ON CERTAIN DIFFICULTIES EXPERIENCED IN THE USE OF THE STEREOSCOPE.
There are many persons who experience great difficulty in uniting the
two pictures in the stereoscope, and consequently in seeing the relief
produced by their union. If the eyes are not equal in focal length,
that is, in the distance at which they see objects most distinctly;
or if, from some defect in structure, they are not equally good, they
will still see the stereoscopic relief, though the picture will be
less vivid and distinct than if the eyes were in every respect equal
and good. There are many persons, however, whose eyes are equal and
perfect, but who are not able to unite the pictures in the stereoscope.
This is the more remarkable, as children of four or five years of age
see the stereoscopic effect when the eye-tubes are accommodated to the
distance between their eyes. The difficulty experienced in uniting the
binocular pictures is sometimes only temporary. On first looking into
the instrument, _two_ pictures are seen in place of _one_; but by a
little perseverance, and by drawing the eyes away from the eye-tubes,
and still looking through them, the object is seen single and in
perfect relief. After having ceased to use the instrument for some
time, the difficulty of uniting the pictures recurs, but, generally
speaking, it will gradually disappear.
[Illustration: FIG. 52.]
In those cases where it cannot be overcome by repeated trials, it
must arise either from the distance between the lenses being greater
or less than the distance between the eyes, or from some peculiarity
in the power of converging the optical axes, which it is not easy to
explain.
If the distance between the pupils of the two eyes, E, E′, Fig. 52,
which has been already explained on Fig. 18, is _less_ than the
distance between the semi-lenses L, L′, then, instead of looking
through the middle portions _no, n′o′_, of the lenses, the observer
will look through portions between _o_ and L, and _o′_ and L′, which
have a _greater_ power of refracting or displacing the pictures than
the portions _no, n′o′_, and therefore the pictures will be _too much_
displaced, and will have so far _overpassed_ one another that the
observer is not able to _bring them back_ to their place of union,
half-way between the two pictures in the slide.
If, on the other hand, the distance between the pupils of the
observer’s eyes is _greater_ than the distance between the semi-lenses
L, L′, then, instead of looking through the portions _no, n′o′_ of the
lenses, the observer will look through portions between _n_ and L, and
_n′_ and L′, which have a _less_ power of refracting or displacing the
pictures than the portions _no, n′o′_, and therefore the pictures will
be so _little_ displaced as not to reach their place of union, and will
stand at such a distance that the observer is not able to _bring them
up_ to their proper place, half-way between the two pictures in the
slide.
Now, in both these cases of _over_ and _under displacement_, many
persons have such a power over their optical axes, that by converging
them to a point _nearer_ than the picture, they would, in the first
case, _bring them back_ to their place of union, and by converging them
to a point _more remote_ than the picture, would, in the second case,
_bring them up_ to their place of union; but others are very defective
in this power of convergence, some having a facility of converging them
beyond the pictures, and others between the pictures and the eye. This
last, however, namely, that of near convergence, is by far the most
common, especially among men; but it is of no avail, and the exercise
of it is injurious when the under refracted pictures have not come up
to their place of union. The power of remote convergence, which is
very rare, and which would assist in bringing back the over refracted
pictures to their place of union, is of no avail, and the exercise of
it is injurious when the pictures have been too much displaced, and
made to pass beyond their place of union.
When the stereoscope is perfectly adapted to the eyes of the observer,
and the _general_ union of the pictures effected, the remote parts
of the picture, that is, the objects seen in the distance, may be
under refracted, while those in the foreground are over refracted,
so that while eyes which have the power of convergence beyond the
picture, unite the more distant objects which are under refracted, they
experience much difficulty in uniting those in the foreground which
are over refracted. In like manner, eyes which have the power of near
convergence will readily unite objects in the foreground which are over
refracted, while they experience much difficulty in uniting objects in
the distance which are under refracted. If the requisite power over the
optical axes is not acquired by experience and perseverance, when the
stereoscope is suited to the eyes of the observer, the only suggestion
which we can make is to open the eyes wide, and expand the eyebrows,
which we do in staring at an object, or in looking at a distant one,
when we wish to converge the axes, as in Fig. 22, to a point _beyond_
the pictures, and to contract the eyes and the eyebrows, which we do in
too much light, in looking at a _near_ object, when we wish to converge
the optic axes, as in Fig. 21, to a point _between_ the pictures and
the eye.
When the binocular pictures are taken at too great an angle, so as to
produce a startling amount of relief, the distance between similar
points in each picture, both in the distance and in the foreground, is
much greater than it ought to be, and hence the difficulty of uniting
the pictures is greatly increased, so that persons who would have
experienced no difficulty in uniting them, had they been taken at the
proper angle, will fail altogether in bringing them into stereoscopic
relief.
In these observations, it is understood that the observer obtains
distinct vision of the pictures in the stereoscope, either by the
adjustment of the moveable eye-tubes, if they are moveable, as they
ought to be, or by the aid of convex or concave glasses for both eyes,
either in the form of spectacles, or separate lenses placed immediately
above, or immediately below the semi-lenses in the eye-tubes. If the
eyes have different focal lengths, which is not unfrequently the case,
lenses differing in convexity or concavity should be employed to
equalize them.
EDINBURGH: T. CONSTABLE, PRINTER TO HER MAJESTY.