230 THREE-DIMENSIONAL PHOTOGRAPHY
the long faces are turned toward each other. If the shorter face
is upward or downward you will see objects inverted, but if in
correct position you will see right and left interchanged.
It is an interesting experiment, but of what practical use is it?
This experiment proves the importance of parallax, in fact it
proves some other facts which have been frequently disputed.
First. Experts in the field of vision are often inclined to dispar-
age the importance of parallax in stereoscopic perception. This is
given as one of several factors which result in distance perception.
Such things as perspective diminution, the overlapping of far
objects by those near at hand, and the effect of atmospheric haze,
to mention but a few, are often said to have quite as much effect
in producing relief perception as the parallax of stereo.
But what happens in pseudoscopic vision? Here every factor
other than parallax remains unaltered. Even when there is a
strong light to give the contour effect of shadow forms, no differ-
ence is made. The inverted parallax overrides all other factors
and produces an obvious effect of inverted distance. Inasm’uch as
the factor of parallax is the only one altered, it seems we are
justified in saying that the distance perception, due to stereo par-
allax is greater in its influence than all other contributory fac-
tors combined!
Second. If the foregoing is accepted, we are then forced to
accept the statement... “The apparent distance of an object is
determined by its stereo parallax.” And that too is true. But
once more:
Third. If it is possible to alter the parallax without altering
the distance of an object, that object will appear to lie at the
distance corresponding to its parallax rather than at its true dis-
tance. Again this is demonstrable, for it is the very factor upon
which we base space control. BUT this statement is attacked often
by writers about human vision; some of whom state that other
factors, principally that of the diminishing size caused ‘by per-
spective will prevent the object from being visually accepted at
an altered distance. All stereographers who have experimented
with space control know how utterly false this is. The realism of
space control is the reality of life-and results in one of the few
pictures of any kind which can and do produce genuine astonish-
ment in the spectator.