50 THREE-DIMENSIONAL PHOTOGRAPHY
appears to be smaller by comparison with other objects at the
same apparent distance.
The same thing applies to the use of hyperstereo. If we have a
vast expanse in a mountainous country, the distances are so great
that the major portion of the scene lies beyond stereo infinity and
we might as well use a planar camera. However, we can introduce
true stereo relief by making use of an increased base. In short, we
increase the degree of parallax beyond the natural. Thus by every
applicable law of stereo, we should have a small sized image at
an apparently near distance. In fact this is the argument ordinarily
chosen to combat the use of the technique. Unfortunately the
argument is not tenable because of the fact that size perception is
always relative, and if we have no other objects in the same stereo-
gram which exhibit a basically different degree of parallax for a
fixed degree of perspective, there is nothing to show that the sub-,
ject is a “miniature view.”
Pseudo-Stereograms.-The pseudo-stereogram, or false stereo-
gram, must not be confused with the pseudoscopic stereogram.
The latter is simply a stereogram which has not been transposed
and which shows the relationships of depth reversed. This is often
used in the study of small, complex solid bodies, the reversal
bringing out points which in the ordinary view are overlooked.
The pseudo-stereogram is simply a slide made of two identical
images. That is, two prints from the same negative are mounted
side-by-side. It is obvious that the result will be flat because it
lacks parallax. However, it is also known that looking at any pho-
tograph through a lens system gives to it a vague appearance of
solidity. This appearance is enhanced in the stereoscope and the
result is that we have every factor of depth perception present
except parallax. Usually the result is the indefinite roundness sup-
plemented by the specific characteristics of perspective, so that
many who are not experienced in stereo are deceived by a well
made pseudo-stereogram. At the same time there is something
lacking, and usually the comment is that the stereogram “isn’t
very good.”
Stereoscopic Dip1opia.-We may well ask what makes stereo
work. There is no conclusive answer to this, but the fundamental
mechanism of stereoscopic vision may be glimpsed when we con-
sider a factor or two which are ordinarily ignored.