THE PARALLAX TEST FOR HETEROPHORIA. 
BY 
By Dr. A. DUANE, New York. 
Reprinted from the Archives of Ophthalmology, Vol. xxiv., No. 2, 1895. THE PARALLAX TEST FOR HETEROPHORIA.1 
By Dr. A. DUANE, New York. 
In the sumnjer of 1889 I published in the New York 
Medical Joitrnal a description of what I denominated the 
parallax test for insufficiencies. Since that time a continu- 
ous experience has convinced me that in this test we have a 
method which is not only among the most satisfactory of 
all that we possess, but also in some cases succeeds when the 
other tests fail or are self-contradictory. As the test is not 
generally known, and as the only detailed account of it ap- 
peared in a general and not a special medical journal, it 
seems not without utility to rehearse briefly the method of 
its performance and the principles upon which it is based. 
To conduct the test, we place the patient in the primary 
position, with head erect and eyes directed straight forward 
or slightly below the horizontal plane (this latter especially in 
making the test for near points). The object of fixation, 
which should be twenty feet distant, may be a candle-flame, 
but preferably is a white spot, I or 2 cm in diameter, upon a dull 
black surface of some considerable extent. By this arrange- 
ment all danger of projecting the image upon a surface be- 
yond is done away with, and the chance of a confusion with 
surrounding objects is prevented. The patient's gaze being 
directed fixedly at the spot, a card is placed before one eye, 
and passed alternately from that to the other,-the patient 
being at the same time asked whether the spot appears to 
move, and, if so, in what direction. If it remains perfectly 
stationary, there can have been no deviation behind the 
1 Paper read before New York Academy of Medicine. 
Reprinted from the Archives of Ophthalmology, Vol. xxiv., No. 2, 1895. 259 
The Parallax Test for Heterophoria. 
card, and the position of fixation of both eyes is perfect. 
If, however, the spot moves, it must occupy a different posi- 
tion as seen by the two eyes ; i.e., there is really a diplopia 
present which our method of observation has unmasked. 
Thus if, on uncovering the left eye, the object (which was 
previously seen by the right eye and is now seen by the left) 
appears to move to the patient's left, there is really a ho- 
monymous diplopia (homonymous parallax), which differs 
from ordinary diplopia only in the fact that the two images 
are seen alternately instead of at the same time; if the 
object seems to move to the right, there is crossed diplopia 
(crossed parallax); if the object moves down, the eye must 
have been higher behind the screen (left hyperphoria, left 
parallax); and if the object moves up, the left eye must 
have been lower behind the screen (right hyperphoria, right 
parallax). In order to determine the amount of this alter- 
nate diplopia, we place prisms of the appropriate direction 
and strength before one eye until the movement is abolished. 
Thus, supposing that when the left eye was uncovered the 
object seemed to move down and to the left (homonymous 
and left parallax, indicating a condition of hyperesophoria), 
two prisms are placed before this eye, with their bases re- 
spectively down and out, and increased in strength until the 
movement has become nil. The strength of the prism hav- 
ing its base down will measure the degree of hyperphoria, 
and that of the prism having its base out will indicate the 
degree of esophoria present. 
For near points the test is made in the same way, a small 
dot on a rather large card being employed, and the move- 
ment of the dot upon the card (and not of the card itself 
projected against some distant object) being observed. 
Such being the nature of the test, and such the simple 
means by which it is carried out, it remains to state the 
reasons for employing it at all. This is requisite, for we 
have already a number of methods of measuring hetero- 
phoria/ and the question may naturally be raised why we 
should multiply tests when enough exist already for the 
purpose in hand. The answer to this is that the complexity 
of the problem presenting itself for solution is often such A. Duane. 
260 
that it is desirable to get confirmatory evidence of every 
kind before deciding, and, moreover, the other tests, while 
indicating the kind of heterophoria that is present, not in- 
frequently give an erroneous impression as to its amount. 
Hence it is useful to have an additional source of informa- 
tion to supplement and check those which we have already; 
and it only remains to inquire if this method that we have 
described answers the demands that we can rightly make 
of it. 
Of any test thus put forward we must ask how far it fulfils 
the requirements of applicability, precision, accuracy, and 
utility. As regards the first requirement, I may say that in 
my hands the test has proved to be almost universally ap- 
plicable. There are very few persons who cannot with a 
little patience on the part of the examiner be got to recog- 
nize the displacement of the image, and to indicate its direc- 
tion with accuracy. If the displacement is not recognized 
at once, it will in most cases be noticed after the patient's 
attention has been called to the artificial parallax produced 
when a prism of 2° or 30 is placed before the eyes. That is, 
the prism by exaggerating the parallactic displacement has 
forced it upon the patient's attention, and has made him 
recognize in what it consists ; and he is then able to say 
whether a similar movement does or does not exist when 
there is no prism before the eye. 
As regards precision the test leaves nothing to be desired. 
A prism of suffices to produce or to neutralize quite a 
decided lateral parallax; and a vertical movement, corrected 
by a prism of even ^°, is clearly noticeable.1 
As regards accuracy, I have become convinced from the 
constancy of the results which the method affords that it is 
one of the most reliable of all that we have at our com- 
mand. And this, in fact, constitutes its reason for exist- 
once; establishes its claim to be used with, and sometimes 
in preference to, the other methods of determining insuf- 
1 This precision makes it useful in determining whether a glass that the 
patient is wearing has or has not a prismatic element, and whether it is centered 
or not. If, for instance, a patient, who has no hyperphoria, shows, with his 
glass on, a vertical parallax, it proves either that the glass has a prismatic ele- 
ment in it, or is faultily adjusted (decentred) so as to produce a prismatic 
effect. 261 
The Parallax Test for Heterophoria. 
ficiency. A word upon this point will make this statement 
clear. 
All the subjective tests for insufficiency depend in some 
way or other upon the production of diplopia. Usually 
this is effected by prisms, either applied roughly in the 
trial-frame, or with more precision in some one of the 
numerous varieties of phorometers. But the difficulty with 
the use of prisms has been that when two images of similar 
character are presented to the patient's notice, and he is re- 
quired to state whether the two are or are not in the same 
vertical or horizontal line, he often unconsciously tries to 
bring them into line, and thus conceals an insufficiency 
which actually exists; or if the prisms are rotated, and the 
position of the images thus displaced, he instinctively tries 
to keep the images in line, and thus simulates an insuf- 
ficiency which does not exist. This is often seen with the 
phorometer, and vitiates the findings of that otherwise most 
accurate instrument. Thus, when testing for near points, 
we are apt to find an exophoria, which increases almost in- 
definitely, although it is often quite certain that no such 
tendency to outward deviation actually exists. And while 
there are ways of obviating this objection that suggest 
themselves to the mind of the experienced examiner, 
they still constitute a source of uncertainty, particu- 
larly when the test is made for near points. Hence the 
attempt has been made to disassociate the images by giving 
one a different appearance from the other-e.g., by convert- 
ing a point of light into a luminous bar (Maddox rod), or 
into a blurred diffusion-circle (Stevens's spherical lens, with 
small aperture). But, as far as my experience goes, the im- 
pulse towards bringing two dissimilar images into line is al- 
most as strong as when the images are alike. I believe that 
this is particularly the case with the Maddox rod. On ac- 
count of»this inaccuracy sometimes attaching to the sub- 
jective tests, some observers have preferred to rely solely 
upon objective methods, and have fallen back upon the old 
exclusion-test-?>., that in which the observer takes note of 
the deflection of one eye behind a screen, while the other 
eye is performing fixation. Now the parallax-test is A. Duane. 
262 
nothing but the subjective correlative of the objective ex- 
clusion-test, and is superior to the latter, both in being far 
more delicate and in admitting of precise measurement. 
Like the exclusion-test, it may be applied while the patient 
is looking in any direction whatever, and thus a difference 
in the amount of deflection in various parts of the field of 
fixation may be observed and estimated? Moreover, the 
results are stabile, and when once the deflection is cor- 
rected, any further attempt at correction with prisms will 
produce a deflection in the opposite sense, so that there is 
not in this test, as in some of the others, a tendency on the 
part of the patient to overcome the prisms, and thus sim- 
ulate an insufficiency which does not exist. In other words, 
the two images, being seen alternately, there is not the 
same impulse to force them together, as obtains in the other 
tests, where the two images are seen simultaneously. 
Hence it will often be found that when the phorometer in- 
dicates an insufficiency of the interni, or exophoria, of any- 
where from 50 to io° for near, the parallax test will show 
constantly a deviation outward of not more than i°; and 
corresponding to this small deviation it will be seen that the 
eyes remain steady behind the screen in performing alter- 
nate fixation. 
Having used the test now for eight years, and upon a 
great number of cases, I am thoroughly convinced of its 
utility, and desire to recommend it to those who busy them- 
selves with the consideration of anomalies of the ocular 
muscles. And whatever may be thought of the role that 
these anomalies play in producing direct or reflex disturb- 
ances, it is at least important that the data upon which our 
judgment of this difficult question is based should be as pre- 
cise and as free from error as possible. 
1 I have already (Archives of Ophthalmology, xxiii., p. 71) signalized 
the importance of observing this difference, which may give us a clue as to the 
nature and etiology of the heterophoria. Ube Iknicfcerbocher press 
IRew Korb