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Page:Popular Science Monthly Volume 49.djvu/323

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plate. Such a plate consists of a sheet of glass coated over with bromide of silver incorporated in a film of gelatin. When single positive or negative discharges are desired, the photographic plate,

PSM V49 D323 A negative discharge.jpg
Fig. 2.—A Negative Discharge.

the back of which is covered with tin foil, is placed between the two discharging points, and the strength of the discharge is regulated so that, while it shall cover as much of the plate as possible, it shall not spread over the edge. The tin foil performs a double function: it does away with the spark on the back of the plate, which would otherwise occasion a confused figure; and it also assists in causing a uniform spreading out of the discharge. It does not seem to modify the character of the figure produced in any way. It, however, affects its form, since, if the foil be smaller than the plate, the discharge seems to have a great disinclination to extend itself beyond the edge of the foil, but would even curve