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SOME EARLY APPLICATIONS OF PHOTOGRAPHY 223
increased by this strengthening process. It can be augmented by a treatment with solutions of chloride of mercury and iodide of potassium, which transform the metallic silver into more bulky compounds. A relief was ultimately obtained nearly as high as the incisions of an engraved plate are deep. If now a collodion positive be taken of a negative from a linear drawing, and treated as above described, we have all the requisites for producing from it an engraved copper plate. The relief -like photographic plate is then brought into an electrotype apparatus. This produces on the plate a coherent copper precipitate, which is in low relief where the plate shows high relief — that is, where there are strokes or outlines. Thus a copper plate is obtained from which impressions can be taken as well as from one that has been engraved. This process was used to reproduce drawings like copper plate.
Maps were prepared in this manner, in which the original could be photographically enlarged or diminished, also writings on an enlarged and diminished scale. Scamoni thus reduced a page of the illustrated journal, Ueber Land und Meer, to sheets of one inch in width, on which the print was perfectly legible through the microscope.
Photography and the Sand-blast Process. — Tilghmann, at Philadelphia, made the observation, during his residence at the watering-place Longbranche, that the windows exposed to sea wind became quickly dim. He found that this was occasioned by fine sand, which the wind drove against the window ; this gave him the idea of making ground glass by blowing sand against it, and he succeeded perfectly. He covered a glass surface with an iron screen, in which figures and letters were cut, and kept this in a current of air and sand. In a short time the exposed parts of the glass were ground, and thus a picture of the figures produced on the glass. A blast of a pressure of only four inches of water and a period of ten minutes are required for this work. If the air pressure is stronger, or