TRICK WORK 23 1Now let us carry this reasoning to its logical conclusion. It
might be said that the external stimulus of stereoscopic percep-
tion is parallax; but this is not enough. For any one given dis-
tance there is a corresponding parallax. But if the stereogram
shows us just one plane in the strictest sense it is not perceptibly
stereoscopic; at the same time if we do have a single plane, the
parallax normal to that plane is present. Therefore it is not
enough simply to have parallax.
If the subject consists of a group of objects all in precisely the
same plane, each exhibits the parallax of its position, but still we
do not have stereoscopic relief, as the image is still confined to a
single plane.
Therefore the mere existence of multiple parallax is not suffi-
cient, although it is obvious that multiple parallax is essential.
If there are objects in different planes, each will exhibit the
parallax normal to its distance, and these various distances will
create a series of different parallax values. Thus we have the
establishment of depth in the sense of the establishment of several
planes, and we are approaching our goal.
Now I must ask you to bear with me in a bit of repetition: If in
viewing a stereogram (or real objects for that matter) we fix our
eyes upon a single point and hold it steadily for several minutes-
(which by the way is almost superhumanly difficult, because most
people do not realize that “steadily” means without shifting the
gaze as much as i/ioo inch)-the eye becomes conscious only of
the spot upon which attention is focused. The side images are
as visible as ever, but they remain motionless and demand no
attention whatsoever. The result is that we return to the condi-
tion of a single parallax, and there is a definite loss of stereo per-
ception. The instant the eyes are moved from the target spot, the
scene jumps back into full relief. In fact this experiment is more
strongly marked when direct vision is used rather than a stereo-
gram.
Thus we come to our final factor, the shifting of the eyes from
point to point, the focusing of attention, no matter how briefly,
upon point after point. Our eyes are constantly in motion, some-
thing of which we are unaware until we gaze fixedly at some
object and so produce eye strain! Therefore parallax must be dy-
namic if depth is to be perceived.