52 THREE-DIMENSIONAL PHOTOGRAPHYthis one spot. Do not allow the vision to shift for a second. This
will be extremely difficult, for the eyes normally shift all the time
from object to object. While maintaining the visual focus upon
the one spot, remain conscious of the images of both objects. At
first they will appear in normal relationship, but after an interval,
perhaps 15 seconds, perhaps a minute, they will be seen to be ap-
parently in one plane, just as in an ordinary photograph. The
specific stereo differentiation has been lost. This occurs very
quickly, but at first it takes a few seconds for us to fully realize
the change. Once the loss of depth has been recognized, shift the
eyes, even as little as to the next bough of the tree, and instantly
the images jump apart into their original stereo separation. This
second change from the non-stereo to the stereo phase is marked
and unmistakable. Thus it seems to be the continual shifting of
the degree of stereo diplopia which is directly responsible for
stereo perception.
To support the Wheatstone theory that static parallax is the sole
stimulus of stereoscopic vision, an experiment is cited in which
a subject is permitted to view a scene “for a very short period
of time” with the (claimed) result that full stereo depth is per-
ceived. In the laboratories of the Stereo Guild several subjects
were used in a similar experiment. The slides were artificial stereo-
grams of graded depth but containing no familiar figures, no ex-
trinsic factors of depth. Illuminated with an electric flash whose
duration was extremely short (so that the period of vision should
be limited to the normal visual persistence), it was found that not
one subject could identify the relative spatial positions in any
slide, although all subjects could correctly classify all slides when
permitted a period of vision of only a few seconds. This seems to
support the theory of dynamic parallax rather than of static.
Parallax in the Stereoscope.-As we have seen, the lateral dis-
tance separating the two images of the same object in the stereo-
gram is known as the homologous distance. In a stereogram the
homologous distance decreases as the object occupies planes nearer
the eye. Thus it is obvious that when the visual attention is
focused upon any one object in the stereogram, the images of all
other objects will fail to superimpose, but will produce stereo
diplopia or ghost images.
The angular separation of these images is, in the orthostereo-