Three-Dimensional Photography - Principles of Stereoscopy

182 THREE-DIMENSIONAL PHOTOGRAPHY
objects which were hidden by the near one. In short, while you
see the distant object apparently maintaining its relationship to
you, the near object visually moves in a direction opposite that in
which you are moving. If this movement were to cease, the near
object would retain its position relative to you, as it does upon the
stereo screen, and it would therefore necessarily move along with
you. Only by so doing could it maintain its position relative to
you.
Again, when you walk toward an object, the parallax of that
object increases according to optical law; and the greater the par-
allax, the greater the visual length of that object in the direction
of depth. Therefore, visually, the nearer you approach to an ob-
ject, the deeper it grows. This is not usually recognized but can be
easily demonstrated to be true.
Therefore in the direct vision of real life we have two visual
movements of stationary objects, (A) The object moves in a direc-
tion opposite your own motion and (B) the nearer you approach
to an object the deeper it grows.
Now what happens in stereo projection? Because the images are
all fixed, there can be no change in relative position, no change in
parallax.
When you walk past an object the normal shifting of relative
position does not occur and you perforce maintain the same line
of view into depth. Therefore the stationary picture, because it
lacks the normal relative motion of real life seems to take on a mo-
tion in the opposite direction, that is it moves with you instead of
against you and so the object “follows” you. Thus the abnormal
motion is after all nothing more than the absence of relative mo-
tion among stationary objects.
Likewise, when you approach the screen, the parallax of an ob-
ject does not increase, but its width does. The effective result is a
decrease in depth, but this too is only relative. As you walk away
from it, it does not lose parallax and so seems to increase in depth.
This has been discussed in another phase in the early part of this
chapter. So it can be seen that the movement of the image which
is objected to is after all nothing but the absence of a motion
which does exist but of which we are not directly aware in real life.
It would be just as sensible to criticize the still picture for not