90 THREE-DIMENSIONAL PHOTOGRAPHY
window for close-ups, but this is obtained only by the sacrifice of
portions of the film area, making the picture field conisderably
narrower than normal. The value of such “special window”
mounts is open to serious question. Most 35mm workers use the
“standard” mount for all types of subjects.
It is essential that the two images be vertically aligned with
great care, and that their true perpendiculars be both perpendicu-
lar and parallel. It is advisable to have the spacing of homologous
objects at infinity equal to the spacing of the stereoscope, but fur-
ther than that the arrangement is purely a matter of personal pref-
erence with no valid arguments for or against any particular style.