46 THREE-DIMENSIONAL PHOTOGRAPHY
of the eyes when we are looking at a far distant object) becomes
really an abnormal condition. Vision at close distances also neces-
sitates convergence of the eyes, and as a consequence we are tend-
ing toward national nearsightedness and convergent squint or
“crossed eyes.” Optometrists find a surprisingly great number of
people who have a tendency to squint which is not manifest in
their ordinary vision, or in their appearance.
It has been found that even a quarter of an hour each day spent
in looking at stereograms will do much to offset this tendency and
give everyone a chance to look into the far distance. The use of
the stereoscope not only provides for the complete relaxation of
the ocular muscles into the condition of rest or infinity vision,
but it also provides for a period of complete relaxation of con-
vergence. While any good stereogram will approximate these
conditions, a standard therapeutic stereoscope and a set of special
exercise slides prepared for use with it, will provide exactly the
relaxation desirable and will also, through the use of other slides,
provide for flexing the muscles or alternating from convergence
to relaxation through specific angles and through specific series.
Thus the therapeutic stereoscope provides a visual gymnasium
which can become one of the most valuable possessions of the con-
sistent user. The reward for consistent exercise is better vision,
freedom from eyestrain and its attendant headaches, greater acuity
of vision, and retardation of the effects of presbyopia or “old age”
vision. *
Non-stereoscopic Stereogrmm-While discussing the subject of
stereograms for visual training, the type of slide which is binocu-
lar but not stereoscopic should be considered. These are of several
types.
The simplest is a picture slide such as is shown here (Fig. 3-10).
The dog is seen by one eye and the pig by the other. The dog
will appear to be jumping over the pig, but it may be just starting
the jump, it may be midway over the pig, or it may be past the
pig and ready to land. The relative positions reveal any tendency
toward abnormal convergence or divergence. Similar slides con-
taining an arrow pointing toward a series of numerals, serve a
like purpose but provide a more exact result.
*Such stereograms are made by Bausch & Lomb, American Optical Co., and
Keystone View Co.