W. LeConte Stevens— Physiological Optics. 359° 
cases. The group of light images in the one eye forms a sum 
total that is different from that in the other, and the sensations 
they arouse are simultaneously conveyed to the brain. Ex- 
perience comes to our aid, and the modified resultant is in- 
stantly recognized, even though we may be unable to perceive 
separately the minute modifications which give character to 
the two main components that form it. 
I doubt therefore the anatomical theory of corresponding 
retinal points, and regard that of partially correspondent retinal 
areas as a substitute more in accordance with observed facts. 
This correspondence moreover must be considered merely the 
effect of association resulting from oft repeated experience, an 
association that is very quickly established, although a new- 
r 
apparent blending of images that are really dissimilar. 
dissimilarity is but slight, clear relief is perceived without the 
production of any sensible duplication of images in any part 
of the binocular picture. 
In the light of these facts it is seen that the explanation of 
stereoscopy with perfectly similar figures, as given in 
last paper,* was inc te. It was known to be so at the 
time it was given, but my intention was to present only a geo- 
* This Journal, April, 1882, p. 297, . 
