HYPERSTEREO 261ance. Our ability to perceive differences among the distances and
the relief of objects in space, is a most acute one. Because this
sense is so well developed, most stereographers assume that it
applies as well to size perception when it is not based upon direct
comparison. This is not true. For example, if, in a single stereo-
gram, two objects are shown, one of which lies only an inch or so
farther from the camera than the other, that difference can easily
be perceived, even though the objects lie at about ten feet from
the camera. But if one stereogram shows an object at eight feet and
another shows a similar object at ten feet, it is hard to differen-
tiate between them when viewing the stereograms in succession.
This inability to carry over standards of comparison from one
period of time to another is familiar to those who try to match
colors by memory rather than by direct comparison.
The perception of relief and distance is purely a relative thing
and not at all absolute. Therefore if in a stereogram the relative
positions and proportions are normal, the lack of absolute dimen-
sions cannot be recognized. The sole exceptions are those in-
stances where the skilled stereographer has some extrinsic factors
to guide him in the perception of the departure from absolute
size and distance.
Because of the great confusion which exists regarding this
subject, an experiment was performed under highly exaggerated
conditions. The subject was a roadway lined with trees and shrubs
which gave a continuous recession of planes. The nearest object
within the scene was the foreground only 20 feet away, and a
limb of a tree at substantially the same distance. It must be re-
called that the stereoscopists who conform to classic rules insist
that ten feet is the minimum distance for using normal base sep-
aration, and that anything nearer must be made with reduced
base. Therefore 20 feet represents a fair, normal near point for
the stereogram.
This scene was stereographed three times, using normal, two-
times normal, and four-times normal base. The four-times base
is that which would be used theoretically for a subject about one
mile distant.
The stereograms were shown to a group of people, laymen, who
were familiar with the original of the scene, and who are accus-
tomed to viewing stereograms, although they were not stereogra-