134 THREE-DIMENSIONAL PHOTOGRAPHY
siderable complaint about color shots. The charge is made that
the color film is defective. Not at all; the film simply records
the normal loss of color intensity which we see but do not ordi-
narily perceive. It is just another example of the photograph
showing details of a familiar object which escape direct vision.
In the stereo color slide, this color recession is so characteristic
of distance that it actually helps in making distance specific. Its
importance is hardly surpassed by any of the other extrinsic fac-
tors. True, it is not as obvious as size diminution or overlapping
contours, but realism is a subtle characteristic and color perspec-
tive or stereo-chrornatisrn does, perhaps, even more than these to
increase the essential realism of the stereogram.
It will be noted that these factors are all purely psychological.
None of them is definite, none (except perspective) is subject to
measurement, none of them is inherent in the object or scene, and
all of them are subject to different interpretations by different
people.
The only factor of depth perception which is inherent in the
physical conditions of the view and the viewer is that of difleren-
tial parallax, that precise difference between the two members of
all pairs of details afforded by the two points of view. This, and
this alone, is the stereoscopic factor. The other factors simply
afford a greater or lesser intensity to the general appearance of
depth perception.
The inherent factor of differential parallax is essential to stereo-
scopic perception. If it is absent there is no stereo; if it is present
there is stereo. This is not true of any of the psychological factors.
They may enhance the stereo effect, but they can neither produce
it nor destroy it by their presence or absence.
Therefore, when making a stereogram, it is essential that differ-
ential parallax be present. This is automatically assured by making
the two exposures from different points of view. If it is con-
venient to include any of the other factors, they will add to the
effectiveness of the result in some measure; but if it is not practical
to include them, the fundamental stereo effect will be present
even if not one of the psychological factors is included. Perspec-
tive, of course, cannot be eliminated.
In making the stereogram, it must always be remembered that
most of the extrinsic factors mentioned are not peculiar to stereo.