Three-Dimensional Photography - Principles of Stereoscopy

44 THREE-DIMENSIONAL PHOTOGRAPHY
normal sensation. At this point the less tangible reactions take
place, The two sensations are synthesized by a process which we
do not fully understand, to produce in the Cyclopean center a
perception of the three-dimensional image corresponding to the
visible surface of the solid hexagonal object. However, this image
is created and exists only in the brain, hence the necessity for the
final step, the projection. The image which is synthesized is then
psychologically projected out into space until it occupies the same
identical position in space occupied by the original solid body,
and we have the sensation of seeing into space for a definite dis-
tance. A more detailed discussion of this “projection” of the im-
age from the Cyclopean center into space may be found in any
authoritative reference book of physiological optics.
The stereoscope is the physical means which we use to make
possible a projection into space identical with that which would
have existed had we viewed the original object. It is well known
that “free-vision” stereograms, such as Vectographs, anaglyphs,
bar-screen stereograms, and the like, do not even approximate the
original distances of depth, as does the true orthostereogram, and
in that respect they are inferior to the viewer type of stereogram.
The projection of the free vision image is strictly limited.
The stereoscope provides a set of physical conditions which
make possible exactly the same projection which would exist in
directly viewing the original object. Thus it is possible for us to
sit in a room, facing a wall only a few feet away yet truly see
hundreds of yards out into space directly through the wall. How-
ever this may suggest fantasy, every competent stereographer
knows it to be a very real fact. That is the basis, not only of the
charm of stereo, but for some very genuine beneficial effects which
result from the consistent use of the stereoscope.
Stereoscopic Idiosyncrasy.-Stereoscopic perception is a charac-
teristic of marked variability. Like those who are color blind,
there are those who have perfect binocular vision (that is, they
have two eyes which are synchronized and which produce perfect
visual images) yet who have little or no stereoscopic perception.
The condition itself is somewhat analogous to color blindness, but
unlike that defect of vision, in many instances stereoscopic per-
ception can be developed by consistent exercise.
Some people have such a marked stereoscopic sense that they