310 THREE-DIMENSIONAL PHOTOGRAPHY
ing slightly rounded corners; 45x107 and smaller with sharp
cornered, rectangular masks. If you choose you may follow the
old convention of using a domed top with a sharp cornered bot-
tom. This is almost universal in the 3x6 size, although no one
has been able to cite a good reason for it. Perhaps the suggestion
of an arched window seemed more “elegant” to the Victorian
s tereographer.
It is also known that commercial paper prints in 3x6 are
mounted upon cards which have a decided curve. We have been
unable to discover any reason for this practice which is certainly
optically inferior to the flat mount.
Window trimming or masking is useful when open type viewers
are used, but of less value in the better grade stereoscopes which
have the field masked by the visual tunnels in the instrument.
Transparencies.-It is difficult to discuss this subject, because it
is one photographic art which has relatively few adherents today.
The stereo transparency, aside from the duality of image, is iden-
tical with the lantern slide. The stereographer can do nothing
except get some materials and work at it until he has mastered
the knack of doing it well. We know of no better guide for the
beginner than the lantern slide brochure in the Kodak Reference
Handbook.
The difficulty lies in the fact that in transparency making, the
fixed exposure and development are abandoned, and the contrast
of the positive controlled by a sliding scale relationship between
exposure and development.
If the highlights have the correct delicate tone but the shadows
are weak, decrease exposure and increase development.
If shadows are strong, but highlights fully clear, increase ex-
posure, decrease development.
If the image is weak throughout with clear highlights, increase
both exposure and development.
If highlights are gray and shadows black, decrease both ex-
posure and development.
That is but a rough guide, and only experiment and experi-
ence will enable you to make a good transparency. It is not, how-
ever, difficult to learn if you really try.
The good transparency is almost black as it lies in the developer,
far darker than a good paper print. The final judgment can only