If we first test the right eye, the gas
jet at twenty feet will be distinctly
seen by the left, while with the right
a light streak will be seen in the
vertical direction. t

If there is perfect orthophoria, the
streak will pass directly through the flame, as is
represented in a (Fig. 141). If there be esophoria,
the streak will be to the right (i); if exophoria,
it will be to the left (c). Each eye being tested,
it will be found that if there be esophoria of the
right, there will also be esophoria of the left; and
if there be exophoria of the right, there will be
the same form in the left and of the same degree;
the amount of which can readily be ascertained by
the. prism, placed before the eye, that will throw
the light streak into the flame, as represented by a.
HBTBROPBOKIA.
Maddox has another test which shows any com-
pound muscular error, as it will indicate at once
any defect of the vertical with that of the hori-
zontal as well as the oblique muscles. This is
his double prism test, consisting of two prisms set
in a rim (Fig. 142) with bases together, having the
effect of doubling objects. This is placed in the
frame before, say the right eye, the left being cov-
I ered by an opaque disc.
The candle, door-knob, or
object looked at, at twenty
feet, will appear double if
the base line of the prisms
cuts the center of the pupil.
The images of the object
-the door-knob — will be
separated to the distance
of twelve or fourteen inches, one being directly
abovs the other, as in Figure 143 a. Now, if the












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Fig. Itf.
