uggestive of surprise as our daily bread."

A stereoscope is an instrument which makes surfaces look solid. All
pictures in which perspective and light and shade are properly managed,
have more or less of the effect of solidity; but by this instrument that
effect is so heightened as to produce an appearance of reality which
cheats the senses with its seeming truth.

There is good reason to believe that the appreciation of solidity by the
eye is purely a matter of education. The famous case of a young man who
underwent the operation of couching for cataract, related by Cheselden,
and a similar one reported in the Appendix to Mueller's Physiology, go to
prove that everything is seen only as a superficial extension, until
the other senses have taught the eye to recognize _depth_, or the third
dimension, which gives solidity, by converging outlines, distribution
of light and shade, change of size, and of the texture of surfaces.
Cheselden's patient thought "all objects whatever touched his eyes, as
what he felt did his skin." The patient whose case is reported by Mueller
could not tell the form of a cube held obliquely before his eye from
that of a flat piece of pasteboard presenting the same outline. Each of
these patients saw only with one eye,--the other being destroyed, in one
case, and not restored to sight until long after the first, in the
other case. In two months' time Cheselden's patient had learned to
know solids; in fact, he argued so logically from light and shade and
perspective that he felt of pictures, expecting to find reliefs and
depressions, and was surprised to discover that they were flat surfaces.
If these patients had suddenly recovered the sight of _both_ eyes,
they would probably have learned to recognize solids more easily and
speedily.

We can commonly tell whether an object is solid, readily enough with one
eye, but still better with two eyes, and sometimes _only_ by using both.
If we look at a square piece of ivory with one eye alone, we cannot tell
whether it is a scale of veneer, or the side of a cube, or the base of
a pyramid, or the end of a prism. But if we now open the other eye, we
shall see one or more of its sides, if it have any, and then know it to
be a solid, and what kind of a solid.

We see something with the second eye which we did not see with the
first; in other words, the two eyes see different pictures of the same
thing, for the obvious reason that they look from points two