PICTORIAL STEREOGRAPHY 151have been substantiated by the only tenable form of proof, actual
demonstration, we discover the full value of the science.
Still, there are discrepancies which cannot be explained clearly
by anything but analogy to other sensual experience.
There certainly is a limiting distance beyond which the eye
does not distinguish parallactic difference. There is beyond ques-
tion a true stereo infinity. Moreover that limit can be precisely
evaluated provided we can devise some accurate measurement of
the fundamental sensitivity of the particular individual involved.
Once the basic evaluation has been made, the physical stereopsis
of that individual can accurately be predicted. All this is of the
utmost value in certain stereoscopic studies.
While officially stereo infinity lies at 670 meters, somewhat less
than a half mile, among outdoor people, experience and observa-
tion indicate that the limit actually exceeds a mile. It is doubt-
ful that anyone has a stereo infinity at a distance greater than
21/2 miles. But, if you make a stereogram of a scene which em-
braces a depth of five miles or more, you can actually see in the
stereogram, the natural visual relief extending to the full limit of
the scene. You can even see this in a stereogram in which the
depth is 20 or 30 miles, as in some scenes which include distant
mountain ranges.
Why can you see relief through a depth several times as great
as the value of stereo infinity?
The explanation is the familiar one. The stereogram shows you
just what you would see if you were looking at the real object.
When you see such a scene, you are not conscious that your per-
ception of stereo relief ceases at about a mile, more or less. You
“see” the relief right back to the limit of vision. Why?
In this chapter we have already discussed several extrinsic fac-
tors which enhance the appearance of relief although they do not
affect the true relief. As true stereo relief becomes less and less
evident, these extrinsic factors take over and there is a gradual
replacement until, at stereo infinity, they have taken on the whole
load of relief.
Stereo perception does not mean the ability to see that depth is
present, it does not mean that contours can be interpreted. That
can be seen in a planar photograph, and a person who has unfortu-
nately lost one eye often has a very good judgment of distance.