56 THREE-DIMENSIONAL PHOTOGRAPHYgram does not in this one respect duplicate direct vision; but this
fact is fortunate as far as the stereographer is concerned inasmuch
as it provides him with increased visual skill after a little practice.
It will be noticed that the foregoing remarks assume the exist-
ence of correctly made and correlated stereoscope and view. It is
of interest to know that the therapeutic stereoscope is so accurately
calibrated that the operator can introduce any desired accom-
modation and any desired convergence, both independently ad-
justable. Note however, that parallax is not adjustable. It is a
characteristic of the stereogram. This makes it possible to measure
the degree of both functions, to study the results of the habitual
bond between the two, and to treat certain tendenices toward ab-
normal vision. “The Telebinocular Manual,” Keystone View Go.,
Meadville, Pa., provides detailed instructions for this manipu-
lation.
Trick and Puzzle Stereograms.-The stereoscope is also a source
of enjoyment for a group. Those present need not have any
knowledge of stereo. In fact, when they do not, the results are
even more baffling. For example through the control of parallax
(space control) see Chapter 16, a stereogram can be made of
someone well known to those present, but the figure is obvi-
ously only six or eight inches tall and standing upon a table at
which another familiar person is seated. The method has already
been described.
The pseudoscopic stereogram may also be made a source of
pleasure when well done. Suitable subjects are rarely found in
Fig. 3-15. Zridescent drawing.