PICTORIAL STEREOGRAPHY 133ogous to the effect of light and shade. In that, we consider the
effect of shadow gradation, and one important purpose of shadow
gradation is to suggest, upon a plane surface, the contour of an
object. In stereo we have the actual contour shown in relief, so
the shadow plays a minor role in that respect. Now the gradation
of both shadow and contour play a purely pictorial role.
If the scene shows scattered rocks, the contour gradation is a
series of isolated rocks, a jump from one to the next, but if the
object shown is the curving wall of a pool, there is a continuous
recession of contour which the eye follows easily and smoothly.
Interrupted relief which might not be observed in a planar
composition, which might indeed contribute to the continuation
of shadow gradation, will in the stereogram be rough and broken.
On the contrary, a continuous contour may change in color and
be spoked with abrupt patches of shadow and highlight alternat-
ing, yet the smoothness of flow is not interrupted. This complete
antagonism between planar and stereo pictorial character must be
observed, because the planar system alone cannot provide the flow
character desired. It is completely overshadowed by the new ele-
ment of contour gradation. The relation between contour grada-
tion and shadow gradation is that existing between substance and
shadow.
- Color Perspective.-Atmospheric perspective has tradition-
ally been a potent device in the hands of the planar pictorialist,
but unfortunately many of the locations which provide the great-
est amount of pictorial material have such clear air that the haze
element is invisible. Here again the stereographer has the advan-
tage because he can always make use of color perspective. In fact
it is one factor which tends to lower planar quality while it en-
hances stereo quality.
Color, like shadow, has an intensity which in part depends
upon the size of the area and the distance of the color. Given a
certain patch of color, it loses intensity as well as size as we recede
from it, and this characteristic persists even when there is no
observable haze. It is also perceptible over the relatively short dis-
tances within which stereo is most effective, something which is
not at all true of haze-unless the haze is a definite mist or fog.
In the stereo color slide, this color recession is so characteristic
toward a common neutral tint, that it has been the cause of con-