When making transparencies by contact use artificial light, either
gas or a common lamp. A deep printing frame is the most suitable,
having a piece of thin plate glass in it to support the negative.
Lay the negative down with the film side up, place upon it the dry
plate, put in the back and press firmly down with the springs;
expose to gas light from six to ten seconds at a distance of twelve
to fourteen inches; expose longer to lamp light, say from ten to
twenty seconds, or the duration of time that may be considered
sufficient, judging from the density of the negative in use. [p215]
The time of exposure and the strength of the developers are the most
important factors in determining the quality and tone of the
resulting transparency. Giving a short exposure and using a strong
developer will ensure a vigorous picture with a rich velvety black
tone. A long exposure, necessitating a weak developer, will produce
fine gradation of light and shade, clear shadows and a warm brown
tone.
When making transparencies for the window, cut a mask, with an
opening, allowing for the border or margin on the ground glass it is
intended to use. A suitable paper for this purpose is of deep orange
color and moderately heavy; place the mask between the negative and
the dry plate to be acted on, and it will protect the margin from
the action of light, leaving it clear glass, and so exhibiting to
better advantage the ornamented border of the ground or etched
glass, which is placed against and covers and protects the film of
the dry plate. When making lantern slides the mask is not necessary.