or three
inches apart. By means of these two different views of an object, the
mind, as it were, _feels round it_ and gets an idea of its solidity. We
clasp an object with our eyes, as with our arms, or with our hands, or
with our thumb and finger, and then we know it to be something more than
a surface. This, of course, is an illustration of the fact, rather than
an explanation of its mechanism.

Though, as we have seen, the two eyes look on two different pictures, we
perceive but one picture. The two have run together and become blended
in a third, which shows us everything we see in each. But, in order that
they should so run together, both the eye and the brain must be in a
natural state. Push one eye a little inward with the forefinger, and the
image is doubled, or at least confused. Only certain parts of the two
retinae work harmoniously together, and you have disturbed their natural
relations. Again, take two or three glasses more than temperance
permits, and you see double; the eyes are right enough, probably, but
the brain is in trouble, and does not report their telegraphic messages
correctly. These exceptions illustrate the every-day truth, that, when
we are in right condition, our two eyes see two somewhat different
pictures, which our perception combines to form one picture,
representing objects in all their dimensions, and not merely as
surfaces.

Now, if we can get two artificial pictures of any given object, one as
we should see it with the right eye, the other as we should see it with
the left eye, and then, looking at the right picture, and that only,
with the right eye, and at the left picture, and that only, with the
left eye, contrive some way of making these pictures run together as we
have seen our two views of a natural object do, we shall get the sense
of solidity that natural objects give us. The arrangement which effects
it will be a _stereoscope_, according to our definition of that
instrument. How shall we attain these two ends?

1. An artist can draw an object as he sees it, looking at it only with
his right eye. Then he can draw a second view of the same object as he
sees it with his left eye. It will not be hard to draw a cube or an
octahedron in this way; indeed, the first stereoscopic figures were
pairs of outlines, right and left, of solid bodies, thus drawn. But the
minute details of a portrait, a group, or a landscape, all so nearly
alike to the two eyes, yet not identical in eac