32 THREE-DIMENSIONAL PHOTOGRAPHY
obtained with the self-illuminated viewer adapted for house cur-
rent illumination.
Conversion cords are available with a tiny transformer built
into the plug, These operate the standard bulb of three volt rating
normally used, so no bulb changes are necessary. Only with this
light is it possible to obtain and maintain the best effect from
color slides.
OTHER NORMAL SIzEs.-These include the 45xio7mm and the
6xi3cm, both of which are normal dual images spaced by ap-
proximately 65mm. Richard makes a 7x13, but its superiority to
the universal 6x13 is open to question, For these normal sizes the
range of available viewers is greatest.
Folding Types.-The folding stereoscope is, at best, an emer-
gency instrument. It rarely maintains a level base, it has no field
mask, it has no interocular adjustment, and the focusing is often
of the most crude type. One of the surest ways to lose interest in
stereo is to habitually make use of a folding viewer.
Book Viewers.-The one logical use for the folding viewer is to
accompany stereo-illustrated books. The German version is to
place a separate folding viewer in a pocket formed in the abnor-
mally heavy binding. One American viewer has a folding post
attached to the binding to support the viewer in correct position
above the page. Neither method is satisfactory to the stereog-
rapher, and they are important only in so far as their novelty
is concerned.
Box Viewem-This is the largest group of all. It starts with a
simple rectangular box equipped with lenses set in one side, and
slide grooves at the other side to accept the stereo diapositive.
The same box may be provided with a hinged lid in the top
which may be raised to admit light to the surface of a paper
stereogram. This is the basic form upon which the usual box
viewers are designed. All of the later forms consist in refinements
and elaborations of this form. Therefore, instead of repeating the
descriptions of the various models, we shall simply note the
refinement and its effect upon the whole instrument, bearing in
mind that all of those described may be, and are, applied sepa-
rately and collectively to the basic box form.
In some box viewers the lenses are set in sliding tubes not
unlike opera glasses, and are usually controlled by a button at the