STEREO PROJECTION 181
appreciable. In fact it could, without too great an error be called
orthoprojection.many problems suggested and some amateurs have even gone so
far as to state flatly that even in paraprojection there remains an
unbearable degree of depth distortion! This is not at all true. The
dimensional accuracy of the para image has been measured geo-
metrically and by direct means; it has also been demonstrated to
be subjectively accurate. The statements to the contrary, upon
investigation, come from those who have seen projection once or
twice and have then based their judgment of projection upon their
own, untrained observation. This is not the type of criticism which
deserves consideration.
APPARENT DIsToRTIoN.-However as long as there is any unde-
sirable effect, it is well to understand what it is and how to over-
come it. Regardless of whether the fault lies with the individual
or with the technique, a remedy is desirable. Therefore we shall
take up the two most frequently mentioned “distortions” those of
abnormal movement of the subject and the often condemned ex-
cessive relief.
APPARENT MoTIoN.-When viewing the stereo screen two things
will be noticed. If you walk across the room parallel to the screen,
any object which points more or less directly at you will follow
you. Also if you walk from a distance toward the screen, the length
of the object will seem to decrease in the direction of depth.
Although these motions have been described as abnormal, ri-
diculous, unnatural and a whole series of similar names, the
strange fact is that they do not exist. Remember, the image is pro-
jected upon the screen and it is motionless! No matter how you
view the screen, the relative positions shown are those fixed upon
the slide. There is no motion of the Projected image whatsoever.
In other words these “abnormal motions” simply do not exist.
The reason, of course lies in the fact that the visual motions
normally seen in direct vision are absent; we therefore seem to see
motions in the opposite directions. You are familiar with the way
in which a landscape appears to flow past you as you look from the
window of a train or bus. That fact alone demonstrates the exist-
ence of visual motion in real life. But what is this motion?
When you walk past a nearby fixed object, you can see farther
QUESTIONS ABOUT PARAPRO JECTION.-NatUrally there have been