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94 THE ADVANCE OF PHOTOGRAPHY
These Albert-types were also known as " Lichtdrucke," and were in appearance very similar to prints obtained with silver salts. The process was well adapted for copying pencil and chalk drawings. It was largely used for preparing views having the appearance of ordinary photographs ; Obernetter of Munich printed many such which had been obtained by the photographic department of the Prussian army during the Franco-Prussian War.
The brilliancy of these pictures was produced by giving them a coat of varnish.
If the prints by this process be compared with Woodbury-types, it is seen that the latter give the shades and dark parts better, but that the white parts often appear discoloured. The Woodburytypes are, however, much more like photographs than the Alberttypes, for the latter have a lithographic tone. It is only by coating with varnish that they are made to resemble photographs.
Aniline Printing — Willis. — Aniline is a substance which resembles ammonia in its chemical relations, but having a different odour and a different chemical composition. The substance is obtained as a brown mass from coal tar on distillation.
If this brown fluid is treated with chlorine or nitric acid, or manganese and sulphuric acid, or arsenious acid, various shades of colour are produced. One specially interesting to us is the colour formed by heating aniline with chromate of potash and sulphuric acid ; the result is a peculiar violet substance — aniline violet. Chromate of potash, as already seen, pla}^ a part in some of our photo-chemical processes, and on this substance is based the aniline printing invented by Willis.
Willis immersed a piece of paper, in the dark room, in a solution of chromate of potash and sulphuric acid. He exposed the paper under a positive picture, e.g. a drawing or copper engraving. The light shines through the white paper, and in these places the chromic acid is