348 W. LeConte Stevens—Physiological Optics. 
can be explained by encroachment of diffusion circles at the 
edge of the retinal image, or on the supposition that the antero- 
posterior diameter of the eyeball has been slightly diminished 
by pressure on opposite sides of its elastic sclerotic coat. For 
reasons already given, the apparent distance is not very deter- 
minate. ithout changing the convergence, let the eyes be 
rolled to the left until A’ is seen with the left eye in the direc- 
tion of its visual lines. The image of <A, seen still more ob- 
liquely, now appears slightly larger and dimmer, while A’ is 
still smaller. ‘The apparent variation in size is thus independ- 
ent of binocular combination. 
3. 
In such experiments one great advantage attained by using 
Wheatstone’s stereoscope is, that by keeping the plane of each 
picture perpendicular to the arm that carries it, any distortion 
of perspective that might be due to changing the direction of 
the visual lines is reduced to a minimum. aking allowance 
being similarly marked. Below A’ is another circle A’’, equal 
in size. By cross-vision the images of A and A’ are combined, 
strong involuntary contraction. On closing one eye, these 
muscles become relaxed, while the direction of the visual line 
for the eye remaining open is easily kept unchanged. The 
elliptic image then apparently recedes, and in doing so it grows 
larger; but an interval of one or two seconds may elapse be- 
_fore normal monocular vision is restored. This process can be 
