THE STEREOSCOPE 35sion. Usually this viewer has an auxiliary table stand, although
it may be held freehand. It is intermediate between the box
viewer and the cabinet type.
Cabinet Viewem-These are table instruments, and while all
models vary somewhat, it may be stated that as an average they
measure about 10 inches square and stand about 15 inches high.
They are usually in the form of a rectangular wooden cabinet
with the lenses set near the upper end of the front side.
The views are ordinarily loaded in trays of 12 or 25. An in-
dexing mechanism on the outside permits the selection of any
desired view. If all the views are to be observed, pressure upon
the operating lever will bring the views into position in succes-
sion without any manipulation of the index lever.
These cabinets are made for use with transparencies only, as
the viewing space is a black tunnel with the view masked to the
actual picture area. In the case of cabinets for the smaller 45x107,
which have the two picture images separated by a space in which
the title is ordinarily written, an auxiliary lever is provided which
lifts a shutter and inserts a prism in the optical path to make
the title visible.
The cabinet viewers usually have both focusing and interpu-
pillary adjustments, and most of them are provided (as extras)
with illuminating boxes for artificial light and with single or
double projection systems. Some of them (e.g., Mattey) are inter-
changeable for both 6x13 and 45x107 sizes. While there are many
very good cabinets, or classifiers as they are called, the Richard
Taxiphot may be regarded as typical.
The Mattey dual size cabinet instrument just described has
been converted, very easily, to take the 35mm slides in both glass
and cardboard mounts. This type of stereoscope is often called a
“classifier.”
Chain Viezuers.-The chain viewer resembles the classifier ex-
ternally, but usually has no interpupillary adjustment. The views
are carried upon a pair of endless chains which hang upon sprock-
ets attached to the operating knob. The knob is turned to bring
the views into position successively. In the simpler type, the chain
is built into the cabinet, and views must be removed from the
chain and replaced when a change is desired. The better type has