MOUNTING, TRANSPOSITION AND SPACING 85Rotary alignment. The worst fault of all is to have the true per-
pendiculars leaning at one side or the other. If the true perpen-
diculars in the two images are parallel and both slightly inclined,
the only immediate effect is a sense of visual discomfort, If the
true perpendiculars to the camera base line of the two images in-
cline in different directions the effect is at once noticeable. If the
inclination is slight, a definitely painful sensation is experienced
when viewing, but if the difference is great the eyes see two dif-
ferent images (diplopia). The term “perpendicular” always refers
to those lines perpendicular to the camera base, common to both
images. If the negative was made with a laterally tipped camera,
the images must be mounted with this degree of tilt. Thus in
stereo mounting, camera side tilt should never be rectified by
trimming as is the case with the planar print.
The negatives, whether made in a stereo camera or in a single
camera with sliding base or stereo reflector, have a common base
line. This is a good guide for correct alignment. If the print bases
are also kept uniform, there is little danger of rotary misalign-
ment. However, if the two images have been made by a single-
lens camera held freehand, then it is necessary to determine the
perpendiculars by visual proof. The two prints are placed in the
stereoscope and turned about their centers until the comfort-
able matching position is found. This is marked and the prints
trimmed to that perpendicular. This operation requires some ex-
perience, and for that reason freehand exposures are best left until
some facility in stereo technique has been acquired..- 0. m.
C AD a00 ;; d’co ie’
Fig. 540.When the distance between infinity images is correct, and both
horizontals and verticals are in correct alignment, there remain
only those factors of mounting which are more or less a matter of
taste.
WINDOW TRIMMING-Figure 5-8a and b show the relative hori-