132 THREE-DIMENSIONAL PHOTOGRAPHY
parallel to the film plane. It is the most potent device yet found
for lending to a picture upon a single plane, a suggestion of depth.
- Contour Interference.-If a tree is shown in front of a build-
ing, the side of the building which lies behind the tree cannot be
seen. Hence we know at once that the tree is in front of the build-
ing. If the lower part of the tree trunk were hidden by the
building, we would know that the tree was standing behind the
building. This is the principle of interfering contours, the hiding
of objects or portions of objects by other objects which are nearer
the observer. Alone, it will give a fairly accurate index of relative
depth.
- Sire Diminution.-This partakes of both the preceding fac-
tors, and is the well-known effect of apparent decrease in size as
an object becomes more distant. It is usually assumed to be an
inherent phase of perspective, and is usually included as such
when the principles of perspective are explained. However, it
should really be considered apart from, or at any rate as a distinct
subdivision of, perspective, because of its great influence upon
the appearance of non-stereoscopic depth.
- Light and Shade.-The forms and intensities of contour
shadows are so important that they, together with the careful ap-
plication of perspective, form almost the sole means for imparting
to a plane image a simulation of depth. Just as an angular or
broad light gives to the plane image a false appearance of depth,
so in stereoscopy does the same light enhance the true stereo
effect. (Compare Contour Gradation.)
- Hare.-The effect of haze, sometimes called aerial perspec-
tive, aids to some extent in the perception of depth. It is com-
monly used in planar photography to afford separation of planes
(i.e, stereo relief), but when applied to stereography the result
is usually an unfortunate appearance of a weak, washed-out print.
(Compare Color Perspective.)
These five factors apply equally to planar reproduction, and in
fact they constitute the principal devices which are used to suggest
depth in the planar photograph. There are two additional factors
which are purely stereoscopic, and which must be given careful
attention by the stereographer who wishes to attain success in
stereo pictorialism.
- Contour Gradation.-Contour gradation is somewhat anal-
