LECTURE  XII
 

§ I. Binocular Vision ; Difference of the Two Retinal Images.
§ II. The Stereoscope; Simplest Stereoscopic Experiments.
§ III. Theory of Stereoscopic Vision.

§1

THE two eyes may be compared to two sentinels who,
viewing the world from different standpoints, impart their
experience to each other, and so complete in idea a common
picture, uniting in itself all that each observer has seen
separately.

It is not very long since the fact was discovered that bino¬
cular vision was different from monocular. The early physio¬
logists thought that the image of an object produced in one eye
was not different from that apprehended with both. And it
was accordingly supposed that the two eyes were simply equi¬
valent to a single eye,—a conclusion which found an apparent
confirmation in the anatomical structure of the optic nerves.
At a certain point in their course after leaving the brain, de¬
cussation occurs. At this point the nerve-fibres are closely
interlaced; and then again two nerve-trunks appear, one ex¬
tending to either eye. It was supposed that at the point of
decussation and interlacing of the two nerves the fibres were
divided. Each fibre, from whichever side of the brain it came,
was thought to divide in such a way that one of its parts ran
to each eye, and within each eye to correspondingly situated
retinal points. It was in 1840 that the English physicist
Wheatstone proved that the images cast upon the retina of
each eye are very frequently dissimilar, without there being
occasioned any disturbance of vision. If we hold an object
close before us, and shut first one eye and then the other, we