54 THREE-DIMENSIONAL PHOTOGRAPHYballs converge toward each other. It is obvious that both actions
are definite, and that for any specific accommodation there must
be an exactly corresponding convergence. There are many people
who believe the two actions are inseparably combined by func-
tional apparatus. This is not at all true. The two actions are
wholly independent and only because long habit has caused them
to synchronize, are they apparently inseparable.
The normal stereoscope, lenticular or Brewster, used with the
correctly made normal stereogram, is so adjusted that the viewing
lenses act as collimators. That is, the eye itself is in the condition
for infinity viewing or zero accommodation. The lens is com-
pletely relaxed. At the same time convergence is also at zero for
those objects lying beyond stereo infinity, but for all nearer
objects convergence is normal. Thus in any stereoscope, conver-
gence is divorced from accommodation.
Naturally it is not easy to overcome a lifelong habit, and those
unaccustomed to the instrument will unconsciously accommodate
as they converge. As a result they find it impossible to hold sharp
focus, and as a compromise will almost always select a point mid-
way between far-point and near-point focus. As a result they see
nothing sharply outside the one plane of compromise focus. This
is literally true. In viewing a stereogram which is needle sharp
throughout, these people will see both distance and near fore-
ground as out of focus if the compromise convergence focus point
lies in the mid distance. This is because their eyes constantly
change focus to keep up with the actual convergence changes
induced by the variable separation of the stereo images. The
condition does not, as a rule, last very long however. It is often
overcome within^15 or^20 minutes, and rarely lasts through as
much as two hours of actual viewing. Such a person finds the use
of the stereoscope uncomfortable, and not until by continued use
the two functions are separated, does he really enjoy stereoscopy.
In some instances, the coupling works the other way, and focus
is maintained but convergence insists upon remaining with focus,
and the two images cannot be fused. No matter what is done, the
subject sees two distinct images. This, too, will readily yield to
practice. Just as soon as the two functions are separated there is
usually experienced a sense of increased visual comfort, not only
in viewing the stereogram but in normal, direct vision as well.