PICTORIAL STEREOGRAPHY 135They are factors present in everyday direct vision, but 9f which
we are not ordinarily conscious. We do not realize that the degree
of stereo relief which we see in everyday life varies with the light-
ing, and with other extrinsic factors. We do not realize that the
farther away an object is the flatter it really appears.
We have had a lifetime in which to balance knowledge against
appearance. Hence a tree trunk five hundred feet distant, looks
as if it were just as round as one ten feet away, yet we know that
to OUT vision it really presents a shape of a flattened oval. In fact
when we know this and train our vision we can actually perceive
the diminishing relief in the distance, but commonly our mental
police step in and say, “Now, you know that tree trunk is round,
so see it that way” and we obey. In fact we do not think about it.
We see the tree, we know trees are round, the tree stands in re-
lief so we simply accept the relief as full relief with never a ques-
tion as to whether it is round or flat.
This visual trick is common in almost every phase of vision.
We see the invisible because we know from experience what it
looks like, but if you want to know the truth about visual per-
ception, show a strange object to a group of five people, let them
look at it ten seconds. Remove it and ask for a description. You
will get five wholly different descriptions. It has been tried too
many times for there to be any question about it.
So we are not introducing technical tricks, merely using in
normal stereo the characteristics of normal vision. For example,
the practical application of some of these extrinsic factors are:
GEOMETRIC PERSPECTIVE.-ThiS is the factor which is ideally
illustrated by a pair of railway tracks or even a roadway extending
in a straight line directly away into the distance. Such a strong
perspective is rarely acceptable pictorially, but the roadway wind-
ing back into the picture is a device commonly used by planar
pictorialists.
In stereo the principle is applied by selecting a point of view
from which several objects are to be seen in relatively receding
planes. It is always advisable to have some object in the immediate
foreground to establish the strongly stereoscopic reference. Then
a succession of objects farther and farther away leads the eye into
the distance and enhances the effectiveness of relief.
It must be borne in mind that this and other devices do not