Читать книгу A Handbook of Illustration - A. Horsley Hinton - Страница 9

As has been already said, it is of course necessary to produce from the original a printing surface of such a kind as shall take ink and print an image therefrom. Now it will of course be obvious that with ink and white paper we can only produce two things—black and white—and that therefore all the intermediate shades must be produced by a greater or less number of black dots. The process under consideration, in common with some others, is based upon the fact that gelatine or albumen sensitised with bichromate of ammonium or potassium, becomes insoluble after being acted upon by light. A solution of bitumen in benzole also forms a light sensitive coating which is frequently used in these processes. If we were to expose such a sensitive film to light under (for example) a photographic negative of a figure taken against a light background and then washed it in a suitable solvent, those parts which had been protected from light by the opaque portions of the negative, such as the background, the face, hands, and white portions of the dress, would dissolve away, leaving the insoluble or light affected portions standing.