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PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 97
scientific pictures, and a thousand other things are produced by lithography.
ChromoLithography. — A development of this process, called chromo-lithography, came into general use, and was at one time by far the most important method of producing coloured pictures mechanically. Chromo-lithography is rather more complicated than common lithography. If it is wished to make a chromo-lithograph of a painted picture, not only one stone, but a separate stone for every colour must be prepared. For example, to prepare a picture of an object in which blue, red, and yellow tones appear, a drawing in blue ink of all the blue parts must be made on a separate stone ; a second and third stone are required for the yellow and red places. Prints must be made from each stone successively on the same piece of paper in the proper position, and thus a picture in colours is obtained. If such a picture is then coated with a brilliant varnish it becomes an " oleograph."
If must not be understood that pictures of real artistic beauty could be prepared by these means by the ordinary printer, an artist's taste and knowledge of colour being necessary for chromo-lithography.
Zincography. — Closely related to lithography is zincography, which we shall glance at here before passing to photo-lithography.
Zinc, curiously enough, has the same property as lithographic stone ; for if drawings in oily chalk be made on a zinc plate, and then the whole moistened with gum water, the plate may be inked with oil colours, and thus a picture be obtained of the chalk drawing. The printing, therefore, presents results similar to those of lithography ; but the preparation for zinc printing has more difficulties than lithography, so that the use of zinc for this purpose is limited.
We have given a brief survey of the principles of lithography and zincography, as far as necessary to understand