THE STEREOSCOPE 37use of enlarging to produce the final American size. The Keystone
Ophthalmic Telebinocular is an excellent example of this type
of viewer and one which can be recommended without reserva-
tion (Fig. 3-5). I use this instrument exclusively for this size of
stereogram.
Cabinet-Type Viewem-The classifier is not available for the
American size print (although it is available for transparencies
in the same size), but chain viewers with reflectors for illuminat-
ing the paper print are made. These are so exactly similar to the
transparency chain viewer that nothing further need be said
about them.
Pantoscope.-The Pantoscope was once a great favorite but is
now rarely found in this country although still available in
Europe. It consists of a folding wooden frame, sometimes elabo-
rately carved, made in three sections. The base rests upon a table.
The center section rises at the front end to slope from the user
down toward the table. The third section rises at right angles to
the middle section. At the rear of the middle section two rods rise
at right angles to support the view.
The top or front section contains a large single lens from four
to seven inches in diameter. Below this lies a pair of stereo lenses.
The large single lens is for viewing ordinary photographs to
give them a “plastic” appearance (a phenomenon to be discussed
later), while the two lower lenses are provided for normal stereo
viewing.
MISCELLANEOUS VIEWERS.-coin-operated viewers are often
found in Penny Arcades. Highly elaborate instruments have been
designed for mapping, for therapeutic use, and for other special
purposes. As these are of little interest to the amateur, we shall
mention none of them except for the Wheatstone viewer, which
is a basic type widely copied for general purposes.
The Wheatstone viewer (Fig. 3-6) consists of a pair of mirrors
set at right angles and placed immediately before the eyes so that
the left visual axis is deflected to the left and the right axis to
the right. The two views are separate and are placed in suitable
supports facing each other. The operator must adjust the posi-
tions to produce final stereo fusion. The advantage lies in the fact
that the size of the prints used is unlimited. This viewer is largely
used in aerial survey work and in X-ray stereography. In the latter