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

PHOTOGRAPHIC IMPORTANCE OF CHROMIUM COMPOUNDS 87 f. x/ ,y jr j& p zL them, the light penetrates in various degrees, according to its strength — most in the transparent places, less in the semi-transparent, and not at all in the opaque parts. Accordingly, insoluble films of different thickness will be formed, as represented in b. The shaded parts in the figure denote the portions that have become insoluble. If now the film of gelatine (fig. 22 b) is plunged in hot water, all the parts left white in the figure become dissolved ; but at the c? a' c^ same time the half-tones J^V^M iy " mmm a not adhering to the substratum P — for instance, paper — become detached and are torn off. Therefore a relief of the form d remains behind ; the half-tones (y y) are wanting. In order to avoid this a support must be given to the exposed surface to retain the half-tones. For this purpose a piece of paper, moistened with white of egg, is laid Fls22 on the exposed surface and becomes firmly attached to it. If the sheet is now immersed in hot water, the film P becomes detached from g, the little portions of gelatine remain attached to the albumenised paper ; the white places in fig. 22 b become dissolved, and all the half-tones y y adhere firmly to the new layer, as in fig. 22 e, and form a relief. This is named the transfer process. If the relief produced by cold water, described p. 85 (fig. 22 c), is compared with that produced with hot water (fig. 22 e), the difference is at once apparent : ^/ . ■■"v>.-J jx/OT jr^c/