302 THREE-DIMENSIONAL PHOTOGRAPHY
slide originals carefully prepared for this purpose, but there is no
significant factor involved which is not present in all stereo.
Visual Cornfort.-Strangely enough, this is a factor almost uni-
versally disregarded. Unless one is vain to the point of foolishness,
a pair of shoes must be comfortable, but a pair of spectacles as
often as not are far from comfortable. This is not the fault
of the ophthalmologist but results from our own failure to co-
operate during the examination. A pair of spectacles should bring
the same comfort to our eyes and to vision that a well fitted pair
of shoes bring to our feet.
It is also true that physical comfort demands a certain amount
of physical activity. Otherwise we become stiff and sore. The same
is true of vision. Eyes which are used normally for looking at ob-
jects at a variety of distances are more comfortable than those
which are strained by long periods of fixed distance vision. Many
of us have only rare opportunities to exercise true distance vision,
and as a result infinity vision becomes impossible to us without
strain. That condition which should be the “rest” position be-
comes an abnormal one. Stereo viewing provides opportunity for
true distance vision with resulting relaxation and the mainten-
ance of more nearly normal visual conditions. This does not make
close vision more difficult. On the contrary, the exercise itself
makes close vision more comfortable through the occasional relax-
ation of the strain of constant near vision. Near vision necessitates
a continued tension for both accommodation and convergence,
and there is the same strain which would result if any muscle
were kept under tension for hours at a time. If you want to
know just what happens, hold your arm extended in front of you
and in a fixed position for five minutes!
Prescription stereograms, if necessary, include some of positive
divergence. Each patient has prescribed those slides which will
provide the type of exercise he needs. Thus, by the simple act of
looking at attractive stereograms for a certain period each day,
visual comfort is tremendously increased and very often quite
remarkable improvements are made in serious malfunctions of
the visual apparatus.
Visual SkiZLThis is even less familiar than visual comfort. Of
course we know that some people are unfortunate enough to have
defective vision. We know that even among “normal” visions some