W. LeConte Stevens—Physiological Optics. 347 
Since in normal vision the contractions of the ciliary and in- 
ternal rectus muscles are assogiated, the considerations just 
expressed show why so acute an observer as Wheatstone 
should have noticed the apparent decrease in size of the binoc- 
ular image when strong convergence of visual lines was induced 
though he observes that the image seems changed in position, 
but does not say whether the change is that of increase or de- 
crease in distance. 
2. Errect or Muscunar Errort oN RETINAL SENSITIVENESS. 
The estimation of size and distance, when there is strong, 
muscular effort, depends in some measure upon the part of the 
retina on which the image is formed, variations being more 
noticeable when the central part is impressed. In a former 
stone's stereoscope. Let A and A’ (fig. 1), be a pair of conju- 
gate pictures adjusted so that the reflected rays, mz and m’7’, 
are sensibly parallel. If the visual lines of the eyes receiving 
these rays coincide with them, the image appears in front, com- 
bined in full relief, with apparent diameter and distance slightly 
magnified, the contraction of the internal rectus muscles being 
less than normal. Let the visual lines now be forcibly crossed. 
The retinal images, 7 and 7’, are no longer upon corresponding 
ici’ 
retinal points; the external projections of them hence appear 
separate and without relief. For the subjective binocular eye 
(ig. 2). the two visual lines are combined into the median line 
- By indirect vision, the image of A appears on the right of 
the median, and that of A’ on the left, each diminished in size on 
account of the disturbance of ordinary retinal sensation by strong 
muscular contraction. This apparent diminution is greater than 
* Philosophical Magazine, 1852, p. 508. + This Journal, Dec., 1881, p. 450. 
