The optical and other reasons for this phenomenon must be sought elsewhere,[A] but the following brief consideration will serve to illustrate what happens. The rays of light which ultimately reach the sensitive plate are acted upon by two lenses, that of the camera and the meshes of the screen, each one of which acts as a lens on the principle of the pin-hole camera. Each mesh, therefore, brings the image of the diaphragm to a focus on the negative, but the lens of the camera focusses the picture as a whole, thus the amount of light falling on the different pin-holes will vary in intensity, and hence the dots produced will vary in size, for it is assumed, with good reason, that each dot is built up from its centre and radially expands according to the amount of light acting upon it. [Footnote A: See Verfasser, _loc. cit._, p. 94.] It is obvious that the quality of the resulting picture will depend, other things being equal, upon the coarseness of the screen employed. Screens are ruled with lines varying from 50 to 400 to the inch: the lower rulings give very coarse reproductions, and are only used for posters, whilst the higher rulings yield very fine impressions and are employed only for the best work. It is hardly necessary to remark that the finer the screen the better must be the skill of the printer. To illustrate the difference in the results obtained by the use of different screens, the two figures on Plate 6 have been prepared; both were made from the same negative, but for the upper figure a 100-line screen was used, and for the lower a 200-line screen. It will be observed that there is more contrast in the former, and more detail in the latter. Authors should therefore mention when sending in their original pictures the qualities they require in the reproduction; it must, however, be remembered that the blocks made from the finer ruled screens will not print satisfactorily except on more or less highly glazed paper, to the use of the "art" varieties of which there are objections on aesthetic and other grounds. [Illustration: PLATE 6. Half Tone reproduction of a photograph by Mr. W. Rowan. Part of a shingle beach shewing plants of Sea Blite (_Suaeda fruticosa_) and a ring plover's nest with four eggs.] Before passing on it may be mentioned that screens with patterns other than that represented in Fig. 4 are sometimes employed; for instance, the wavy-line screen gives the impression of coarse collotype. The preparation of the blocks may now be briefly dealt with. A negative of the picture, using a screen suitable for the purpose, is taken on a special dry gelatine plate ("process" plates) or on some other form of negative, _e.g._, wet collodion which is most commonly employed. This negative requires very careful development in order to get the dots right.