36 THREE-DIMENSIONAL PHOTOGRAPHYinterchangeable chains so that any number of loaded chains may
be kept in readiness for use.
VIEWERS FOR THE AMERICAN SIzE.-This is the old parlor size
in which the stereogram measuring approximately 3x6 inches is
mounted upon a card measuring 3'/2x7. It is the size most familiar
to American and British amateurs and to those who are not
stereographers, but the size most rarely used by the European
stereo enthusiast. The views are almost universally paper, al-
though a few transparencies are encountered now and then.
Hand Viewer.-The common viewer is the old parlor hand
viewer known variously as the American, Mexican, Holmes, or
Parlor viewer. It belongs to the same class as the folding viewer,
and is to be regarded with the same suspicion. The wood construc-
tion and the inaccurate fitting produce a shaky contraption which
is more often out of adjustment than in. Even those made by
reputable firms are acknowledged to be designed only for occa-
sional or demonstration use.
Table Viewers.-The table viewer is the Holmes instrument
made of metal and carefully fitted. The whole is mounted upon
a sturdy base and has in general those physical characteristics ex-
pected of an optical instrument of practical quality. These instru-
ments are inexpensive in the simpler models, and should always
be used for the American size when no better instrument is
available. Inasmuch as these viewers are optically of the Brewster
type, focusing is accomplished by moving the view rather than
the lenses, an arrangement which is convenient.
Therapeutic Models.-One of the most practical of all Holmes
viewers is that made for use in opht.halmic treatment. These in-
struments are similar to the table models, but they are heavier
and more rugged. The infinity homologous distance is greater, the
lenses are of better quality, the focusing scale is accurately cali-
brated, the body is fully adjustable for height and angle to
provide the most comfortable viewing, and the eyepieces are
grooved to take correction lenses so that even those with defective
eyesight may use the instrument without wearing spectacles.
Inasmuch as the therapeutic viewers cost about the same as
the automatic box viewer and much less than the cabinet classi-
fiers, they are well within the reach of every serious amateur who
works in this size, or those who use a small camera and then make