256 THREE-DIMENSIONAL PHOTOGRAPHY
is ONE and only ONE parallactic value for any given distance.
Thus, when you decide the distance at which you wish the object
to appear, you have this value. Ordinarily we should choose the
arbitrary “reading distance” of 16 inches, but because 15 inches
10 II- The micro arrangement for lateral displace-
ment. Object S is displaced first to 0, then to 0‘
for the two exposures. XY is the plane of motion
and OB J the microscope objective. - The “half angles” ordinarily used in stereo
computation as explained in the text.
is very close to a half angle value of five degrees, we’ll adopt
it. Therefore, when you wish the object to appear to lie at 15
inches from the eye, the deviation on each side of zero will be
five degrees. In Fig. 11, A and B are the eyes and 0 an object at
any distance. Assuming the distance AB to be 65mm, as is usually
done, then AB/2 will be the half base, and by marking the center
of AB by X, we have either AX or BX as the half base. Then
AOX or BOX will be the visual half angle or the parallactic value
for one eye (for 15 inches this will be almost five degrees). By
solving the problem for any distance XO, the Parallactic value is
obtained which will permit you to place the object at any de-
sired apparent distance.
It is inadvisable to make use of any value for XO, which is less
than eight inches, as the depth of parallax may be disturbing to
fuse.