secrets of double vision, which could never
have been either thought out or discovered
by a mere watching of nature at her daily
work, have been wormed out of her by such
tricks or such experiments.

Place any irregular or angular solid body
on the table before you. Close each eye in
turn, while you observe the object accurately
with the other. You will not fail to observe
that a slight—but very sensible—difference
exists between the results of the two sights
taken from two points in the same head at
the same object The points of sight in the
two eyes are of course different, and by
the laws of perspective it is easy to
determine that the views of the same thing
taken from those two points could not be
identical. That is very obvious and very
simple. Yet that simple observation is the
whole basis of the theory of the stereoscope,
and it had not been made or rather
when made had been always set aside as
immaterial, before Professor Wheatstone built
upon it one of the most beautiful little
discoveries that grace the science of our day.
There is a reason, thought Mr. Wheatstone,
for this difference. It had been commonly
supposed that single vision with two eyes only
resulted from the falling of the same point of
the picture formed by an object on the same
point in each eye. But that is what can take
place only in the case of a painted landscape.
If we look at a Claude or a Canaletto the
eyes both see the same picture, and both see
it in precisely the same way, but the result
is that they see it as a flat painting on
canvas, and are so convinced of its flatness, that
the best skill in shadow and perspective will
not cause the houses to look really solid,
the hills really to appear as lumps arising
on a broad flat earth. The best picture will
not, as an illusion, stand the test of two
eyes. But if we look at it with one eye,
the painter can cheat that. If one eye be
not allowed to compare notes with its neighbour,
and to see the objects which profess to
lie one behind another from a second point of
view, then accurate lights and shadows in a
picture, corresponding to the real light in the
room, will be assumed as evidence of actual
solidity. In a landscape that consisted of
real fields and trees, or in a real street, one
eye could have obtained not much more
evidence than that, and the mind, satisfied to
get the utmost evidence attainable, would
upon that have founded a conclusion. For
this reason, connoisseurs may be seen often
shutting one eye when they examine a
painting. If use be made of a hollow tube,
or a roil of paper, which is the same thing, in
such a way that the frame, and all surrounding
objects of comparison are carefully
excluded, the cheat perpetrated upon one eye
by a really good picture is very complete
indeed.

Leonardo da Vinci noticed this method of
examining a picture with one eye, and is the
only person who before our times had
reasoned on the matter. He pointed out, that
if you look at a solid globe with one eye
it conceals a certain piece of background,
which to the other eye is visible; and if you
change the eye you change the background,
so that, as he said, except a certain part
behind the globe invisible to both eyes, the
solid body is in a certain sense transparent.
He thought that the impossibility of cheating
two eyes with a picture lay in the impossibility
of getting at this state of affairs in the
background. Mr. Wheatstone observes justly,
that had the philosophic painter taken any
other solid than a ball on which to found
his illustration, he would have observed not
only the difference in the background, but
also the difference between the two
perspectives. But he did not. Mr. Wheatstone,
therefore, was the first who called distinct
attention to this very obvious, but, nevertheless,
practically new fact in the theory of vision.

Then the experimenter said to himself:
The old theory which supposed an identity
between the pictures painted at the same
time on the two eyes being false, there
must be something more in the disparity
than a mere necessary awkwardness resulting
from the impossibility of having two
eyes in one place. If the possession of two
eyes only caused a confusion to be got over
by habit, we two-eyed people should be all
really worse off than Polyphemus. Why
have we two eyes? That was the question
which Mr. Wheatstone entrapped Nature
into answering. The trap set by him was the
stereoscope.

One could not easily imagine any apparatus
simpler in its construction. Since it was not
possible twenty years ago, by aid of
photography, to obtain on paper or silver two
sketches of the same scene, having only the
minute difference in the point of view that
would exist between the two points of sight
furnished to man by Nature—which are
about two-and-a-half inches distant from
each other in an ordinary adult head—Mr.
Wheatstone took the simple forms of cubes
and other solid mathematical figures, placing
them before him, and carefully making two
sketches of each, corresponding to the two
appearances presented by it to the two eyes.
They were obvious and easy of depiction.
They were made simply in outline, and in
each case, of course, were evidently flat
copies. Let us take the example of the cube.
These, the experimental philosopher then
reasoned, are the images of the cube
separately presented to each eye; flat outlines
evidently. Let me contrive now to look at
them in such a way that the right eye shall
see only its own proper picture as I have
drawn it from its own proper point of view,
and the left eye the other picture, and that
they shall fall as they do in nature with their
respective differences upon corresponding