APPLIED STEREOSCOPY 209to everyday vision, and it has a characteristic quality quite differ-
ent from the conventional soft focus, and one which cannot be
imitated by uncorrected lenses, or aberrating glass discs. You can-
not obtain it from a sharp negative during the enlarging process.
The only way in the world to obtain this original type of “soft
focus”-which is still sharp enough to reproduce every hair on the
head-is to use a lens whose actual aperture exceeds two and one-
half inches. If you use that aperture you cannot help obtaining
the soft-focus effect. It is a matter of stereo parallax, not loss of
definition in the usual sense.
This fact has been recognized from time to time by “inventors”
who reason that if the picture includes the necessary differentiated
images of stereo parallax, then the image must be stereo. Some
of them went so far as to place a vertical bar across the front of the
lens to produce two images differentiated in the same print, and
then insisted that these were stereograms. All the time they neg-
lected the one fundamental point: In any true stereogram, the
eyes must be able to fuse the images of any object within the
entire field, and at the same time the “double” or “ghost” images
of other objects must be separated as in normal, direct stereo-
scopic vision. Although this fact is elementary, not less than a
dozen inventors have at one time or another “invented” this half-
way stereo which is not stereo in any sense of the word.
PSEUDOSCOPIC PIcTums.-One interesting phase of stereoscopy
is the pseudoscopic picture. This is a stereogram in which the
left and right pictures are left in their original negative positions,
that is, untransposed. The effect is to reverse all depth dimen-
sions, bringing the far distance close to the eyes and removing
nearby objects to a remote position in space,
This phenomenon is interesting, but it also has a practical
value. Through familiarity, we are prone to fail to appreciate true
spatial relationships and take them more or less for granted. We
may grasp the fact that the stereogram is faithfully reproducing
three-dimensional space, and still fail to appreciate the various
relationships which go to make up this spacing. The pseudoscopic
picture so emphasizes these differences that planar separations
unperceived in the stereogram become very distinct. This makes
the pseudoscopic picture of definite value when examining cer-
tain microstereograms and other stereograms of scientific interest.