4 
W. LeConte Stevens—Notes on Physiological Optics. 293 
A’. These arms move upon a pivot, each through an arc of 
60°, under a divided circle. When so adjusted that the angle 
of incidence on each side is 45°, the direction of the reflected 
rays is such as to necessitate parallelism of visual lines for 
those which come from the centers of A an ‘ respectively. 
If the arms are pulled forward for example to B and B’, the | 
angle of incidence becomes such that the eyes must be made 
to roll inward to retain binocular combination of images; if 
ushed back toward © and ©’, divergence of visual lines is 
necessitated. The value of the optic. angle, positive for con- 
vergence, negative for divergence, is obtained, with but trifling 
error, from the circle. On each side let the picture be kept at 
a fixed distance, while the eyes are as near as possible to the 
sivas for example, so that Am + mR =50™. For this dis- 
inocular image BAe in full relief about 50™ in front. 
ard of comparison, the optic angle, focal adjustment and 
ul angle all conducing to the s ame judgment of distance. 
Modifying slightly the formula hitherto employed, we have, 
for the distance, D, of the optic vertex from each eye, deter- 
mined by intersection of visual lines 
D = fi cosec fa, 
where 7 is the interocular distance, and @ the optic angle. If 
this equation be expressed as a curve, fig. 2, ta ing values of 
a for abscissas and values of D for ordinates, the axis of ordi- 
nates is obviously an asymptote 
t the arms now be pulled forward until 2 = 37° 20’. The 
corresponding value of D is 10™, while the visual angle is un- 
ses and the focal adjustment, if perfectly distinct vision 
cured, must still be fora distance of 50°. These two ele- 
ie therefore tend to counteract the suggestion due to strong 
convergence, and the image appears perhaps 15™ or 20 dis- 
tant. Its apparent diameter varies directly as the estimated 
distance, PB is diminished to ‘3 or 4 of the original diameter. 
The influence of axial convergence, though partially counter- 
acted, preponderates over that of the other elements in deter- 
mining the judgment. 
et the arms now be pushed back until a=—5°. The theo- 
retic value of D is negative and hence physically impossible, 
but practically pa contraction of the external rectus muscles 
