THE STEREOSCOPE 45are always painfully aware of the flatness of the ordinary photo-
graph; there are others who have so little stereo perception that
the perception of depth depends upon such extrinsic factors as
the diminution of size due to perspective, and similar effects. In
fact, this has led in the past to the use of the word “perspective”
when stereo depth is meant. The words, far from being synony-
mous, are complementary. These people see depth in a good
planar photograph or when viewing a motion-picture screen.
Such people see nothing unusual when viewing their first stereo-
gram. They see just what they do when looking at a transparency
in an illuminated viewer for example. This not uncommon lack
of stereopsis is one reason that stereo has not become even more
widely popular.
Several optometrists and ophthalmologists, who specialize in
stereoscopic treatment, were asked about the average conditions.
These men are located in the East, Southeast, Southwest, and
West, fairly representative of the nation. The consensus was that
about one-third of all adults have low stereoscopic perception,
and of the remaining two-thirds, from 12 to 15 percent have fully
developed stereopsis. How much of this is due to physical or or-
ganic defects in the visual apparatus is open to question, but all
are agreed that thorough training will show a marked increase in
stereopsis in the case of anyone who has the slightest degree of it
with which to start; that those who practice stereoscopic pho-
tography are usually in the high bracket; and most significant of
all, that the use of stereograms in the home and office results in a
marked improvement in vision. Incidentally, this answers those
who refuse to work with stereo for fear of injuring their eyesight.
Stereoscopic Exercise.-It is well known that exercise, either
physical or mental, is of value in sustaining health and in develop-
ing ability. Visual exercises are often regarded as purely subjec-
tive, but the visual organs, the eyes, operate through the agency
of a complete system of delicate and highly co-ordinated muscles.
That these muscles will respond to exercise just as will any other
muscle is not only logical, but also a proven fact.
With each succeeding generation more of our major effort is
expended within doors, and the maximum range of vision is per-
haps 50 feet, with 10 to 15 a more common average. With the
eyes adjusted for such distances, the position of rest (the position