THE STEREOSCOPE 51The activating stimulus of stereo vision is undoubtedly tha! of
dynamic stereo diplopia. Diplopia means seeing two images of one
object, and the diplopia normal to stereoscopic vision is known
as stereo diplopia to differentiate it from the pathological form.
Hold one finger vertically about a foot in front of the bridge of
the nose and in line with a window. Look at the window. You
will see two ghost images of the finger, one at each side of the
window. Look at the finger and pu will see two windows, one at
each side of the finger. Thescare examples of stereo diplopia.
Look from the finger to the window. During the act, the images
of the finger separate into two and the two images of the window
merge to form one. In the midway period you see two images of
both objects. This motion of the diplopic images toward or away
from one another is dynamic stereo diplopia. It will also be seen
that the nearer any secondary object is to that which is the center
of visual atention, the smaller is the separation between the
ghosts. This diplopia is a function of dynamic parallax.
When we view an open scene, not only does the visual attention
shift constantly from object to object, but with each shift there is
a complementary alteration of the distances separating each pair
of ghosts, so that stereo vision embraces an intricate, interweaving
dance of ghostly pairs. It is this contraction and expansion of the
degree of stereo diplopia which provides the stimulus to depth
perception. The amount of distance between two ghost images
which must be overcome to fuse the images is the subjective key
to its position in space. This must not be confused with the physi-
cal, muscular tension involved in converging the eyeballs. The
latter is of extremely minor importance in producing stereoscopic
vision.
This fact is subject to the most striking proof. Take up a com-
fortable position which can be maintained for several minutes
without discomfort. Select such a position that two objects, such
as two trees, are aligned in such a manner that both may be seen
simultaneously without shifting the eyes. It is better if the dis-
tance separating the two objects in the line of vision be not more
than half the distance separating the nearest from the eyes of the
observer, although this is not an essential condition.
Select some fairly prominent spot in the nearer object, perhaps
a broken twig, a dead leaf, or the like. Keep the eyes fixed upon