When we look at any round object, first with one eye, and then with
the other, we discover that with the right eye we see most of the
right-hand side of the object, and with the left eye most of the
left-hand side. These two images are combined, and we see an object
which we know to be round.
This is illustrated by the _Stereoscope_, which consists of two mirrors
placed each at an angle of 45 deg., or of two semi-lenses turned with
their curved sides towards each other. To view its phenomena two
pictures are obtained by the camera on photographic paper of any object
in two positions, corresponding with the conditions of viewing it with
the two eyes. By the mirrors on the lenses these dissimilar pictures
are combined within the eye, and the vision of an actually solid object
is produced from the pictures represented on a plane surface. Hence the
name of the instrument, which signifies _Solid I see_.--_Hunt’s Poetry
of Science._