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

88 THE ADVANCE OF PHOTOGRAPHY in the former case the parts not affected stand out in relief, in the latter case those exposed to light. Woodbury Type. — A new printing process founded on the preceding, was invented by Woodbury in 1865, and has been called Woodburytype, from its inventor. The heliographic methods of printing previously described are apparently very simple. Pictures of all objects cannot, however, be prepared by these methods. A linear drawing, such as a map or letter -press, can be reproduced, either on an enlarged or diminished scale, by these methods ; but pictures with half-tones, such as sepia drawings or photographs from nature, cannot be so copied.1 The soft half-tones become rough and hard, rendering the picture very ugly. According to Osborne, the cause of this is found chiefly in the nature of halftones in copper-plate printing, which are formed by a series of lines at various distances from each other or by roughening the plate ; the latter method produces a series of points, which, according to their distances from each other, give a lighter or darker shade or half-tone. The half-tone of sepia drawings and photographs is quite different. It is not formed of strokes or points, but is a homogeneous light or dark colour. Accordingly, it was first necessary to break up the photographic half-tone into a series of strokes or points, to make it a copper-plate half-tone, and this constitutes the difficulty. Woodbury conceived the idea of producing, by a new printing process, homogeneous half-tones, perfectly similar to those of photographs or sepia drawings. He produced a relief by exposing a film of prepared gelatine under a negative, and then treating it with hot water. This relief shows the dark parts of the original in high relief, and the light parts in low relief. For the negative is transparent where the original 1 See half-tone process.