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100 THE ADVANCE OF PHOTOGRAPHY
portant means of preparing thousands of copies from one original map, and thereby contributing to the successful advance of the German army, which, with these maps in hand, showed itself better acquainted with the enemy's territory than the enemy's troops themselves. The photo-lithographic establishment of the brothers Burchard, at Berlin, produced in the war of 1870-71, 500,000 maps. Plate I. is a specimen of this process.
PhotoZincography . — The nature of photo-zincography will now be clear to the reader : for the treatment of the zinc plate and of the stone is the same. The negative is either copied directly on the zinc plate, coated with gelatine and bichromate, or a copy from the negative is prepared on chromo-gelatine paper, and the paper is then inked and transferred by pressure to the zinc plate. In both cases the prints may be taken directly from the zinc plate.
It must be remarked that, even without photography, direct mechanical copies of maps, writings, etc., can be made by a transfer process if the original be executed in oil colours. The back of the original is moistened with acidulated gum water, and then the face is inked with an oily ink which adheres only to the oily strokes of the drawing or printing. The original, thus freshly inked, is then placed on a fresh stone, or a freshly cleaned zinc plate, and pressed. The drawing passes over to the stone or the zinc, and can be easily multiplied by inking and printing. It is difficult to preserve the original, which is often much damaged by the pressure. Still more difficult is it to obtain clean lines, for the ink is often squeezed out by the pressure, and if the lines are close, as in the mountain lines of maps, they run together ; therefore the process has been more applied for copying old books, which have been reproduced page by page in this way.
It is self-evident that only reproductions of the original size can be made by this process.