202 THREE-DIMENSIONAL PHOTOGRAPHY
practice of advertising slide viewers, projectors, lenses and the
like on the basis of “three-dimensional effect” (when they are not
truly stereoscopic) is to be deplored.
No other pictorial art has ever been subjected to the false pub-
licity, the inoperative systps and inventions and the intentionally
false representations which have dogged stereoscopy throughout
its life of more than a century. Hardly a year passes without the
announcement of the discovery of some revolutionary three-
dimensional process, only to prove it has nothing “three-dimen-
sional” in it other than the name. This has been one of the most
popular of pastimes in the motion-picture field.
SINGLE IMAGE RELIEF.-It is well known that when a single
picture is viewed in a concave mirror or through a convex lens it
does take on an appearance of “unflatness,” but so vague is the
relief that it cannot by any stretch of the imagination be called
stereoscopic, yet that is just the word used to sell these inoperative
devices. Even some of our reputable magazines permit the use of
the term for selling concave mirrors, probably because of editorial
ignorance of true stereo.
A stereoscopic effect is one in which the true, natural quanti-
tative relationships of all objects and planes within the field of
view are normally reproduced. Nothing less than this can truth-
fully be called stereoscopic. As we have seen that this effect can
be obtained only when the two eyes each receive a differentiated
image, which in turn necessitates two photographs, it is obvious
that a true “single image” would send identical stimuli to both
eyes, and that therefore stereoscopic perception of a single image
is fundamentally and basically impossible.
INTEGRATED STEREOGRAMS.-HOWeVer, the single image ideal is
approximated by the integrated type of stereogram. Loosely ap-
plied, this may be used to describe the anaglyph and the Vecto-
graph as well as the bar-screen type of stereogram, which is
ordinarily called the Parallax stereogram. But inasmuch as all
stereograms are made possible only by parallax, the term becomes
vague and meaningless. “Bar-screen” or “grid” describes the proc-
ess specifically and leaves no room for confusion. In all of these
processes the two images occupy substantially the same positions
upon the supporting base, and are differentiated by color (an-
aglyph), polarization of light, (Vectograph), or by an opaque