Three-Dimensional Photography - Principles of Stereoscopy

SPECIAL PHASES 297

And of course, nothing in the field of medicine has ever been

attempted which can ever approach it.

STEREO DRAWING

Because the characteristic stereo-subjective reaction has its
stimulus in definite geometric relationships, it is possible to pro-
duce drawings based upon such relationships and to have these
drawings appear in full three dimensions. Once no more than a
novelty, it is now somewhat commonplace to construct true stereo
elevations from floor-plans.
Geometric Drawing.-This should not be misunderstood. The
geometric drawing itself is easily constructed by stereo methods,
but many freehand artists make a “net” or “cage” in stereo by
geometric methods and then use this net as a guide for the stereo-
freehand drawing. This is similar to the use of cross-section paper
for the purpose of reducing or enlarging a freehand sketch.
Stereo drawing is based upon the principles of geometric per-
spective. Inasmuch as full sized textbooks have been devoted to a
discussion of this art, we do not have the space to develop the
theory fully. However, it is possible to give a superficial explana-
tion of the process.
Perspective.-Perspective drawing is based upon a horizon line;
and somewhere along that line there are two vanishing points.
Usually one vanishing point is nearer the working center of the
line than the other, so that (assuming the object to be a cube), the
perspective view will show more of one side of the cube than of
the other. If the vanishing points are equally spaced, one edge of
the cube will be directly before the eyes and an equal portion
of both sides will be seen.
There are really two distinct steps in the drawing. The first
step is one which starts with the undistorted plan and converts
this into a plan in perspective.
The pivotal point is the station point or point-of-view. This is
the assumed position of the spectator who is looking directly at
the nearest point of the object.
If we wish to draw a cube in perspective, we start with the plan,
which is a simple square. The first step is to convert this plan into
the shape which it would have if we were above it and looking