Processes in which the picture is printed from a metallic relief surface include the ordinary photo-engravings or photo-electrotypes. In the former a plaster mold is taken of the gelatin film and a east reproduced in type metal, when it is called a photo-engrucing, or in copper, when it is known as a photo-electrotype. These include those processes of in which the pic tures are printed front zinc plates to which the design has beeu transferred in adhesive transfer ink front paper. similar to the transfer process referred to under LITHOCHAPII Y. section on Photolithography. The zinc plate thus ob tained is then treated with acid in order to lower the white portions, producing a low relief.
Processes in which the picture is printed from an intaglio copper plate had their origin in France, and were known variously as photo gravures, photo-aquatints, etc. These consist of gelatin relief films similar to those of the Wood burytype (see above), with certain modifications which caused the production of a grain in the re lief film; thus in the case of the photogravures of Goupil, the material producing this effect is said to be pulverized glass. A mold is then made from the gelatin relief, from which a copper plate is obtained by electrotyping. from which the prints are then carefully made.
The invention of a transparent and flexible film pellicle for supporting the sensitized photo graphic surface was made by Hannibal Goodwin, and his application for patent was filed May 2, 1SS7, but the patent was not issued until Septem ber 13, 1S9S. The substance itself was entirely new-, and the manufactured article consisted of a film support of a dried and hardened celluloidal solution of nitrocellulose. The film must be non-greasy, insoluble in developing fluids, insen sible to heat and moisture, hard, smooth, brilliant of surface, exceedingly thin and and abso lutely transparent. It was no easy matter to obtain all these properties in the new support, and it is only after many years that the film has succeeded in supplanting, to any great extent, glass as a supporting substance. It was a very important invention and has made possible the widespread modern employment of photography by amateurs. The roll holder, by means of which a long strip of film can be carried in a compara tively small space, was first suggested by W. .1. Stillman, and has led to the modern hand camera for films, and later to the so-called system of 'daylight photography,' which still further in creased and simplified the use of photography by amateurs.