228 THREE-DIMENSIONAL PHOTOGRAPHY
of such untransposed slides. Such slides are called “pseudograms”
to distinguish them from “stereograms.” This term is often con-
fused with “pseudo-stereograms.” The pseudogram (picture in
the method of pseudoscopy) is a differentiated pair mounted in
reverse. The pseudo-stereogram is a pair of identical pictures,
made from one negative which of course exhibit only that peculiar
indefinite relief common to all pictures viewed through a lens and
which so many people consider to be truly stereoscopic. So, to
prevent confusion, we shall refer to the pseudogram as an inverted
s t ereogram.
The basic effect is that of transposing distances. Those objects
far away seem to be near; while nearby objects seem to be far
away. Of course, nearby objects obscure parts of those objects be-
hind them, so we have silhouettes of odd shape hanging in the
air nearby. But because of the absence of perspective diminution,
the fact that the distant objects conform exactly to the outlines of
these patches is not at once apparent. This effect has been briefly
discussed in the chapter on pictorial work.
In fact, unless there are marked planes and a fairly great
variety of them, the effect is appreciable only to the experienced
stereoscopist. There is great excuse for the beginner who cannot
see what is wrong with one of these slides.
One of the worst types of subject is that which includes several
planes, closely spaced, and with a considerable distance between
the camera and the first significant plane. Subjects of this kind
have confused even experienced stereo technicians for a time, be-
cause the inversion of distances is not immediately apparent. Very
often the stereoscopist states that the slide is made of two identi-
cal prints.
So, if you find among your slides one which for some obscure
reason does not look just right; which appears to be somewhat
“jumbled” without being fuzzy just try changing places with the
two films and see if that does not solve the problem.
It has been said (and written) that pseudoscopic vision is purely
a phenomenon of photography, but this is definitely untrue. It is
quite easy to see inverted distance by direct vision. One of the
easiest ways is to remove both ocular and objective lenses from a
pair of prismatic binoculars, and to look through the prisms alone.
There is, of course, no magnification, but the pseudoscopic relief