The advance of photography : its history and modern applications (1911)

98 THE ADVANCE OF PHOTOGRAPHY what follows. Both processes resemble the Alberttype printing in many respects ; in both cases the surface has the peculiarity of retaining ink in some places and repelling it in others — but, of course, lithography was invented long before Alberttype. When photography was invented it deprived lithography of an important branch — that of portraits. Even as late as 1850 it was still customary to produce lithographic portraits. The lithographs from oil paintings also suffered through photography. Photo-Lithography. — Poitevin, who allied the two by inventing photo-lithography, endeavoured to economize the labour of the lithographic draughtsman, and to replace it by the chemical action of light. He coated lithographic stones with chromate of potassium and gelatine, and exposed them under a photographic negative. The picture thus obtained was then washed and inked. All parts affected by light took the colour, and gave an impression in the press. The first attempts of the kind were very imperfect ; the pictures were especially wanting in half-tones, which were lost in washing, as they are in the pigment process. An improvement was introduced by Asser and Osborne ; they printed the negatives on paper prepared with bichromate of potash coated with gum, gelatine or albumen, and then they inked this. Such paper has the peculiar property of retaining printer's ink on the exposed places only. After inking, the paper was carefully washed and then pressed on a lithographic stone. This absorbed the colour, and thus the picture was perfectly transferred to the stone, from which excellent lithographic prints could be obtained in the ordinary way. Though half-tones were thus produced, the impression fell far short of photographs in quality. The lithographic half-tone differs essentially from the photographic, which forms a homogeneous surface, while the lithographic half-tone appears as a mass of