34 THREE-DIMENSIONAL PHOTOGRAPHYcamera lenses be of substantially the same focal length, and used
at the same separation, It is an ideal condition when one can look
through the centers of the lenses, but this is possible only when
the interpupillary exactly corresponds to the separation of the
lenses, Although in theory the stereoscope lenses act as colli-
mators, making it unimportant to use the lens centers, it is visu-
ally more comfortable to have the lenses movable so that the
separation corresponds to the interpupillary rather than to con-
form to stereo theory and remain exactly opposite the image
centers. Normally the eyes converge and accommodate (focus) at
the same time. Thus when viewing images of near-by objects the
accommodation changes. Many persons, particularly those not
accustomed to stereo viewing, adjust the focus for a nearer dis-
tance than infinity. Thus the true collimating effect is lost, and
interpupillary adjustment becomes more of a necessity.
Stereoscopes of the full lenticular type should have lenses not
less than 15mm in diameter to provide free vision for interpu-
pillary distances between^60 and 70mm. When the lenses are not
set at infinity focus, the adjustable separation becomes desirable.
Stereoscopes whose lenses are 30mm or more in diameter and
of variable separation may be used either as full lenticular or
semilenticular (Brewster) instruments. They may be used with
stereograms of widely varying image separations.
It is an advantage to have cups for shielding the eyes when the
viewer has no self-contained illuminating system. Achromatic
lenses are advisable in all types of viewers, and are well worth the
slight extra cost. Both full lenticular and Brewster instruments
are available so equipped.
VuriubEe Focus.-Some elaborate European viewers are equipped
with interchangeable lenses of various focal lengths. Although
these are sold for the purpose of obtaining variable magnification,
the feature has value only when a single viewer is to be used with
stereograms made with different cameras having different focal
lengths. Other than this, the use of interchangeable lens viewers
is to be avoided.
Automatic Hand Viewers.-The automatic viewer is simply a
box viewer of the better class, fitted with a simple mechanism into
which several views are loaded. Pressure upon a convenient lever,
or turning a button, causes the views to be presented in succes-