CHAPTER 15TYPES OF STEREOSCOPYNE ORDINARILY DOES NOT THINK of asking of stereoscopy,
0 “What kind is it?”, simply because stereoscopy is not some-
thing which we think of as being capable of division and classifi-
cation. One thinks we have stereo or we do not have stereo, but
this is not entirely true.
When the term is not qualified, we consider stereoscopy as be-
ing orthostereoscopy, but already we have had occasion to discuss
stereoscopy which is really the genuine thing but which is cer-
tainly not ortho.
ORTHosTEREoscoPY.-This, the normal form, actually repro-
duces upon the retinas images which are identical with those
which would have been produced by the original objects, disre-
garding of course the non-essential factor of motion which may be
present, but not necessarily so.
The orthostereogram, or rather the stereogram viewed ortho-
stereoscopically shows us the appearance of the original object in
full life size at full natural distance and in full, natural color.
It is true that color is not usually included in the definition of
orthostereoscopy, but it is hardly less important than other fac-
tors, because it is an essential factor of stereo relief, wholly aside
from the added attractiveness it gives the picture. This has been
fully discussed elsewhere in this volume. Inasmuch as ortho-
stereoscopy is by definition, fully correct stereoscopy, the term
can hardly be accepted if it does not include color.
HETEROSTEREOSCOPY.-ThiS is a mixed form, such as using a
viewer unrelated to the camera, or any of the “free-viewing” de-
vices. The projected image (in the absence of controlled specta-
tor position), the Vectograph, and the bar-screen processes all fall
into this class. The heterostereogram shows us true parallactic
relief which bears a definite relationship to the real relief, but
not necessarily in normal proportions. The relief may be exag-
gerated much more than normal, as in some aerial views.
Ordinarily the reproduction presents neither full life size nor
true natural distance. In using viewerless methods, the perception
is that of a small relief model held in the hands.
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