204 THREE-DIMENSIONAL PHOTOGRAPHYlogically projected into space so that it occupies the same position
as the real object. Only thus can we have stereoscopic vision. Inci-
dentally this is accepted theory and will be found in authoritative
textbooks of physiological optics.
So if you “see” an automobile, what you actually see is an arti-
ficial image projected from your own mind. If you see a stereo-
gram, you see an identical artificial image, synthesized in your
brain and projected in exactly the same way. The image is placed
in space at exactly the same distance. In fact, the only difference is
that existing between two different external stimuli, but from the
retinal reception on the two processes are identical.
The true fact is that you never actually “see” any object stereo-
scopically, you only see the artificial “mockup” which you have
created yourself. Thus in stereo, although you will not want to
accept the fact, you actually and literally see exactly the same thing
you would see in observing the original object.
The difference between the free-vision stereograms and those
seen in the stereoscope will always remain the difference between
a tiny model and the real thing. As a matter of fact, free-vision
stereograms have very little advantage over the ordinary flat pho-
tograph, aside from their novelty. Make them commonplace for
five years and people will not care whether they have them or the
old planar type.
The stereoscope is a fundamental and essential part of that
stereoscopy which actually re-creates realism in its fullest detail.
Second is the use of a good projector in a fully darkened room.
Other systems fall too far short of realism to have more than
novelty appeal, with the exception of the Vectograph whose stereo
quality makes it a true member of the stereo family.
THE BAR-ScREEN.-Ives invented the system in 1903, and four
years later Lumihe Autostereo plates were marketed. These plates
were backed with a grid of fine lines spaced from the emulsion by
the thickness of the glass base. A dual diaphragm in the lens
caused the images to fall upon this grid from the two sides so that
the “shadow” of a grid line from one lens aperture fell exactly
between two grid shadows from the other aperture. Thus, al-
though the whole image was composed of alternate fine strips
from the two stereo images, the image as a whole would appear to
be normal, except for a suggestion of a line pattern caused by