parts of the two eyes. What will be the
result?

The instrument was soon made. Two bits
of looking-glass placed back to back were
arranged in the form of a broad letter V,
their angle a right angle and their mirrors
looking outwards. On two little walls placed
at equal distances beyond the mirrors, the
two pictures of the cube were hung and carefully
adjusted so that the two images should be
reflected in precisely the right way. Then an
observer, placing his nose at the point of the V,
and looking with one eye into one mirror, and
with the other eye into the other mirror
would, of course, see with each eye its own
distinct view of the cube, as it had been
sketched. What, then, was the result? Not a
confusion of two sketches, but a complete
reproduction of the cube itself in all its wholeness
of length, breadth, and depth. The illusion
was perfect. The instrument so constructed,
and here rudely described, was a reflecting
stereoscope; and, by its use, Mr. Wheatstone
was able to demonstrate so simply that all
could understand, and no man could dispute
the fact, that the use of two eyes is to obtain
two pictures from different points of view,
and that the use of the differences that exist
in the two images of every solid object so
seen is to assure to the mind the idea of
depth or distance.

Mr. Wheatstone reflected in his mirrors a
pair of real cubes. When they were so
placed that they threw upon the eyes in the
due way two pictures so differing, that they
represented the two aspects of a single cube as
seen by the two eyes, there was a single
cube seen in relief: when they were so
adjusted that each eye received a precisely
similar impression, though two solid forms
were looked at, the mind believed that it saw
only the flat picture of a cube. I need not
multiply such illustrations of a fact already
placed beyond dispute.

A great many experiments could be made
with the reflecting stereoscope by a
philosopher gifted with Professor Wheatstone's
ingenuity; a great many experiments were
really made, and more secrets were in fact
discovered.

Of course the nearer any object is to
the two eyes, the greater is the discrepancy
between the pictures of it seen by
them, and the more vivid the notion of relief.
Of distant objects the views taken by both
eyes are almost identical, and we judge of the
reality of the whole distant scene as the one-
eyed man judges of all things visible. We
judge by experience and comparison, by the
effects of light and shade, and by conclusions
drawn from the movements of the head,
which enable us to note how the view
changes as we change the point of observation.
In looking with a single eye through a
microscope at crystals or other objects, every
observer knows how difficult it is to avoid
misconccption as to which parts of an object are
nearer to the eye, which are more distant
from it.

Since the same object, say a jug of punch,
throws a larger image on the eye in
proportion to its nearness, and since there are
few positions in which it is not nearer to one
eye than to the other, the two images seen at
one time by the two eyes can rarely be quite
alike in size, and so there occurs another
interference with the identity of the two
pictures. Having reflected upon this matter,
Mr. Wheatstone drew two circles differing
somewhat in their size, and presented by
means of his stereoscope one to each eye.
He did not see two circles. Though different
they coincided, and presented the impression
of a circle intermediate in size between the
two. Beyond certain limits; that is to say,
beyond the utmost difference of this kind that
can occur in any case of vision with two eyes
—when each eye squints outwards; no such
coincidence can take place in the stereoscope
between two outlines of unequal magnitude.
The mind, however, never does more than its
assigned work in the way of fusion. Whoever
wears a pair of spectacles with one glass
blue and the other yellow, will not see
surrounding objects coloured green. The different
impressions made upon his two eyes will
not in that case mingle, but— sometimes one
predominating, and sometimes the other—
he will see things always tinged either with
blue or yellow, sometimes with one colour
and sometimes with the other, but always
with only one of the two colours at one time.

One of the oddest and most instructive
results of experiment with the reflecting
stereoscope, detailed by Mr. Wheatstone—one
which creates artificially a complete chaos of
the laws of vision—we must endeavour in the
next place to explain. In order to do so, we
must make use of and first understand a
technical expression—optic axes. What are
optic axes? Place upon the table before you
one small stone, and look at it with both
your eyes. The line drawn from the stone
at which you are looking through the centre
of one eye-ball is one optic axis, and the
line from the same point, through the other
eye-ball, is the other axis. On the stone,
when you look at it, the lines of course
converge. Look at the stone from a considerable
distance, and the two lines or axes run
for a long way side by side; look at it from
a distance of three inches, and the lines
converge very rapidly; in other words, they
form, when they meet on the stone, in the
first case a small angle, and in the last case a
large one. Very well. Now, as you come
nearer to the stone in walking from a corner
of the room towards the table, the optic axes
converge upon it gradually more and more,
at the same time that the image of the stone
enlarges on the retina. It is a familiar
experience that things in motion become larger
on the eye as they approach us, smaller as
they recede. At the same time, while they