYSIS OF CASES. 
31 
Summary of Table 3. 
Of 64 cases of R. hyperexophoria at the pre-occlusion 
test- 
Exophoria increased 1° to 14° in ... 56 cases or 87. per cent. 
No change in 7 " "11. " " 
Decreased in 1 " " 1.5 " " 
Hyperphoria increased %° to 5° in ..25 " " 39. " " 
No change in 12 " " 18.5 " " 
Decreased in 12 " " 18.5 " " 
Changed to L. in 15 " " 23.5 " " 32 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 4. 
Exophoria with L. hyperphoria before. 99 cases 
Amount and kind of change after. 
6° 
1 
5° 
AMOUNT OF 
4° 
CHANGE TO 
R. HYP. 
3° 
1 
2° 
2 
1 
1 
li° 
1 
1 
1° 
1 
NO CHANGE 
0° 
1 
-2 
2 
3 
2 
1 
1 
1° 
-l 
1 
3 
-l 
-i 
-1 
1® 
-3 
1 
-1 
-1 
1° 
1 
-a 
1 
i 
2© 
1 
1 
2 
-1 
i 
-i 
i 
-i 
CHANGE IN 
L.HYP. 
l£° 
2 
2 
4 
2 
1 
2° 
2 
-i 
O' 
2 
2 
2 
1 
1 
1 
3° 
1 
1 
|Q 
2 
1 
1 
1 
1 
1 
1 
4° 
1 
1 
J 
1 
5° 
1 
1 
1 
6° 
7° 
8° 
1 
1 
9° 
1 
10° 
11° 
1 
0° 
i°2° 
3° 
4° 5° 6° 7° 8° 
9° 
10° 11° 12° 13° 
26° 
Change in exophoria. 
- Decrease in hyperphoria. 
(-) Decrease in exophoria. 
(=) Decrease in both. 
No mark = increase in both. TABULATION AND ANALYSIS OF CASES. 
33 
Summary of Table 4. 
Of 99 cases of L. hyperexophoria at the pre-occlusion 
test- 
Exophoria increased in, 84 cases or 84.8 per cent. 
Decreased in 6 " " 6. " " 
Remained unchanged in 9 " " 9. " " 
Hyperphoria increased in 59 " "59. " " 
Decreased in 18 " "18. " " 
Changed to R. in 9 " " 9. " " 
Unchanged in 13 " "13. " " 
i.e., 22 per cent, changed from L. to R. hyperphoria. 
In 163 cases of hyperexophoria, 99 are L. hyp. previ- 
ous, and 105 after the test; 15 cases having changed 
from R. to L. and 9 from L. to R. The exophoria has 
changed to almost the same degree as in pure exophoria. 34 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 5. 
R. hyperphoria. No lateral deviation before. 74 cases. 
Changes after. 
3° 
1 
2° 
1 
1 
1 
2 
-1 
1 
1 
CHANGE IN |£° 
1 
-1 
1 
1 
R. HYP. |0 
2 
3 
-1 
1 
2 
1 
1 
2 
1 
-1 
-i 
i 
-1 
-3 
-2 
-1 
NO changeO0 
2 
1 
5 
1 
1 
2 
1 
i° 
1° 
AMOUNT OF ' 
CHANGE TO |z 
1 
1 
3 
2 
1 
L.HYP. 2° 
1 
1 
1 
1 
1 
2 
1 
3° 
1 
4° 
2 
5° 
6° 
3° 2° 1° 0° 
E50. 
1° 
2°3°4° 
EXO. 
5° 6° 7°8° 9° 10° 11° 12° 13° 14° 
- Decrease in hyperphoria. TABULATION AND ANALYSIS OF CASES. 
35 
Summary of Table 5. 
Of 74 cases of pure R. hyperphoria at the pre-occlusion 
test- 
Exophoria of 1° to 13° developed 
in 65 cases or 88. per cent. 
Esophoria of 1° to 3° developed in .. 3 " " 4. " " 
No lateral deviation change in .... 9 " " 12. " " 
R. hyp. increased to 3° in 29 " " 39. " " 
Decreased W to 2° in 14 " "19. " " 
Changed to L. in 19 " " 25.5 " *' 
No change in 13 " " 17.5 " " 
R. hyperphoria increased or remained unchanged in 
54% per cent., decreased or changed to L. in 44% per 
cent., and a lateral deviation developed in 92 per cent, of 
the cases. Exophoria in 88 and esophoria in 4 per cent. POSITION OF REST AND OCCLUSION TEST. 
36 
TABLE 6. 
L. hyperphoria. 72 cases. No lateral deviation before. 
Changes after. 
AMOUNT OF 2° 
2 
1 
CHANGE TO |£° 
1 
1 
I 
E.HYP. |o 
1 
1 
1 
4° 
NO CHANGE 0° 
1 
2 
5 
1 
2 
1 
i° 
-1 
2 
-1 
-1 
f 
1 
-1 
1 
1 
1° 
3-< 
-1 
2 
2 
-1 
1 
2 
1 
INCREASE IN l*° 
1 
1 
-1 
1 
2 
L.HYP. 2° 
1 
1 
1 
2 
1 
1 
3° 
1 
1 
4° 
2 
2 
1 
5° 
1 
1 
1 
6° 
1 
7° 
1 
8° 
-1 
9° 
10° 
1 
8° 7° 6° 
E50. 
504030 
2° 
1° 
0° 
1° 
2° 
3040 5o6o 
EXO. 
7° 
8° 9° 
10° 11° 12° 13° 
- Decrease In hyperphoria. TABULATION AND ANALYSIS OF CASES. 
37 
Summary of Table 6. 
Of 72 cases of pure L. hyperphoria at the pre-occlusion 
test- 
Exophoria of 1° to 12° developed 
in 48 cases or 66% per cent. 
Esophoria developed in 8 " "11% " " 
No lateral deviation developed in .. 16 " " 22% " " 
L. hyperphoria increased to 10° 
in 42 " " 58% " " 
Hyperphoria decreased 1° to 2° in.. 9 " " 12% " " 
Did not change in 12 " " 16% " " 
Changed to R. hyp. in 9 " "12% " " 
In 77 per cent, of pure hyperphoria, exophoria i° to 130 
is latent. In about 20 per cent, the hyperphoria is reversed, 
this happening twice as frequently to R. as to L. hyperphoria. 
This would suggest that it is distinctly safer to prescribe 
prisms for a L. than for a R. hyperphoria, if no occlusion 
test be made. 
L. hyperphoria increased or made no change in 75 per 
cent., decreased or changed to R. in 25 per cent. 38 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 7. 
Esophoria with no hyperphoria before. 55 cases. 
Amount of decrease in esophoria. Amount of change to 
exophoria after. Amount and kind of hyperphoria added. 
3° 
1 
2° 
1+ 
2 
1 
1 
I 
AMOUNT OF H° 
1 
2 
R.HYP. |° 
,+i 
1 
2 
1 
1 
i° 
a 
1 
1 
0° 
1 
1 
2+ 
l+2 
2 
1 
l 
1 
l 
±0 
a 
1 
1 
1 
1° 
1 
2 
2 
1+ 
1 
1 
AMOUNT OF li° 
1 
1 
L.HYP. 2° 
1 
1 
1 
1 
3° 
1 
1 
4° 
1 
1 
12° 11° 10° 9° 8° 
E50. 
7° 6° 5° 4° 3° 
2° 
1° o° 1° 2°3° 
NO 
CHANGE 
4° 
5° 6° 7° 
EXO. 
8° 
9° 10° 
+ Signifies increase in esophoria. TABULATION AND ANALYSIS OF CASES. 
39 
Summary of Table 7. 
Of 55 cases of pure esophoria at the pre-occlusion test- 
Esophoria increased 1° to 12° in ... 7 cases or 12+per cent. 
Decreased 1° to 6° in 21 " " 38 " " 
Did not change in 6 " "11 " " 
Changed to exophoria 1° to 10° 
in 21 " " 38+ " " 
R. hyperphoria developed 1° to 3° 
in 21 " " 38+ " " 
L. hyperphoria developed 1° to 4° 
in 21 " " 38+ " " 
O. hyperphoria developed in 13 " " 23+ " " 
Primary measurements of esophoria exaggerate it in 
77 per cent, and in one-half that percentage there is a 
latent exophoria. Hyperphoria of from i° to 40 is present 
in 77 per cent, of the cases. (80 per cent, in pure exo- 
phoria.) 40 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 8. 
Esophoria with R. hyperphoria before. 44 cases. 
Changes after. 
4° 
2 
1 
CHANGE IN30 
-1 
R. HYP. 2° 
1 
14° 
1 
1+ 
1 
1° 
1 
1 
2 
-1 
-1 
2° 
1 
-1 
-2 
-1 
-a 
i 
-1 
-1 
NO CHANGEO0 
1+ 
1+ 
1 
1 
1 
1 
1 
1 
±o 
2 
1° 
2 
IF 
1 
1 
2° 
1 
2 
1 
1 
AMOUNT OF3° 
1 
CHANCE 4° 
1 
• ° co 
L HYP 0 
unir. 
1 
12° 10° 9° 
8° 
7° 6° 5° 
4° 
3° 
2° 
1° 
0° 
1° 
2° 
3° 
4° 
5° 
6° 
7° 8° 
9° 10° 11° 
Decrease in exophoria 
Amount of change to exo. 
- Decrease in hyperphoria. 
+ Increase in esophoria. 
No mark - decrease in esophoria. TABULATION AND ANALYSIS OF CASES. 
41 
Summary of Table 8. 
Of 44 cases of R. hyperesophoria at the pre-occlusion 
test- 
Esophoria increased in 4 cases or 9. per cent. 
Decreased in 17 " "39. " " 
Did not change in 4 " " 9. " " 
Changed to exoph. 3° to 10° in ... 19 " " 42.+ " " 
Hyperphoria increased J4° to 4° in 12 " " 30. " " 
Decreased in 11 " " 25.5 " " 
Did not change in 8 " " 16.+ " " 
Changed to L. 1° to 6° in 13 " " 28.+ " " 
Primary esophoria is exaggerated in 8i per cent., in 
one-half that percentage masking exophoria of from 3° to 
u°. The hyperphoria decreased, or changed to L., in 54 
per cent. Therefore, in R. hyperesophoria there is a 
tendency to exaggerate the hyperphoria. 42 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 9. 
Esophoria with L. hyperphoria before. 34 cases, 
Changes after. 
2° 
AMOUNT OF .% 
CHANGE TO '2 
R. HYP, 1° 
±0 
2 
NO CHANGE 0° 
10 
a 
AMOUNT OF 1° 
CHANGE IN |j° 
L.HYP. 2° 
3° 
4° 
5° 
6° 
7° 
1 
1 
2 
1 
1 
1 
2 
1 
-2 
•M 
-2 
1 
-1 
-'1 
1 
2 
1 
1 
1 
"I 
1 
1 
|+ 
1 
1 
12° ~ 4°3°2° 1° 0° l°2° 3° 4° 5° 6° 7° 
CHANGE IN NO AMOUNT OF CHANGE 
ESOPHORIA CHANGE TO EXOPHORIA 
- Decrease in hyperphoria. 
+ Increase in esophoria. 
No mark = Increase in hyperphoria. 
No mark = decrease in esophoria. TABULATION AND ANALYSIS OF CASES. 
43 
Summary of Table 9. 
Of 34 cases of L. hyperesophoria at the pre-occlusion 
test- 
Esophoria increased in 2 cases or 5.7 per cent. 
Decreased in 14 " "40. " " 
Did not change in 4 " "11. " " 
Changed to exophoria in 14 " "43. " " 
L. hyperphoria increased in 15 " "43. " " 
Decreased in 8 " "23. " " 
Did not change in 6 " "20. " " 
Changed to exophoria in 5 " "14. " " 
Primary esophoria exaggerated in 83 per cent, and in 
one-half that percentage masking an exophoria from i° to 
70. Tables show a less tendency for L. to change to R. 
hyperphoria than the reverse, 37 per cent, against 54. POSITION OF REST AND OCCLUSION TEST. 
44 
Table No. 10 summarizes the general results of the test, 
showing the relative frequency of the different forms of 
heterophoria before and after it, and also the changes oc- 
curring in each form. Thus, there were 154 cases of orth- 
ophoria before the test, and only 24 after it. Eleven of 
BEFORE OCCLUSION AFTER OCCLUSION 
No.OF CASES 
zz 
UJ 
cy 
llJ 
Q. 
ORTHO. 
EXO. 
R.HYP.EXO. 
6 
<n 
Ld 
d 
<n 
u_! 
X 
a 
Cl 
X 
O' 
A 
ORTHO. 
154 
22 
13 
21 
37 
60 
5 
1 
3 
5 
9 
EXO. 
104 
14.9 
0 
19 
28 
54 
O 
l 
2 
o 
O 
R..Hyp. EXO. 
64 
9.1 
0 
10 
39 
15 
O 
0 
0 
o 
0 
L. " 0 . 
99 
14.1 
1 
9 
9 
80 
o 
0 
O 
o 
o 
E5O. 
55 
7.9 
2. 
4 
8 
9 
7 
13 
12 
0 
o 
R.HVP. ESO. 
44 
6.2 
4 
7 
6 
6 
2 
13 
G 
0 
0 
L. " 11 
34 
4.9 
2 
2 
1 
II 
2 
4 
II 
o 
1 
R.nyp. 
74 
10.5 
O 
10 
35 
17 
0 
2 
1 
8 
1 
L.Hyp. 
72 
10.2 
2 
4 
8 
35 
o 
1 
7 
o 
15 
NO.0FCA5E5 
700 
100 
24 
86 
171 
287 
16 
35 
42 
13 
26 
PERCENT (AFTEROCCLUSION) 
3.4 
12.2 
24.4 
41 
2.2 
5. 
6. 
1.8 
3.7 
TABLE 10. 
these 24 had, however, shown some form of muscle im- 
balance (esophoria in the large majority) previous to the 
test, so that of the 154 cases of pre-occlusion orthophoria 
only 13 remained orthophoric at the end, the large majority 
developing hyperexophoria. The percentage of orthophoria 
falls from 22 before the test to 3.4 afterwards. Similarly 
esophoria almost disappears from the list, being reduced 
from about 8 per cent, to 2.2 per cent. The large majority 
of cases of apparent esophoria must be cases of pseudo- 
esophoria. TABULATION AND ANALYSIS OF CASES. 
45 
Pure exophoria falls from 14.9 per cent, to 12.2 per 
cent., and pure hyperphoria from 20.7 per cent, to 5.5 per 
cent. In other words, the deviating tendencies are more 
complex than they appear to be, the apparently pure lateral 
deviations having a latent hyperphoria, the apparently pure 
hyperphorias having a lateral deviation, usually exophoria. 
It is noteworthy that 574 cases or 82 per cent, showed 
hyperphoria at the final test, instead of 55 per cent, previ- 
ous to it, cases of L. hyperphoria being 60 per cent, more 
numerous than those of R. hyperphoria. The percentage 
of cases of exophoria also rose from 38 to 77. The rela- 
tive frequency of exophoria and hyperphoria will be re- 
ferred to later. 
It must be re-emphasized that these figures and per- 
centages apply only to cases in which there are a priori 
reasons for suspecting the existence of a muscle error. 
If Bielschowsky's definition of the relative position of 
rest be accepted, and no greater criticisms can be made of 
the prolonged occlusion method than those already ad- 
vanced, and if the results tabulated above be conceded to 
be an approximation to the truth, which I see no reason 
to doubt, it must be admitted that tests for the muscle 
balance as ordinarily made leave the observer very much 
in the dark as to the degree of error present, and often 
mislead him as to its character. Consequently, the neces- 
sary correct data upon which alone a satisfactory diagnosis 
and therefore, therapeutic measures, can be based, are 
lacking in a very large percentage of cases. 
This goes far to explain the unsatisfactory results com- 
monly experienced in the treatment of these cases, a situa- 
tion so unsatisfactory that so careful and experienced an 
observer as E. C. Ellett says: 
"My personal experience is that the vast majority 
of cases of eye-strain are due to errors of refraction, 46 
POSITION OF REST AND OCCLUSION TEST. 
and not to errors of muscle balance. From the point 
of view of the results obtained and the methods by 
which they were obtained, I am sure that most of 
the time relief has followed the use of glasses, and 
when the glasses did not give the desired relief, no 
amount of muscle treatment by exercise, prisms or 
operation has given any considerable assistance." 
(Journal of The Tennessee State Medical Association, 
Feb., 1920.) 
The method is free from the objections which are some- 
times urged against the use of prisms either for diagnosis 
or treatment, being entirely passive in its mode of action. 
It is, of course, true that the wearing of prisms to cor- 
rect a portion of a latent deviation is followed in many 
cases by the manifestation of a higher degree of error, but 
it is also true that such manifestation occurs with greater 
rapidity and completeness under prolonged occlusion. 
It seems desirable, therefore, to emphasize the great dif- 
ference in the point of view brought about by the test and 
to consider the difference in the treatment which would 
be recommended before and after occlusion. The methods 
of dealing with these cases, aside from those concerning 
general health are three in number. (1) Exercises with 
prisms to strengthen the weaker muscles, (2) The con- 
stant wearing of prisms to diminish the work of the 
weaker muscle and (3) Operation to bring the visual lines 
nearer to parallelism. 
First as to exercises-I have rarely seen any material 
or prolonged benefit from any form of prismatic exercises 
in my own practice, and in cases reported to me by patients 
previously seen by other men there has usually been no, 
or only temporary, improvement. Moreover, it must not 
be forgotten that no method of treatment carries with it TABULATION AND ANALYSIS OF CASES. 
47 
so large an element of suggestion as this. The patient 
sees two lights combining with greater facility and with 
stronger prisms from day to day, and naturally thinks that 
he is better, and it is unnecessary, in these days when 
psychic influences are recognized to play so large a part 
in matters of health, to lay any further stress upon this 
point. 
Moreover, part of the apparent improvement is due to 
the patient's learning what may be called the knack of 
overcoming diplopia. An act essentially involuntary be- 
comes partly voluntary. 
Furthermore, even when improvement seems to have 
taken place in the muscle balance, as shown by a diminu- 
tion in the degree of heterophoria, the symptoms are not 
always relieved, may indeed be aggravated, and an oc- 
clusion test may show that the improvement is apparent 
and not real, as the following cases show. 
Mrs. L. E., age 35, came under observation in 1896, 
suffering from asthenopia and headaches. Examination 
revealed astigmatism of high degree and exophoria of 40. 
She was given a full correction for her astigmatism and a 
i%° prism base in, in each lens. She remained comfort- 
able until 1912, when her symptoms recurred. She then 
showed exophoria of 8° and the prisms were increased to 
2%° each eye. This failed to give the desired relief and 
in 1913 she showed an exophoria of 90. Another change 
failed also to give relief, and she consulted another ophthal- 
mologist, who prescribed prism exercises for distance, 
which she carried out for some months, without benefit. 
I saw her again in 1914 and found her exophoria reduced 
to 30 for 6 m. 140 to 16° at 1 /3 m. She was instructed to 
add exercises for the near point, but derived no benefit 
therefrom. A week's occlusion test brought the exophoria 48 
POSITION OF REST AND OCCLUSION TEST. 
up to 8°. Tenotomies (partial) of the externi gave a 
high measure of relief. (See Chart IV.) 
Mrs. M. F. W. (see Chart II), came under my obser- 
vation at 39 years of age on account of intense sick head- 
aches and other symptoms, which were thought by her 
physician to be of ocular origin. Twenty years previously 
an exophoria of 20 ° had been diagnosed by a well-known 
New York ophthalmologist. A long course of prism exer- 
cises was carried out, apparently reducing the amount of 
error and finally a 20 prism base in, over each eye, was 
prescribed for constant wear. The patient thought at 
the time that she received some benefit, but on looking 
back does not think the improvement amounted to much, 
and at best was temporary only. 
When first seen by me, she showed an exophoria of 70, 
which diminished to 20 under cycloplegia. A four day oc- 
clusion test brought the exophoria up to 120, and a second 
test of 11 days brought it up to 190, or within one degree 
of the original measurement. 
Tenotomies of the externi, her p.p.c. being quite normal, 
effected a great improvement in her condition. 
Moreover, if these errors are anatomical and congenital, 
as no doubt a very large majority of them are, it is quite 
unreasonable to expect that the position of rest can be 
changed by any amount of exercise, certainly not by that 
ordinarily prescribed, especially when consideration is 
given to the fact that the nature of the case itself has 
compelled constant exercise of the same muscles for the 
purpose of maintaining single V., from a very early period 
of life. The small therapeutic addition would appear to 
be negligible. From the standpoint of this test, the change 
in position of rest caused by exercises is apparent and not 
real. TABULATION AND ANALYSIS OF CASES. 
49 
CHART IV. 
q° 
1896 1912 1913 1914 
8° 
7° 
6° 
5° 
4° 
3° 
2° 
1° 
0° 
*6 
o 
n / 
i 
*> / 
I 
V 
ui \ 
J) \ 
Q 
O / 
4 
4 
V 
ct 
u 
s 
\ > 
o / 
if \ 
/ 
y 
t 
A 
k / 
X I / 
0-\ / 
MRS. L. E. 23 - 41 CASE NO. 35 193 50 
POSITION OF REST AND OCCLUSION TEST. 
Exercises for convergence insufficiency with a white 
pin head or a black dot on a white card have resulted only 
exceptionally in an approximation of the near point of 
convergence. 
The use of prisms for constant wear has been consid- 
ered undesirable by Duane and others, especially in lateral 
deviations. They are said to increase the amount of error, 
and to weaken instead of strengthen the muscles. I have 
seen no reason, in cases coming under my observation, to 
attribute any importance to these considerations. There 
are obvious and unavoidable limitations to the use of 
prisms for constant use, the chief one being that 
they cannot be worn for the relief of one form of 
deviation without putting the eyes out of their normal 
alignment with the muscles acting at right angles to that 
deviation. Thus, if prisms are worn for a lateral devia- 
tion the visual lines lose their normal relation to the verti- 
cally acting muscles and a new source of strain is intro- 
duced. The practical effect of this varies with different 
individuals, some tolerating much higher degrees of pris- 
matic correction than others. The dispersal of light by, 
and the weight and appearance of, strong prisms constitute 
other but less important objections. 
The increase in the measure of the deviation following 
the constant use of prisms is better explained by the 
gradual manifestation of the latent error, and continues 
only until the latter error becomes completely manifest. 
A far more rapid manifestation of the error can be brought 
about by a prolonged occlusion test, which is free from 
the objection which may be raised against the possibly 
active effect of prisms. I have been in the habit of pre- 
scribing prisms for many years, and have yet to see a 
case in which the apparent increase of error could be bet- 
ter explained by the latter than by the former theory, TABULATION AND ANALYSIS OF CASES. 
51 
and in numerous cases no increase at all has taken place. 
Briefly, the claim is made that the prolonged use of prisms 
causes an actual increase in the error, i.e., changes the 
anatomical position of rest. If this contention were true, 
it would place at our disposal a most effective means of 
curing these deviations. The problem of treatment would 
indeed be solved. But so far, no satisfactory evidence 
has been brought forward in favor of this position. What 
is commonly called the "Therapeutic Test," which is an- 
other variant of the old proverb that "the proof of the 
pudding is in the eating", also can be utilized to furnish 
evidence as to the value of prisms for constant wear. 
There are a great many cases, among which I must in- 
clude my own, in which the patient obtains no, or very 
incomplete, relief until the faults in the muscle balance are 
detected and a prismatic correction included in the formula, 
and in a very large number of these cases the relief af- 
forded is just as satisfactory and as permanent, as that 
which is attributed to the correction of hypermetropia and 
astigmatism. 
The usefulness of prisms for constant use is limited for 
the reasons just stated to the lower degrees of error, the 
exact limits of which cannot be defined on account of in- 
dividual variations of tolerance. (Further and more 
definite suggestions on the prescription of prisms will be 
made in the section on Treatment.) 
By far the most important point in connection with 
the constant use of prisms and the occlusion test is, that 
the latter opens up what is practically an entirely new field 
for this method of treatment. Cases appearing by the 
ordinary tests as orthophoria or low degrees of hetero- 
phoria are shown to have degrees of error far greater 
than that shown by the primary tests, and consequently 
prisms can be prescribed in cases in which, without this 52 
POSITION OF REST AND OCCLUSION TEST. 
test, no indication for their use would be apparent. More- 
over, as already suggested, cases measured in the ordinary 
way and dealt with by the constant use of prisms have er- 
rors often really of such high degree that the prisms pre- 
scribed correct too small a fraction of the total error to have 
any effect: hence the failure to give satisfactory or per- 
manent relief. 
As regards operative treatment it is obvious in the first 
place that the kind and degree of error, and if possible 
the particular muscle or muscles at fault must be known 
if the operation is to be anything more than a haphazard 
procedure, and a little experience with the prolonged oc- 
clusion test will show that none of these factors can be 
estimated with sufficient precision without it. The muscle 
balance after operation also cannot be determined without 
it. These two considerations, ignorance of the condition 
previous to operation and ignorance of the effect of the 
operation go far to explain the ill repute into which these 
operations have fallen. 
The great diversity of opinion as to the effect of a 
partial tenotomy is to be accounted for in the same way. 
As is well known the degree of error is often apparently 
unchanged. If, for example, we consider the case of Miss 
M. N. (Chart V), who showed at her first examination 
4° of exophoria, which came up to 8° after prismatic 
measurement of her abduction, we shall note certain points 
of great practical importance. A week's occlusion raised 
the exophoria to 30°. If previous to occlusion partial 
tenotomy had been done, aiming at an 8° correction, dis- 
appointment would have been inevitable, both as regards 
the effect on the symptoms, and on the degree of error. 
No doubt an amount equal to that first noted would soon 
have become manifest, and the conclusion that partial ten- 
otomy has no effect would appear to be justified. Similarly TABULATION AND ANALYSIS OF CASES. 
53 
1920 
APRIL MAK 
19 20 212223242526 II 12 13 1415 16 17 1819 20 
Si 
31' 
30' 
29° 
28° 
27° 
26° 
25° 
24° 
23° 
22* 
21° 
20° 
19° 
18° 
17° 
16° 
15° 
14° 
13° 
12° 
11° 
10° 
9° 
8° 
7° 
6° 
5° 
X=EX0PH.4° 
3° 
2° 
1° 
ORTHO. 0° 
1° 
0L.HYPERPH20 
3° 
4° 
5° 
6° 
- 1 u 
*- Q 
r 2 
C > UL 
t 
co 
a.- 
T/T" 
=□ 
o 
Z 
CC 
UJ 
-!*» 
X 
UU 
tu 
h- 
Ol 
uu 
-J 
r 
Q 
LU 
c 
1 
| 
txx~ 
z 
z 
o 
o 
ID 
X 
CD 
c 
3 
o 
v 
Sn 
' 2 
c 
r 
M 
6 
z. 
2 
H 
oc 
z 
z-1 
o 
Q 
F- 
-1 
z 
o 
o 
bs 
LU 
CQ 
xT 
04 
LU 
LU 
z: 
_ t 
3r 
U 
>_ 
r 
J 
g 
o 
X- 
3 
-J 
o 
c- 21 
8/ 
3 
Ud 
t 
CHART V. 54 
POSITION OF REST AND OCCLUSION TEST. 
had prisms been prescribed for constant wear, disappoint- 
ment would have ensued as to the relief of symptoms, and 
the error would have appeared to increase as a direct 
result of their use. These disappointments and erroneous 
conclusions can be avoided, as in this case, by a prolonged 
occlusion test. 
The case of Mrs. C. A. T. shows what may underlie an 
apparently perfectly successful operation. 
She was the subject of sick headaches of the most in- 
tense type, and eighteen years before I saw her con- 
sulted an ophthalmologist of national reputation who made 
an operative correction of her heterophoria, without giv- 
ing any relief from her symptoms. This was in 1898. 
When I saw her in 1916, examination showed no lateral 
deviation and L. hyperphoria %°. Under cycloplegia, the 
R. eye was emmetropic and the L. hypermetropic 0.50 D., 
and the muscle condition was Ex. i°, L. hyperphoria i°. 
An 8-day occlusion test brought the exophoria up to 140, 
and the L. hyperphoria to 4U>°. 
On inquiry I learned that before operation she had an 
esophoria of 70, and L. hyperphoria i%°, and that opera- 
tions were done on an internus and one of the vertical 
muscles. 
The operative result was apparently good. 
In the light of the occlusion test there must be some 
doubt as to whether this patient had originally an eso- 
phoria or not, and also what change was affected by 
operation. 
I append here a chart of a case showing the effect of 
partial tenotomies, as demonstrated by occlusion tests 
before and after operation. 
What has been said of the constant use of prisms and 
the new field opened up for their use by the "Occlusion 
Test," applies equally emphatically to operative treatment. TABULATION AND ANALYSIS OF CASES. 
55 
It becomes obvious after the occlusion tests that operation 
is the only means of relief in many cases, in which the pre- 
occlusion test would not indicate it at all. 
CHART VI. 
1919 
DEC. 
15 16 17 18 19 20 21 22 
921 1922 
MARCH APRIL MAY JUNE JULY NOY. FEB. 
30 28 2 9 30 1 2 3 4 5 6 9-16-25 2519 11-21 6 
12" 
11" 
10° 
9° 
8° 
7° 
6" 
5' 
4" 
y 
2° 
1° 
0° 
1° 
2° 
3' 
4° 
5° 
6° 
7° 
8° 
ll°- 
10° 
9° 
8° 
7° 
6° 
5° 
4' 
3° 
2' 
1° 
0° 
1° 
2° 
3° 
4° 
5° 
.6° 
7° 
QO 
ro 
X 
a 
z 
c 
z 
co 
o 
u-> 
=5 
a 
LU 
CD 
§ 
co 
5 
a. 
CZ 
o 
- 
/ 
f-o- 
o 
Z 
a 
UJ 
c» 
LU 
o 
cn 
c 
■Qi* 
S 
L-J 
zz 
LU 
z 
tn 
UJ 
V_j 
£ 
b- 
z 
o 
s. 
O 
LU 
LU 
-r 
r. 
>- 
Vs 
g 
J 
O 
a 
z 
z 
z 
h- 
t- 
-J 
H- 
UR 
U-L 
HYF 
-1° 
- r 
DR 
QI 
-3* 
■ y 
£ 
O' LINE « ORTHOPHORIA REFRACTION' DEC. 1919 
X ABOVE O'LINE "EXOPH R*O. 12= 35° 
• BELOW '• " = E50PH L»O. IZC 35' 
0 ABOVE •• - "R.HYPERPH SEP. 1921 
• BELOW - -I - RtO. 75s 
tO. 12 = 35' 
L+O. 75s 
♦0. 50' 170' CHAPTER VI. 
MUSCLE BALANCE IN RELATION TO REFRACTION. 
A correlation of the different forms of post-occlusion 
heterophoria with those of ametropia results as follows: 
The cases have been arranged in three groups. 
(1) Those in which the refraction was hypermetropic. 
(2) Those in which it was myopic. 
(3) Those in which mixed astigmatism was present. 
TABLE 11. 
HY. AND MY. AND MIXED 
HY.ASTIG MY. ASTIG ASTIGMATISM 
ORTHO. 
4. P.C. 
2.9 P.C. 
4.4 P.C. 
EXOPH. 
"•6177 
16 5 La r 
I6'66b™ 
HYPER. EX. 
65.4J77 
Or.J 
68.00 
57.2 J™ 
E50. 
2.2 
1.9 
1.20 
HYPER. ES. 
8.8 
3.9 
6.00 
HYP. 
8.8 
68 
14.40 
These figures cannot be said to have any value in con- 
firmation of the theory, that the form of heterophoria has 
any relation to the kind of refraction present. The per- 
centages of heterophoria vary but little in the three groups. 
They show a slightly greater incidence of exophoria in 
myopia than in hypermetropia, but the difference is dim- 
inished if the cases of mixed astigmatism, which are es- 
sentially cases of incipient myopia, are added to those of 
myopia. Then we find the muscle balance to be exophoric 
in 77 per cent, of the hypermetropic cases, and 79 per cent, 
of the myopia cases, a difference quite negligible. 
56 MUSCLE BALANCE AND REFRACTION. 
57 
The incidence of esophoria of all kinds in hypermetropia 
is ii per cent., in myopia and mixed astigmatism 6% per 
cent., but as the total number of cases of myopia and mixed 
astigmatism is small (187) compared with those of hyper- 
metropia (500), inferences drawn from these percentages 
may be misleading. 
The incidence of pure hyperphoria in myopia and mixed 
astigmatism is slightly greater than in hypermetropia, 10.6 
per cent, in the former, 8.8 per cent, in the latter, and when 
all hyperphorias are considered we find 83 per cent, in 
hypermetropia, and 78.7 per cent, in myopia, a difference 
without significance. On the other hand the absence of 
difference is significant because it suggests that hyperme- 
tropia may diminish and finally change to myopia without 
change in the average muscle balance. As one would ex- 
pect upon evolutionary or developmental principles, exo- 
phoria is the commonest and esophoria the rarest in all 
forms of refraction. 
If it be borne in mind that almost all eyes are hyper- 
metropic or hypermetropic and astigmatic at birth, and 
that myopia is an acquired condition it will be seen that 
the tables quite harmonize with the supposition that hetero- 
phoria is of congenital origin. 
We see cases of hypermetropia passing through what 
Risley called the "Turnstile of Astigmatism" into myopia, 
but this transformation is accompanied by no change in 
the position of rest. In other words what the position of 
rest was in hypermetropia it remains when the refraction 
changes from hypermetropia to myopia. 
Variation is a well-recognized law of nature, and it 
would be quite unreasonable to expect that the ideal would 
be attained in the matter of the relative position of rest 
any more frequently than in other characters. Attainment 58 
POSITION OF REST AND OCCLUSION TEST. 
most frequently falls short of the ideal, occasionally 
reaches it, and occasionally overshoots it. 
Those who regard the various forms of heterophoria 
as acquired conditions developing out of an original or 
congenital conditon of orthophoria, necessarily assume a 
degree of perfection or uniformity in the binocular func- 
tion, rarely if ever attained in any other character, physi- 
cal or mental. To emphasize this point it is only necessary 
to refer to the color sense and the sense of musical pitch. 
Aside from well-recognized degrees of color blindness it 
is well known that the color sense of average normal in- 
dividuals varies enormously, and that the ability to recog- 
nize and remember the pitch of musical tones varies simi- 
larly. These are simply variations in the perfection of 
nature's attainment. They are only two examples of the 
well-known law of variation. There is no justification for 
the assumption that nature will furnish each or any in- 
dividual with orthophoria as a basic condition at birth. CHAPTER VII. 
AGE, HEREDITY AND EVOLUTION. 
The ages of the patients included in this series of cases 
vary from 9 to 78. 
High degrees of error may remain latent until extreme 
old age, as in the case of Mrs. B. Mrs. K. P. B., aged 78, 
was subjected to a prolonged occlusion test because the 
correction of her refraction failed to make her comfortable. 
R. V. 6/9 with + 4.50 Sph. O + 1-37 C 50 
L. V. ®/9 with + 4.00 S. O + 1.00 C 170° 
No lateral heterophoria. Hyperphoria varied from low 
L. to low R. At the end of a seven day occlusion period, 
tests showed L. hyperphoria 50 and exophoria io°. 
That errors in the muscle balance in children may re- 
main latent, until one eye becomes defective from trau- 
matism or disease or has to be occluded by a bandage, is 
well known. 
Table 12 shows the incidence of the various forms of 
heterophoria at different ages before and after occlusion. 
The question whether heterophorias are congenital or 
acquired receives some illumination from an examination 
of this table. 
If congenital they should be as frequent at one age as 
at another. 
If acquired they should be of increasing frequency as 
age advances. 
Particularly it should be true that occlusion would re- 
veal a larger percentage as age advances, but the reverse 
rather is true. 
59 POSITION OF REST AND OCCLUSION TEST. 
60 
In the second decade 32 per cent, were orthophoric 
at the pre-occlusion test, and one per cent, after it. 
In the third decade 25 per cent, were orthophoric 
before, and 2j4 per cent, after. 
In the fourth decade 21 per cent, were orthophoric 
before, and 5 per cent, after. 
TABLE 12. 
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HYPERPHORIA 
A scrutiny of the table will show that the percentages 
of the different forms of error vary but little at different 
ages. 
Comparing the prevalence of all exophorias, the com- 
monest form at the post-occlusion test, at different ages 
we find: 
78 per cent, from 10 to 20 years. 
82% " " " 20 " 30 " 
77 " " " 30 " 40 " 
78 " " " 40 " 50 " 
70 " " " 50 " 60 " 
45 60 " 78 " (20 cases only.) 
Briefly, the table suggests that age has no bearing on the 
prevalence of heterophoria and that its congenital origin 
is the more probable. So far as it goes the table tends to AGE, HEREDITY AND EVOLUTION. 
61 
show a diminishing functional orthophoria with increas- 
ing years, and also a diminishing latency of imbalance. 
That several members of the same family often present 
the same kind of error is a matter of frequent observation. 
A boy, aged 6% years, the subject of ophthalmia nodosa 
complained, after the removal of the bandage worn for 
several days after operation, of diplopia which was found 
to be due to an exotropia of 8°, and a right hypertropia 
of 40, which disappeared within two days after the band- 
age was left off. This boy's mother had been previously 
under my observation for symptoms due to an exophoria 
of 12°, and right hyperphoria of i°. 
The well-known occurrence of exophoria early in life, 
in several members of the same family, and in succeed- 
ing generations, would seem to suggest that it is con- 
genital and anatomical in nature. 
The great prevalence of exophoria as compared with 
esophoria can be best explained by evolutionary considera- 
tions. The visual lines have come up to a relation of 
parallelism from one of divergence, and a reversion to an 
earlier type is more probable than that nature should over- 
shoot its mark. 
Treacher Collins, in his exceedingly valuable and inter- 
esting Bowman Lecture, says: 
"Animals, such as the terrestrial herbivorous mam- 
mals, who require panoramic vision, have their eyes 
set laterally in the head so as to obtain the largest 
circumferential effect of the combined monocular 
fields. Lindsay Johnson (3) measured the divergence 
of the optical axes in a large number of representa- 
tive animals, and constructed a diagram graphically 
. showing how it varies in the different natural orders, 
families, genera and species. 62 
POSITION OF REST AND OCCLUSION TEST. 
"This movement forward of the optical axes to- 
ward parallelism is clearly in the interests of binocular 
vision, and at the sacrifice of range of simultaneous 
circumferential vision. As ontogeny is a condensed 
recapitulation of phylogeny, it is interesting to note 
that in the human embryo the optic vesicles when first 
formed are directly opposite to one another from 
which position they gradually turn forwards, so that 
at the third month of fetal life the optic axis of each 
eye diverges 45° from the middle line; before birth 
they become parallel." 
Just as few eyes attain that ideal refractive condition 
known as emmetropia, just indeed as few individuals at- 
tain the ideal in stature, proportion, mentality or any 
other character, so few individuals attain that ideal and 
most desirable perfection of muscle balance-orthophoria; 
the great majority fall short of it and are, therefore, 
exophoric. 
Similarly prism-abduction is probably a survival from 
an earlier evolutionary period in which an active abduc- 
tion served a useful purpose in panoramic or circumfer- 
ential vision, and is therefore essentially a vestigial func- 
tion; an assumption which would explain its great vari- 
ability. The alternative hypothesis that it has been devel- 
oped in response to the necessity of adjusting the visual 
lines to a point beyond infinity is unthinkable. May not 
the frequency of hyperphoria be accounted for in the 
same way? With panoramic vision it cannot be essential 
that the visual lines be in the same horizontal plane. In 
some animals they certainly have the power of moving 
out of it as in the chameleon.* 
curious reptile, Gadow, in the Encyclopedia Britannica says, 
'The right and left eyes are incessantly moved separately from each other AGE, HEREDITY AND EVOLUTION. 
63 
May not sursumduction and deorsumduction also be due 
to the survival of this power of independent movement? 
Sursumduction and deorsumduction apparently serve no 
useful physiological purpose in man and those higher ani- 
mals possessing binocular single vision. In whatever 
position the head is placed, single vision requires that the 
visual lines remain in that plane of the head in which the 
eyes converge for fixation of objects at different distances. 
If one visual line departs from that plane to the slightest 
degree confusion or diplopia must arise. Hence, hyper- 
phoria, sursumduction and deorsumduction also, may repre- 
sent a reversion to an earlier type.* It is perhaps not 
without significance that the percentage of hyperphoria is 
and literally in every direction, up and down, forwards and straight 
backwards, producing the most terrible squinting." And yet "chameleons 
alone, of all reptiles, can focus their eyes on one spot, and conformably 
they alone possess a retinal macula centralis or spot of acutest binoc- 
ular vision." 
* These purposeless conditions and activities, namely exophoria and 
abduction, hyperphoria, sursumduction and deorsumduction may be 
placed in the same class as the persisting functions of the ear muscles 
in some human beings, which are conceded to be vestigial or rudimentary 
in character. "The power of erecting and directing the shell of the ears 
to the various points of the compass, is no doubt of the highest service 
to many animals, as they thus perceive the direction of danger." (C. 
Darwin : The Descent of Man.) Moreover, cases occasionally occur, and 
some are recorded in the literature of the subject, in which there are 
more definite reversions to an earlier evolutionary type. Thus, W. L. 
Phillips, in Am. Med., March 24, 1906, quoted in the Practical Medicine 
Series for 1907, reports two cases showing the preservation of ocular 
function existing normally in some animals. Thus X has V% in each 
eye and can fix with both eyes, and at the same time turn either one out 
and back again to fixation while reading, or converge or diverge both 
simultaneously, etc., without diplopia. The second patient could set type 
with one eye and read proof at one side with the other. 
The writer has under observation a man with divergent strabismus, 
normal vision in each eye, ability to use either eye at will but with 
no convergence power. Two other members of his family are said to be 
similarly affected. 64 
POSITION OF REST AND OCCLUSION TEST. 
very close to that of exophoria at the post-occlusion test, 
85 of the former, 75 of the latter. The causes 
which produced exophoria seem to have been even more 
effective in producing hyperphoria. Presumably the eti- 
ology is the same in both cases, for it would be quite 
superfluous to assume a different causation for each form 
when one will satisfactorily explain both. 
If it be conceded that exophoria and hyperphoria are 
vestigial conditions arising from similar causes then it is 
unnecessary to assume a dependence of one upon the other. 
Consequently, treatment need not be influenced by this 
assumption. The fact that hyperphoria seems in the large 
majority of cases to be due to weakness or insufficiency of 
one or more muscles does not necessarily invalidate this 
theory. It seems entirely improbable that an actual paresis 
should occur in anything like the percentage of cases in 
which hyperphoria can be demonstrated. 
Whatever weight these considerations may possess must 
be placed in the scale against the theory that exophoria 
or hyperphoria is often an acquired condition. 
It seems most probable that a developing exophoria or 
hyperphoria is a latent anatomical error undergoing 
gradual manifestation. 
If exophoria is a congenital anatomical survival from 
an earlier evolutionary period it would seem extremely im- 
probable that exercises of any kind would bring about an 
organic change, and this would be truer as age progresses. 
As already stated, the occlusion test furnishes a ready 
means of testing the actual effect of exercise on the posi- 
tion of rest. A small number of cases have come under 
my observation in which the exophoria has been appar- 
ently diminished by exercise, without, as a rule, any 
improvement in the symptoms. An occlusion test has 
brought out error of about the same degree as that present 
before using the exercises. CHAPTER VIII. 
ABDUCTION* 
Abduction may be defined as the divergence of the 
visual lines from parallelism brought about by placing 
prisms, base in, before the eyes; the measure of it being 
represented by the strongest prisms with which single 
vision can be maintained. 
It is not demonstrable under any condition except this 
artificial one. 
The question is raised elsewhere as to its origin, and 
purpose or utility. 
Stevens has pointed out that a low power of abduction 
does not necessarily exclude the presence of exophoria. 
"It happens not infrequently that with an abduction of 
only 50 there is, as shown by the phorometer, exophoria 
of one or more degrees." This is rather a vague state- 
ment as to the amount of exophoria which may exist 
under such conditions, but presumably the degree of exo- 
phoria would not be expected to exceed the degree to which 
the eyes could be forced outwards by the use of prisms 
placed base in; in fact such a relation of abduction to exo- 
phoria during the same examination is unthinkable. Yet 
observations made as to the degree of abduction before oc- 
clusion and the degree of exophoria after occlusion show 
that the latter not infrequently exceeds the former and 
sometimes very greatly. In other words the spasm of the 
interni brought on by constant efforts to overcome a 
diverging tendency cannot be induced to relax in response 
* Used here only in the sense of prism divergence. 
65 66 
POSITION OF REST AND OCCLUSION TEST. 
to the brief use of abducting prisms, but a prolonged an- 
nulment of binocular fusion will gradually effect this re- 
laxation, raising both the degree of exophoria and that of 
the abduction. Thus, in the most extreme case I have 
seen, in which an exophoria of 40 (8° after testing 
abduction) and an abduction of 130 could be demonstrated 
at the primary examination, a week of occlusion raised 
the exophoria to 30° and the abduction to 310. 
Evidently, then, the pre-occlusion abduction does not 
necessarily bear any quantitative relation to the post-oc- 
clusion exophoria. 
In recording abduction the points at which doubling 
occurs, and fusion is resumed have both been recorded. 
The point at which diplopia occurs (the ascending point) 
has been taken for the purpose of comparison with the 
final exophoria. Had the fusion point (the descending 
point) been regarded as the measure of the abduction, the 
number of cases in which the final exophoria exceeded the 
primary abduction would be far higher. 
Table 13 shows the abduction before and after occlusion 
in 47 cases. In all but four cases there was in increase, 
sometimes a very great one, as from i° to io°, 6° to 130, 
9° to 170, etc. 
In three cases there was no change. 
In one case there was a decrease of i°. 
It is evident that the pre-occlusion tests for abduction 
fail to reveal the total abducting power and are of low 
value. As a guide to the possible state of equilibrium, 
they are of still less value. 
These observations raise two questions: 
(1) Which is to be the measure of abduction, the 
doubling point or the fusion point? In many 
cases they are not more than a degree or two ABDUCTION, 
67 
apart, but in others there is a difference of 
several degrees. In regard to this point it seems 
to the writer that the measurement just preced- 
Change in abduction, 106 cases. 
TABLE 13. 
18° 
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17° 
16° 
1 
15° 
14° 
13° 
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1 
12° 
1 
1 
1 
11° 
I 
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1 
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2 
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1 
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ABDUCTION 
7° 8° 
AFTER 
9° 
10° 
11° 
12° 13° 14° 15° 16° 17° 18° J9° 20°2l°22° 31° 
ing the doubling point is the true measure of the 
abducting power. Were the power of abduction 
less than this, diplopia must necessarily occur 
sooner. On the other hand the point at which 
the images fuse would seem to indicate the activ- 
ity or sensitivity of the fusion center. To put 
it in other words the point of doubling is a sign 
of the limit of endurance, the point at which 68 
POSITION OF REST AND OCCLUSION TEST. 
fusion is resumed of the activity or alertness 
of the fusion center. It would seem that both 
the points should be recorded. 
(2) Is the abducting power properly represented by 
its measurement before or after occlusion? The 
analogy of the power of accommodation seems 
apposite. The power of accommodation is repre- 
sented by the difference between a minimum and 
a maximum innervation. We must know the 
latent refractive error in order to measure the 
power of accommodation. Similarly we must 
have the extrinsic muscles in a state of complete 
relaxation to measure the abduction. 
The post-occlusion test is the true measure of the ab- 
duction. Therefore, without an occlusion test we do not 
know the power of abduction. 
Table 14 shows the relation between the pre-occlusion 
abduction to the post-occlusion exophoria as observed in 
299 cases. 
In 195 cases the final exophoria measured less than the 
pre-occlusion abduction. 
In 28 cases the two were equal, and in 76 or 25 per 
cent., the former exceeded the latter, sometimes very 
greatly, the most extreme case showing abduction 130 
before and exophoria 30° at the end of the test, the abduc- 
tion becoming 31 °. 
It becomes evident that although the post-occlusion exo- 
phoria is more commonly less than the pre-occlusion ab- 
duction, exceptions to this rule are sufficiently frequent (25 
per cent.) to make any dependence upon this relation 
quite unreliable. 
The great variability in the relation between abduction 
and exophoria is worthy of emphasis. Thus in 36 cases ABDUCTION. 
69 
in which the post-occlusion test showed an exophoria of 
20 the pre-occlusion abduction varied from i° to 140. In 
TABLE 14. 
Relation of exophoria after test, to abduction before test. 
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19 cases in which the post-occlusion exophoria measured 
io° the pre-occlusion abduction varied from 2° to 12°. 
On the other hand, of 61 cases in which the abduction 
was 8° before the test, the exophoria after it varied from 
i° to 12°, and in 19 cases showing pre-occlusion abduc- POSITION OF REST AND OCCLUSION TEST. 
70 
tion of 40, the post-occlusion exophoria varied from i° 
to n°. 
The truth is that there is no relation between the pre- 
occlusion abduction and the post-occlusion exophoria ex- 
cept that in 75 per cent, of the cases the total exophoria 
does not exceed the primary abduction, if the doubling 
point be taken as the measure of abduction. CHAPTER IX. 
INDICATIONS FOR THE USE OF THE TEST. 
In a broad sense, if any inference can be drawn from 
the observations here detailed, the test is indicated when- 
ever it is desirable to obtain the greatest knowledge pos- 
sible of the position of rest, but in a narrower sense there 
are certain definite groups of cases in which it is specially 
indicated, and some in which no progress is made, or any 
solution arrived at without it. 
In the first place there is the very definite group of cases 
in which the symptoms are aggravated by an apparently 
accurate correction of the refractive error, especially 
astigmatism. Long ago the writer formulated for him- 
self the rule that, when a correction of refractive error 
makes the symptoms worse, a latent muscle error is pres- 
ent. The explanation presumably is that the formation of 
better defined images on the retina necessitates more exact 
binocular fusion and so a greater demand is made on 
the extrinsic muscles. The same principle no doubt is 
the explanation of the photophobia, sometimes intense, to 
which heterophoria gives rise. The brighter the light, 
the more clearly defined is the retinal image, partly on ac- 
count of the smaller pupil and partly because the accom- 
modation is stimulated to greater effort, and so an in- 
creased muscular effort has to be made to accomplish 
accurate fusion. 
Some of the worst cases of photophobia seen, have been 
due to latent faults in the muscle balance, and relief has 
followed their correction after detection by the occlusion 
test. 
71 72 
POSITION OF REST AND OCCLUSION TEST. 
Many patients give the history of having consulted one 
or more ophthalmologists of repute, being prescribed for, 
and of finally leaving the glasses off because they found 
themselves more comfortable without them. 
CHART VII, 
H.J.H 33 
CASE NO 
8 15 4 7° 
6' 
5° 
4° 
3° 
WEARING 
R +1.005 2° 
< J.25C 95° 
L+l. OOS. |0 
* 1.00 5.75° 
0° 
UNDER SCOP 
R + I.OOS. |0 
FEB. 1920 
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y* 
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3° 
HYPERPH: L 
PRIMARY P05 - 2j° 
UP. RIGHT 2? 
U.L. J° 
D. R. 4° 
D. L. 2° 
Some wear their glasses with comfort for a time, then 
leave them off to rest their eyes" from discomfort pre- 
sumably due to muscle strain, and after a while have to INDICATIONS FOR USE OF TEST. 
73 
CHART VIII. 
1915 1919 
TO NOV. 
1919 3 4 5 6 789 10 II 
8° 
7° 
6° 
5° 
4° 
3° 
2' 
1° 
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1° 
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4° 
5° 
6° 
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MISS D.A. 
20-25 70 
CASE NO. 
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5' 
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X-EXOPH. 
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ORTHOPHORIA 09 
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POSITION OF REST AND OCCLUSION TEST. 
replace them to relieve discomfort of some other type, 
presumably due to accommodative strain, so that the com- 
plaint is sometimes heard that since beginning to wear 
glasses, they are comfortable neither with nor without 
them. 
Chart VII shows in a graphic way the effect of the oc- 
clusion test in a case of this type. On account of severe 
and constant sense of strain this patient had consulted io 
to 12 ophthalmologists of national reputation in different 
parts of the country with invariable failure to obtain relief. 
His refraction, carefully measured, under cycloplegia, was 
found to be corrected with essential accuracy by the glasses 
he was wearing. The pre-occlusion test showed ortho- 
phoria, the final after eight days' occlusion, exophoria 6° 
and L. hyperphoria A prismatic partial correction of 
the error gave him a high degree of relief. 
In another group the correction of the refractive error 
does not aggravate the symptoms, neither does it relieve 
them. In this group the indication for the use of the test 
is not so strong as in the first, nevertheless, if the symp- 
toms are indubitably those of eye-strain, and cannot be 
accounted for by any other local condition or by any 
general disorder, occlusion will furnish an explanation in 
a great many cases. 
In still another group in which the presence of some 
fault in the muscle balance can be detected in the ordinary7 
way but its correction with that of the refraction fails to 
give the desired relief, an occlusion test will often show 
a much higher degree of error, or an error of a different 
kind; for instance the hyperphoria may change from R. 
to L. Chart VIII illustrates a case of this type. 
Although strictly monocular pain is rarely due to hetero- 
phoria there are exceptions to the rule, and it is therefore 
worth while, in cases in which there is no other evident INDICATIONS FOR USE OF TEST. 
75 
cause, to ascertain more definitely the state of the muscle 
balance. 
Thus, Mrs. D. had suffered for many months from 
constant pain in and around the right eye. A full cor- 
CHART IX. 
MRS. D 9° 
8694 g0 
7° 
6° 
5° 
4.0 
MARCH 
IO II 
1921 
12 13 
14 
15 
16 
17 
18 
19 
9' 
8° 
7° 
6° 
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z 
Q 
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0 
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CD 
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1° 
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1° 
1 
rection of her hypermetropia and astigmatism gave no 
relief whatever; muscle tests showed functional ortho- 
phoria. Finally an occlusion test being suggested and ac- 
cepted, the result was as shown in Chart IX, the develop- 
ment of 70 of exophoria and R. hyperphoria i°. On the 
third day of occlusion the pain ceased entirely, and did not 
return. 
Patients are frequently referred to the ophthalmologist 
by the general practitioner on the ground that some form 76 
POSITION OF REST AND OCCLUSION TEST. 
of eye-strain may be responsible for symptoms of a neu- 
rasthenic type, which do not yield to the correction or 
treatment of such non-ocular errors as can be demon- 
strated. A final judgment cannot be arrived at in these 
cases without as complete a test of the muscle balance, 
as of the refraction and accommodation. 
A very summary examination of the statistics given 
above will show that the ordinary short methods of in- 
vestigation may completely fail to reveal errors of the 
highest importance, or may be misleading as to their 
character. 
Mrs. S., aged 28, Chart X, was referred by her phys- 
ician because he thought that every possible cause of her 
neurasthenic symptoms had been eliminated with the ex- 
ception of eye-strain. Examination of her eyes revealed 
practically no error. Under cycloplegia the R. eye ac- 
cepted + 0.75 sph., and the left + 0.50 S. Tests of the 
muscle balance showed orthophoria. Before giving a final 
opinion, however, an occlusion test was decided upon. At 
the end of a week's occlusion the test showed exophoria 
8° and L. hyperphoria 70. The patient volunteered the 
statement that the week of occlusion was the most com- 
fortable she had ever had. 
The use of the method is also indicated by those not 
very uncommon cases in which patients after prolonged 
trial will not accept a full correction* of hypermetropia 
after cycloplegia. Blurring of distant objects persists in 
spite of the constant wearing of the glasses for a long 
period. This persistent accommodative spasm is associated 
in some cases with overaction of the extrinsic muscles in 
the interests of fusion. This is simply the converse of 
what occurs in the production of convergent squint or 
convergence excess in hypermetropia. Here the effort to 
accommodate causes excessive convergence, i.e., converg- INDICATIONS FOR USE OF TEST. 
77 
CHART X. 
1918 
NOV 
21 
22 
23 
24 
25 
26 
27 
28 
29 
30 
8° 
7° 
6° 
o 
MRs.E.p.s.aa 
CASE NO. 7o 
6927 
5° 
5° 
3° 
ABOVE ZERO LINE 
X = EXOPHORIA 
o=r,hvpfrph.2 
4° 
3° 
2° 
1° 
orthophoriaO0 
J 
0° 
i° 
BELOW ZERO LINeI 
x=esoph: 
O=LHyPERPH: 2° 
2° 
3° 
4° 
3° 
REFRACTION 4° 
UNOER SCOPOLAM. 
z 
5° 
5® 
R+O.15 D 5PH. J 
L*O. 5OD5PH. 
6" 
6° 
7° 
- 
7° 78 
POSITION OF REST AND OCCLUSION TEST. 
ence is subordinated to accommodation in the interest of 
clear vision. In the cases just referred to of accommoda- 
tive spasm, accommodation is subordinated to fusion in 
the interest of single vision, and blurring in the distance 
results. 
In any particular case the result seems to depend upon 
which of the two centers is the more active, the center for 
accommodation, or the center for fusion. Or to put it in 
another way the relative accommodation (or convergence) 
is insufficient or not elastic enough to permit the accom- 
modation to adjust itself to the muscle balance so as to 
permit clear as well as single vision, and the more pow- 
erful center dominates the weaker. If the center for 
accommodation is the stronger we have clear vision 
and excess of convergence, latent or manifest; if the 
fusion center is the stronger we have single vision, but 
blurring in the distance. A case of this kind which was 
very instructive to me is published in the May, 1922, num- 
ber of the Archives of Ophthalmology. 
Many years ago the great frequency with which the 
presence of astigmatism, anisometropia and heterophoria 
were observed in cases of myopia-it is possible to go 
further than that and say that the fact that cases of myopia 
were practically never seen without one or more of these 
conditions being present-necessarily suggested that they 
might be important factors in its development and progress. 
The above statistics indicate that the assumption that an 
ordinary short test of the muscle balance reveals the truth 
is not justified and consequently when myopia progresses 
in spite of accurate correction of the astigmatism and 
anisometropia as far as practicable, and the adoption of 
such hygienic measures as are indicated, an occlusion test 
may reveal the presence of a latent deviation. It is. as- INDICATIONS FOR USE OF TEST. 
79 
sumed that the presence of a latent deviation necessarily 
causes increased tension of the extrinsic muscles and there- 
fore increased compression of the globe. 
The use of the method is also particularly indicated in 
cases selected for operative treatment, it being then ex- 
tremely important to know as accurately as possible the 
kind and degree of error, to determine the amount of 
operative correction. It is also indicated after healing 
has taken place, for the degree of correction cannot be 
determined with any certainty without it. CHAPTER X. 
EFFECT OF OCCLUSION ON SYMPTOMS. 
The effect of occlusion upon the symptoms varies in 
different individuals and its nature cannot always be re- 
garded as a reliable indication of the future effect of treat- 
ment based upon its findings, though it must be admitted 
that it is extremely suggestive. In many cases the pa- 
tients experience an immediate amelioration or even a 
complete cessation of their symptoms during the occlusion 
period, and it is reasonable in such cases to expect a satis- 
factory result when the error is corrected. But it must 
not be forgotten that occlusion completely annuls all 
binocular strain, and that it is rare indeed when any cor- 
rection, prismatic or operative, can do the same thing. 
There are many patients who are more comfortable with 
one eye occluded than under any other conditions, and 
this may be true when no fault in the muscle balance can 
be found at the end of occlusion. Thus, a young man, 
the subject of severe asthenopia, preventing him from 
doing any reading, in spite of a very careful correction of 
his refraction, found himself able to read indefinitely dur- 
ing the occlusion period. At the end of this period, how- 
ever, tests showed perfect orthophoria, as previous to the 
occlusion. The binocular function was evidently at fault, 
but the fault could not be detected, perhaps because a 
week s occlusion was too short a period to effect a relaxa- 
tion of his muscles. The symptoms returned on the use of 
both eyes being resumed. 
Many patients, however, tolerate occlusion with great 
difficulty. The headaches and other symptoms may con- 
80 EFFECT OF OCCLUSION ON SYMPTOMS. 
81 
tinue unchanged, more or less vertigo, nausea and even 
vomiting may be superadded, though such symptoms are 
rare, and apparently psychic in origin, due to the nervous- 
ness and impaired orientation associated with the loss of 
binocular vision and often to the glare of the ground glass. 
These and other annoyances are greatly diminished by 
using a black patch instead of a ground glass, or pasting 
a piece of black paper over the back of the latter. 
The occurrence of such unfavorable symptoms during 
the test need not necessarily be interpreted as of bad omen, 
as some of these at least are followed by a finally satis- 
factory result. CHAPTER XI. 
NEAR POINT OF CONVERGENCE. 
The near point of convergence is practically unaffected 
by the test. Thus, in two only of 82 cases in which the 
p.p.c. was taken before and after was there more than 2 
cm. increase. In 30 cases there was no change, and in 
22 the distance was shortened. In fact such variations as 
occurred were within the normal limits of variation on 
different days. 
TABLE 15. 
Exophoria at % M and near point of convergence be- 
fore occlusion. 117 cases. 
19' 
18° 
17° 
2 
1 
1 
16° 
a 
1 
15' 
1 
1 
14° 
I 
2 
13° 
1 
EXOPHORIA 
AT i M " , 
I 
3 
1 
1 
1 
2 
1 
3 
1 
3 
1 
2 
2 
1 
9° 
1 
1 
1 
8' 
a 
I 
2 
1 
1 
1 
1 
7° 
2 
1 
1 
1 
1 
6" 
I 
1 
1 
1 
2 
1 
1 
1 
1 
5° 
2 
1 
1 
1 
1 
4' 
2 
1 
3 
2 
3 
1 
2 
1 
1 
3' 
1 
2 
2 
1 
2" 
2 
2 
1 
2 
1' 
i 
J 
ORTHOPHORIA 
1 
3 
4 
1 
ESOPHORIA '° 
AT | M 2° 
2 
1 
5 
CM. 
6 7 8 9 10 II 12 13 
NEAR POINT OF CONVERGENCE 
14 
>5 
16 
17 
18 
19 20 21 22 
Table 15 shows that a convergence insufficiency (7 cm. 
from the base line being accepted as the arbitrary 
standard) may exist either with an actual pre-occlusion 
esophoria at the near point or with orthophoria, or with 
82 NEAR POINT OF CONVERGENCE. 
83 
any degree of exophoria, and on the other hand that a 
normal p.p.c. may co-exist with any degree of exophoria 
at the near point. Thus, the table shows 23 cases in 
which the p.p.c. was 7 cm. from the base line or less with 
a near point exophoria of from 70 to 17°, and other 
cases in which a low near point exophoria was associ- 
ated with a too distant p.p.c., thus one case of near point 
exophoria 30 and p.p.c. 21 cm. 
In view of these facts, can the near point of converg- 
ence alone be considered as evidence for or against con- 
vergence insufficiency ? Apparently not, except in ex- 
treme or well marked cases, and the fact that many people 
who have a normal p.p.c. suffer from asthenopia in spite 
of correction of all error, raises the question as to whether 
the measure of the momentary maximum convergence 
effort has so much importance as usually attributed to it, 
and whether the question of endurance, a factor not so 
easily determined and probably often depending on condi- 
tions not localized in the eyes, is not of greater importance. 
Table 16, in which the post-occlusion near point exo- 
phoria is shown with the p.p.c., emphasizes still more 
strongly the absence of any constant relation between 
them, many of the highest degrees of exophoria being as- 
sociated with normal or supernormal convergence. 
But it may be well at this point to emphasize the fact 
that many patients have a near point of convergence far 
more distant than the arbitary standard, who appear to 
suffer no inconvenience from it and the question arises 
whether an exaggerated importance has not been given to the 
distance of the p.p.c., or whether the standard fixed is not 
too arbitrary, in other words whether what may constitute a 
good working p.p.c. may not differ very materially in dif- 
ferent individuals. 84 
POSITION OF REST AND OCCLUSION TEST. 
TABLE 16. 
Exophoria at % M and near point of convergence after 
occlusion. 70 cases. 
25° 
1 
1 
22" 
2 
1 
1 
20" 
1 
2 
19° 
1 
1 
A 
18" 
1 
1 
1 
-1- 
17° 
1 
1 
1 
1 
16' 
1 
1 
1 
1 
1 
<- 
15' 
1 
1 
1 
EXOPHORIA 
AT 
14" 
1 
1 
CL 
13° 
1 
1 
2 
1 
Q 
12" 
2 
2 
1 
1 
II" 
1 
1 
I 
1 
□l 
10" 
1 
2 
1 
9' 
1 
I 
1 
1 
1 
8' 
1 
1 
1 
1 
7° 
1 
-1- 
6" 
1 
1 
V 
5" 
1 
1 
1 
4" 
1 
3° 
1 
1 
1 
1 
2° 
1 
ORTHO. AT 3 
1° 
M 1 
1 
E50. AT j M 
1° 
1 
5 6 7 
CM. NEAR 
8 9 10 II 12 13 14 
POINT OF CONVERGENCE 
15 
16 
17 
18 
19 
20 
T.he cases seem to group themselves into four classes- 
(1) Cases of marked near point exophoria with normal 
convergence. 
(2) Cases of marked near point exophoria with sub- 
normal convergence. 
(3) Cases in which there is practically no near point 
imbalance but the convergence is subnormal. 
(4) Cases in which there is practically no near point 
imbalance and the convergence is normal. 
Symptoms may arise in Classes (1), (2) and (3). 
In Class (1)-because the load is too great. 
In Class (2) because the load is too great and the con- 
vergence too weak. 
In Class (3) because the convergence is unequal to a 
normal demand. CHAPTER XII. 
TREATMENT. 
Although the chief object of this essay has been to 
show the influence of prolonged occlusion in determining 
the position of rest, in other words in making a more 
accurate diagnosis, it seems desirable to comment with 
more precision on the methods of dealing with the condi- 
tions found. 
The general condition of health of the patient, includ- 
ing the reserve of nervous energy with which he was en- 
dowed at birth, has a large influence beyond any doubt in 
determining whether a given form or degree of muscle 
imbalance will give rise to symptoms or not. A great 
many people are the subjects of faults of this nature with- 
out giving any symptomatic evidence whatever of their 
presence. On the other hand symptoms of very marked 
character arise in patients of the neurotic type or of a 
low state of general health, from errors of very low 
degree, and it is consequently advisable to correct such 
errors in the presence of otherwise unexplained symptoms. 
It may be noted, moreover, that in most of these cases 
resort is had to the ophthalmologist because of the previ- 
ous failure of general treatment to afford relief. 
Assuming that questions of the bearing of general health 
upon the symptoms have been settled, the question of the 
treatment of local ocular conditions is next in order. 
No doubt the refraction should be accurately corrected, 
but exception must be taken to the common statement that 
such correction has a sedative effect upon the muscle con- 
dition or symptoms. This may be true of some cases of 
85 POSITION OF REST AND OCCLUSION TEST. 
86 
esophoria associated with hyperphoria, i.e., cases of pseudo- 
esophoria, but the effect of refractive correction in many 
cases of latent exophoria and hyperphoria is to make the 
symptoms worse, for reasons already stated. 
Aggravation of symptoms following correction of the 
refraction is in itself strong presumptive evidence of the 
existence of a latent heterophoria, and relief must be 
sought, not in modification of the refractive correction, 
but in an intensive examination and analysis of the oculo- 
motor system, and especially in the determination of the 
relative or anatomical position of rest. 
If the presence of a muscle imbalance has been deter- 
mined, the decision has to be made between the three well- 
recognized methods of treatment. 
(1) Exercises-prism or other. 
(2) The constant wearing of prisms to permit the 
visual lines to fall into or approximate their ana- 
tomical position of rest. 
( 3) Operative treatment. 
The following observations and suggestions are made 
purely on the basis of the writer's own experience, and 
their value is limited accordingly. 
First as regards exercises. In spite of the widespread 
use of various methods with the underlying principle of 
strengthening the supposedly weak muscles or stimulating 
the nerve centers, very few well-authenticated cases of 
improvement by this method in the hands of others have 
come under my observation, and in these few the benefit 
has usuahy been temporary only. In my own practice 
benefit has been still more exceptional though not entirely 
absent. Even in cases of convergence insufficiency, which 
would seem to be especially suitable for this method, any 
material improvement has been rare, though cases have TREATMENT. 
87 
occurred in which satisfactory improvement both in the 
p.p.c. and symptoms have resulted. When I prescribe 
exercises now, it is less with the expectation of benefit 
than for the purpose of determining by actual trial its 
futility in the particular case in point, and leaving the 
field open for the adoption of some other more promising 
method of treatment. 
My experience in the prescription of prisms for constant 
wear has been far more satisfactory, extending over a 
long period of years, during which the series of 700 con- 
secutive cases upon which this essay is based constitutes 
only a fraction of the total number of cases observed. 
This experience leads to the following suggestions: 
That in low degrees of exophoria, i.e., up to 40 or 50, 
the greater part or all the error found after occlusion can 
often be corrected with advantage. I have ceased to con- 
sider some of the lower degrees as of no importance. The 
prismatic correction of even 30 has been followed by sur- 
prisingly good results. 
Thus, Mrs. H. E. M., age 43, suffering from extreme 
nervousness, severe headaches accompanied by nausea and 
vomiting, apparently related to the use of her eyes, never 
having been able to get satisfactory glasses, was subjected 
to the test for 7 days. At the end of this period she 
showed R. hyp. , Ex. 30. Under cycloplegia each eye 
accepted + 0.50 sph. She was ordered: 
R: + 0.25 S. C iM° prism base down-1550 
L: + 0.25 S. O Ye 11 " in 
She returned four years later, having become presbyopic 
in the interval and in reply to a question as to what the 
effect of the glasses amounted to, said: "What you did for 
me then completely relieved me of my symptoms. 88 
POSITION OF REST AND OCCLUSION TEST. 
The higher the degree of error the smaller is the pro- 
portion of it that can be corrected, but many cases of 
8°, io° or 12° get great benefit by the correction of a 
half or two-thirds of the error. It is rare when prisms 
stronger than 40 on each eye can be worn with benefit for 
the reasons previously stated. There are exceptions to the 
rule, however. Thus, a patient consulted me a few years 
ago wearing a 90 prism over each eye prescribed by the 
late Dr. Alleman, formerly of Brooklyn, three years previ- 
ously. With these glasses she had had satisfactory relief 
from her symptoms. These prisms were re-ordered with 
a change in her refractive correction, relief again resulting. 
The degree of prismatic correction which can be worn 
with advantage varies in different patients, a variation of 
half a degree sometimes making all the difference between 
comfort and discomfort, and some patients do not tolerate 
prisms at all. No doubt a large psychic element is here 
present. 
My experience in the prescription of prisms in esophoria 
is naturally much smaller than in exophoria, and it is only 
when the evidence shows that the case is one of esophoria 
and not pseudo-esophoria that I prescribe them at all. 
In hyperphoria the use of prisms for constant wear is 
generally conceded to be of high value though the prescrip- 
tion of them has to be tentative as to the degree of cor- 
rection. The very irregular distribution of the error over 
the field of fixation explains the lack of uniformity in the 
results. As a general rule, the error found in the primary 
position can be taken as a guide to the amount of cor- 
rection, all or the greater part being corrected, but this 
rule must be modified in cases in which great differences 
in the degree of error are present in different parts of the 
field. The relative deorsumduction and sursumduction 
may be used as a control. The careful measurement of TREATMENT. 
89 
these ductions both before and after occlusion is, indeed, 
often of great value in determining the character of the 
prescription. 
After all it must be conceded that the prescription of 
prisms has to be done in a tentative manner. The length 
of time it takes for patients to get used to the new rela- 
tion between accommodation and convergence varies with 
different people, and failure often results because the 
necessary length of time is not devoted to the attempt. 
In the field of operative treatment my experience is 
not great enough to attempt any but tentative generaliza- 
tions, but I am satisfied that the degree of error which 
it should be the aim to correct, or materially diminish, 
is that which is found at the post-occlusion test. 
In cases of exophoria due to pure divergence excess that 
tenotomies of the externi should be done. 
In cases of exophoria in which the near point of con- 
vergence is too distant and cannot be brought near the 
normal by exercise, advancement of the interni would be 
the operation of election. 
Having abandoned the idea that lateral deviations 
depend upon the vertical, at any rate at all commonly, I 
am consequently under no intellectual compunction to 
correct the vertical previous to attacking the lateral devia- 
tion. In fact the operative correction of the lateral devia- 
tion has been followed in some cases by a marked diminu- 
tion in the hyperphoria (see Chart II). APPENDIX. 
In the 7th Edition of Fuchs, the translator expresses the 
opinion that such Prolonged Occlusion "tends to make the 
eyes revert to their infantile state in which, not being con- 
trolled by co-ordinative influence, they exhibit a varying 
imbalance." 
In spite of the keen appreciation on the part of the 
writer of the very high value of Dr. Duane's opinion on 
any muscle problem, he cannot refrain from making the 
following observations. 
First: That the P. O. Test is simply an extension in 
point of time of the screen and parallax test of which Dr. 
Duane himself has been the most strenuous and eminent 
advocate. It is evident from the above quotation that 
there is a point at which he thinks this test becomes un- 
reliable. Applied for a short time (how short ?) it is 
reliable; applied for a longer period (how much longer ?) 
it is unreliable. I am unable to find in the numerous cases 
I have examined any adequate reasons for the acceptance 
of this position. Although in many cases the development 
of the error does not show a steady and uniform curve, the 
general trend is in the same direction, the variations from 
day to day are usually slight, and explainable by assum- 
ing an occasional partial resumption of the old habitual 
spasm during the process of gradual relaxation. In the 
presence of brain tumors, and presumably other organic 
brain changes, great variations may be present, of which 
the case shown in Chart XI is a good example. 
90 APPENDIX. 
91 
Miss C. S. P., age 18, first seen on Jan. 7, 1920, when 
she complained of severe headache, aching of eyes, and 
stomach disturbance since reading a magazine story in 
Christmas vacation. Had never worn glasses. 
OO 
MISS C.C.S.P.Ift 
CASE NO.? 
&O 72 
6° 
5° 
1920 
FEB. 
13 
14 
CHART 
15 16 
XL 
17 
18 
19 
20 
21 22 25 
8* 
7a 
6' 
5° 
£ 
£ 
V O 
II II 
-i*> t 
4 
ABOVE ZERO LINE -to 
5 3 
Z 
T 
X® 
X - EXOPHOR1A J 
0 • R.HYPERPH: 
2° 
IM 
S3 
□ 
21 
o 
■3 
r 
ORTHOPHORIA no 
8 
i 
BELOW ZERO LINE 
X = ESOPHORIA 
0 - L. HYPERPH: |° 
2° 
X 0 
?7 
1® 
\ y 
2° 
-- 
3° 
3° 
UP TO R. 
R|- 
poxpos 
DOWN TO R 
Rif 
hyperph: on 
UP JO L. 
: Li' 
-DOWN TOL 
R2° 
PEB23"' 
CHART XL 
Refraction under scopolamine 
R. 4- 2.50 S 4- 0.37 C ax 1150 6A + 
L. 4- 2.50 S 4- 0.25 C ax 70° 6/e + 
Oph. o.d.s. somewhat blurred, vessels best seen with 
4- 3.00 D. 92 
APPENDIX. 
Glasses (full correction -0.50 sph.) made her comfort- 
able for two weeks. After that she had headache on us- 
ing eyes and was in bed off and on for two weeks. 
A ten day Prolonged Occlusion test was then made, 
during which she had a very severe headache. 
On Feb. 23d, oph. showed definite papilloedema, the ves- 
sels being clearest with + 7.00 or + 8.00 D. sph. 
She developed other symptoms of cerebellar tumor, 
which upon surgical exploration it was found impossible 
to remove, and died a few months later. 
As a rule there is a fairly uniform and gradual develop- 
ment of the error until the degree remains about the same 
from day to day, and there is no point at which it seems 
proper to say that up to this point the test shows the real 
facts, and beyond this point is misleading. 
Moreover, in cases of blindness of one eye, in which the 
test is therefore carried to its ultimate limit, the deviation 
which develops is not a variable one, and Bielschowsky 
and others have assumed that it represents the relative 
position of rest. 
Second: The infantile state is naturally that state out of 
which functional orthophoria, or rather binocular vision, 
has developed, and if it be conceded that the Prolonged 
Occlusion Test tends to permit a reversion to it, should not 
this be regarded as evidence that this infantile state has 
persisted, and is also an adult state, obscured only by the 
instinct or reflex of binocular fusion? "With the removal 
of all attempts at fusion an approximation to the condition 
antedating binocular vision is to be expected, and is not a 
knowledge of it necessary to the understanding of the 
problem which the muscle balance includes? Do we, as a 
matter of fact, know much about the muscle balance in 
infancy and the development of binocular vision? While 
it is true that the eyes of some infants show occasional APPENDIX. 
93 
deviation, chiefly in, sometimes out, the eyes of the major- 
ity appear to be straight, and to remain so, and is it not 
just as likely that the occasional deviations above referred 
to indicate the true position of rest, as that they show a 
"varying imbalance"? 
If it be true that the Prolonged Occlusion Test does 
develop a reversion to the infantile state, then it gives us 
very surely important information as to what that condi- 
tion is or was, and the 700 cases here recorded show how 
greatly this state varies in different individuals, and out of 
what varying conditions functional orthophoria or bin- 
ocular single vision has to be developed, and it is not sur- 
prising that the muscle work and expenditure of nerve 
energy required to accomplish and maintain the function 
at the necessary and habitual point of perfection should 
give rise, at any rate in those whose endowment in nerve 
energy is not up to the average, to symptoms of one kind 
or another, practically to symptoms of nervous exhaustion. 
Third: The chief proof of the reliability in the main of 
the observations made by this test consists in the fact that 
the results of treatment based upon them have proved in a 
large number of cases to be far more satisfactory than 
those previously obtained, and that the facts thus dis- 
covered prove to be the key to the solution of the problem. 
Many of the cases included in this series have been under 
my observation many years, without obtaining satisfactory 
relief until after the P. O. Test showed the presence of 
errors previously undetected; and many others had passed 
through the hands of ophthalmologists of national reputa 
tion with a similar result. In fact all these 700 patients 
were examples of failure in greater or less degree to obtain 
relief by full correction of the refraction and such muscle 
errors as were demonstrable by the usual tests. 94 
APPENDIX. 
Is it true that binocular fusion is acquired only by 
experience (the empiristic theory) or that it is an inherent 
function due to anatomical conditions, the presence of the 
fovea, in which alone a keen sense of form resides, and 
inborn physiological processes (the atavistic theory) ? 
A case came under my observation some years ago 
which strongly supports the latter theory. 
A young man, age 18 years, had been the subject of 
convergent alternating strabismus at 3 weeks after birth, 
and at 15 months both interni were cut. At 18 years his 
eyes were divergent, and the right deviated upwards. It 
is exceedingly unlikely that this patient had ever had the 
opportunity of acquiring binocular fusion by experience. 
The right superior rectus was partly divided and at a later 
date the R. int. was advanced with a single stitch. The 
next day the patient announced that he could use the two 
eyes together and examination showed this to be the case. 
The writer referred to the use of this method in a 
paper on "Obscure Cases of Eye Strain," published in the 
N. Y. Medical Journal in Sept., 1893, and published a 
preliminary note on the subject entitled "A Note on the 
Use and Non-use of the Occlusive Bandage in Cases of 
Heterophoria," in the Ophthalmic Record of March, 1897, 
and also the following papers: 
"The Determination of the Relative Position of Rest, by Prolonged 
Occlusion of One Eye," N. Y. State Journal of Medicine, April 28, 1915. 
"The Detection and Measurement of Latent Ocular Deviations. The 
Inadequacy of the Ordinary Methods Used." N. Y. Medical Journal 
November, 1918. 
"The Influence of Prolonged Monocular Occlusion in Revealing 
April" 1920 MUSde Balance" British J°urnal of Ophthalmology, 
"Prolonged Monocular Occlusion as a Test for the Muscle Balance." 
Trans. Am. Oph. Soc., 1920. INDEX 
PAGE 
Abduction 65 
definition of 65 
recording of 60 
relation of exophoria to60 
Stevens on 65 
Accommodative spasm I 
due to heterophoria 76 
Asthenopia, ocular causes of 1 
Chameleon (the foot-note) 62 
Collins Treacher, on evidence of binocular vision 61 
Convergence near point 82 
in relation to near point exophoria 82 
Cycloolegia and prolonged occlusion test, comparison of 2 
Dark adaptation, analogous 25 
Deorsumduction and sursumduction 63 
Dolman on master eye 12 
Duration of test 16 
Ellett on the relative importance of errors of refraction and the 
muscle balance 45 
Emmetropia, definition 24 
Esophoria before and after test 38, 39 
Exercise, prism and other, results of 86 
Exophoria before and after test 29 
Extrinsic muscles, sources of innervation 4 
in spasm of & 
General health in relation to symptoms 85 
Hyperesophoria before and after test 40, 43 
Hyperexophoria before and after test 30, 33 
Hyperphoria before and after test ' 
a survival 
Master eye 
Muscle balance and refraction 
60 
age 
95 INDEX. 
96 
PAGE 
Muscle balance and heredity 61 
evolution 61 
Myopia and heterophoria 78 
Myoplegic, Reber on 2 
Neurasthenia and eye-strain 76 
Orthophoria, varying definitions 22 
definition 24 
Hansen Grut on functional 24 
before and after occlusion 26 
Operation 52 
Pain monocular 74 
Photophobia due to heterophoria 71 
Prisms 46 
for constant wear 46, 87 
exercise 46 
improvement from prism exercise apparent but not real 47 
limitations to constant use of prisms 50 
Prism abduction a survival 62 
Prolonged Occlusion Test 23 
artificial dissociation produced by 3 
definition 23 
duration of test 16, 19 
indications for use 71 
repetition of test 17 
Refraction, effect of curvature on muscle balance and symptoms .. 85 
symptoms aggravated by correction of 71 
Relative position of rest, Bielschowsky's definition of 19 
Sighting eye 12 
Dolman on 12 
Squint following loss of sight 10 
iSursumduction a survival 63 
Symptoms, effect of occlusion on 80 
Tenotomy, partial, effect of 52 
Treatment gg 
Vertical deviations, frequency of g