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HOW TO MAKE PHOTO. PRINTING PLATES.


The drawing intended for reproduction is pinned on a board and placed
squarely before a copying camera in a good, even light. The lens used
for this purpose must be capable of giving a perfectly sharp picture
right up to the edges, and must be of the class called rectilinear,
i.e., giving straight lines. The picture is then accurately focused
and brought to the required size. A plate is prepared in the dark room
by the collodion process, which is then exposed in the camera for the
proper time and developed in the ordinary way. After development, the
plate is fixed and strongly intensified, in order to render the white
portions of the drawings as opaque as possible. On looking through a
properly treated negative of this kind, it will be seen that the parts
representing the lines and black portions of the drawing are clear
glass, and the whites representing the paper a dense black.

The negative, after drying, is ready for the next operation, i.e.,
printing upon zinc. This is done in several ways. One method will,
however, be sufficient for the purpose here. I obtain a piece of the
bichromatized gelatine paper previously mentioned, and place it on the
face of the negative in a printing frame. This is exposed to sunlight
(if there is any) or daylight for a period varying from five to thirty
minutes, according to the strength of the light. This exposed piece of