The Essentials of Illustration / A Practical Guide to the Reproduction of Drawings & Photographs for the Use of Scientists & Others

by Thomas George Hill

Fig. 3

This illustrates the principle which obtains in the making of half-tones in which the image is made up of a large number of dots varying in size but all equally dense, so that when viewed from a suitable distance the dots are individually invisible but compose to give gradations of light and shade. In other words, the structure obtaining in a photographic negative is, in a sense, realised by optical chemical means, although the dots in a half-tone block are much coarser than those in a negative (Plate 5, Fig. 3).

This result is obtained by interposing between the diaphragm of the camera and the negative—for the half-tone process is a photo-mechanical one—a glass screen covered with intersecting engraved lines (Fig. 4). As a matter of fact, each screen consists of two plates of glass similarly ruled and cemented face to face so that the lines intersect.