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

268 THE ADVANCE OF PHOTOGRAPHY settled by experience ; it is the rule, however, to place the finer screens nearer the plate, also the less the camera extension the nearer the plate must the screen be placed. As before stated, the fineness of the lines varies very much in different screens. The better the paper on which the print is to be made and the more finished the picture it is desired to produce, the finer must be the screen ruling. For the very rough kind of illustrations in common use in some newspapers, screens with 60 to 80 lines per inch are quite fine enough, while for good book illustrations, screens having from 150 to 200 lines to the inch must be used, and these by no means represent the finest grade of this kind of work. A good deal of controversy has taken place on the theory of the action of these ruled screens. The most generally accepted view is that each little space of clear glass which lies between four of the crossed lines of the screen, acts in forming an image upon the plate just as does a pinhole in the pinhole camera. This explanation is questioned by some authorities, but space does not permit of a detailed account here.1 Some idea of the building up of the picture from the effects of the light received through the little spaces of the screen may be obtained by looking carefully at a half-tone illustration through a small hand-lens (see fig. 109). In the high lights it will be seen that there are a number of very tiny dots, any suitable neighbouring four of which will be easily recognized as forming the corners of a square. In the half tones these dots become larger and spread out into the square, while in the darker shadows it may be almost impossible to pick out the individual black dots, and unless a more powerful lens is used, all that appears is an even black tone with here and there a clear white dot. 1 Photographic Journ., Feb. 1908, p. 77.