approach the optic axes converge more
towards them, and again the said axes become
more nearly parallel as they are departing.
Now it was no hard matter for Professor
Wheatstone so to adjust pairs of pictures on
the moveable walls of his reflecting stereoscope
as that all ordinary experience should
in this matter be contradicted.

In the first place, he arranged the stereoscopic
pair on arms moveable only in a circle,
so that the images in the two mirrors should
always be of the same size, being formed by
pictures always at a like distance from the
mirrors, but that the eyes should be obliged
in following the movements of the pictures to
vary the degree of convergence of the optic
axes. He found that as the convergence of
the optic axes lessened (suggesting distance)
the perceived size of the image grew upon the
mind, and it seemed to become smaller as the
convergence was increased. The real size of
the image was, as we have said, unaltered. In
nature, as the convergence of the axes lessens,
the size of the image lessens, but its
perceived magnitude remains the same; because
the mind, at all reasonable distances, insensibly,
through habit and experience, forms a
pretty equal and just conception of the size
of objects.

The experiment, just cited, was then
reversed. By simply sliding the two pictures
nearer to the mirrors, the size of the image
thrown upon each eye was enlarged, but the
position of the images upon the mirrors not
being shifted, in observing them the inclination
of the optic axes was not altered. The
alterations in size were perceived accurately,
and while the pictures were moved to and
fro, the image, enlarging and diminishing,
cheated the mind in a fresh manner; it
appeared in the most evident way to be moving
backwards and forwards. And yet observe
the curious distinction, whenever it stood
still, and whatever might be then its
perceived size, there was no apparent change in
its position, it never seemed to have moved
at all. It always appeared, when motionless,
to be at one and the same distance from the
eye, because the chief measure of distance—
the amount of convergence of the optic axes
—never altered.

A similar delusion was elicited in the
companion experiment, wherein though the real
size of the image never altered, the degree of
convergence of the axes being made constantly
to vary, caused it apparently to increase and
decrease. In that case, while the picture
grew or dwindled, as we know by experience
that it would increase upon the eye or
dwindle if advancing or receding, yet, for all
that it never seemed to move. It stood still
enlarging like the dog that grew into a
hippopotamus before the eyes of Dr. Faustus.
Nevertheless, whenever the trial ceased,
whatever change has been made in the
position of the stereoscopic plates was
represented to the eye as a difference of
distance; the image had got, apparently, into a
new place, because the inclination of the axes
ceased to be the same. Thus, we may be
told to look at an object in this magic instrument
advancing and receding without changing
place, and changing place without being
observed to move. A state of things utterly
contradictory and confusing, scarcely or not
at all conceivable, because it never has been
in the experience of any man from Adam
downwards, until Mr. Wheatstone learned to
detect and re-combine and make experiments
upon the first principles of vision in his new
instrument, the stereoscope.

Enough has been said to show the great
value and importance of the stereoscope to a
philosophical investigator of the laws of sight.
When we before spoke of this instrument we
said that, apart from its philosophical use, it
was employed only as a toy. It is to be
purchased now—in its less perfect forms—
in all toy-shops; and the use to which it is
put commonly by the photographer, though
agreeable, is unimportant. The stereoscope
itself, however, is not only of philosophical
importance, it admits of many really valuable
practical applications. We need refer only
to what has been already said of the difficulty
experienced by the microscopist in determining
with one eye whether crystals and other
objects seen by him are hollow or solid. If
a sovereign be looked at through a microscope,
the Queen's head upon it will as often appear
to be sunk into the coin as to stand out in
relief from it. Now, however, when photographic
copies can be taken of objects seen in
the field of the microscope, it will suffice to
take two copies of the same object, with the
due angle of difference between their points
of view, and place them in a stereoscope.
The power of two eyes will be then brought
to bear upon the object seen with one eye
only through the glasses of the microscope,
and a correct impression will be formed of
its relative dimensions.

Having explained their principle, we do not
think it worth while to discuss the construction
of the different forms of stereoscope now
in use. In the refracting instrument,
invented afterwards by Mr. Wheatstone, as
convenient for the examination of small
pictures, prisms are used to deflect the
rays of light proceeding from the pictures;
refracted arc there substituted for reflected
images.

Of this instrument the small portable
stereoscope in common use is a modification
suggested by Sir David Brewster. Its pair
of prisms are the two halves of a common
lens. An ordinary lens having been cut
in half, the cut edges are turned outwards,
and the two half circles, or thin edges of the
two prisms so made, are directed towards
each other. They are placed about two
inches and a half apart, with a power of
adjustment that enables them to be presented
accurately to any pair of eyes, so that each