42 THREE-DIMENSIONAL PHOTOGRAPHYBrewster instrument of 85mm separation, anyone so abnormal as
to have this as their interpupillary would look through the outer
edges of the Brewster lenses, that is, through the optical centers
just as through a full lenticular. The Brewster is a full lenticular
stereoscope in which the optical axes are separated by more than
the interpupillary and in which the unused half lens is not physi-
cally present.
CHOICE OF A STEREOSCOPE.-The choice lies primarily in the
size of the stereogram. For all sizes with normal interpupillaries,
the full lenticular type of viewer is used, including those for 35
and 16mm film stereograms. This refers to all stereograms, regard-
less of the dimensions of the print, as long as the homologous
separation is substantially 65mm.
For perfect results, the stereoscope should theoretically remain
with fixed separation and fixed focus. Any compensations made
for focus should be made by the use of supplementary ophthalmic
lenses rather than by altering the distance between lens and
stereogram. However, as this is not often practical, focusing by
the conventional method is acceptable, and supplemented by
variable separation does not produce obvious distortion. For the
most comfortable use, experience has shown variable focus to be
essential in all but the smallest sizes, and variable separation often
convenient.
The stereoscope should have a rigid mechanical structure,
should be equipped with achromatic lenses, and if possible, it
should be equipped with some kind of illuminator to provide uni-
form illumination to both images of the stereogram.
THE STEREOSCOPE IN USE.-The stereoscope is too often re-
garded as simply a peephole device for looking at stereograms, no
thought being given to the optical and neuro-optical phenomena
involved. Because the stereographer can do much better work by
understanding the fundamental principles of stereoscopic vision,
we shall devote some space to this consideration, even though
every attempt has been made in this book to eliminate theoretical
discussion.
Cyclopean Vision and Stereo-pro jection.-Stereoscopic vision is
based upon the perception of an image which has no physical
existence. It is synthesized within the brain from two dissimilar
sphero-planar images, and then projected into space. This involves