PICTORIAL STEREOGRAPHY 149dents, by city dwellers, by those in whom we should expect to find
the lowest degree of stereopsis. When the average of outdoor
dwellers, foresters, hunters, seamen, farmers, small town and vil-
lage dwellers, and particularly those who habitually make use of
stereograms are examined we find the actual value of stereo in-
finity jumping to from two to three times the stated limit!
As for near distance, there seems to be even less logical reason
for its imposition. There is no near limit for stereo. The nearest
distance the writer has employed is (theoretically) i.gmm, using
a micro objective of that focal length. However, not to quibble,
the nearest distance at which stereo may be used with a normal
lens separation is the nearest distance at which the homologous
images fall upon the film area provided, a matter of inches, not
of feet. Of course, as explained in the chapter on closeup work,
this involves a discrepancy in field and loss of picture area; the
tall, narrow panel is not esthetically attractive, but as far as stereo
is concerned, the image is reproduced and it is reproduced with
that degree of relief which would be seen by the eyes at the same
distance.
The trouble lies in the “mental policing” which has been men-
tioned before. We are not consciously aware of the flattening of
round objects by distance and similarly we are not aware of the
exaggerated relief which we actually see when an object is held
close to the eyes, but stereography is certainly not responsible for
the vagaries of human vision. The stereogram represents just that
degree of relief which would actually be seen by direct vision,
always provided of course that the lens separation is equal to the
interpupillary. The mechanism of parallax is as automatic as that
of perspective. There is one factor, however, which does exist in
the extreme close-up: those made at a distance of about one foot
or less. The dissociation of accommodation makes the extreme
convergence noticeable, and the actual muscular tension, as it
changes to follow the extreme depth changes, can be felt-but
not painfully. The effect is rarely noticed unless the convergence
depth is considerable; and it is rarely experienced when the
original distance exceeds 18 inches.
True, many stereographers, particularly those of the old school
who pride themselves upon their orthodoxy, will refuse to look at
a stereogram made at two or three feet, exclaiming that it is “a