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BOOK ILLUSTRATIONS 267
E. and J. Bullock for methods of obtaining a grain. J. W. Swan was also one of the first to use a cross line screen for photo-engraving. About the same time (1867) similar methods were being experimented upon in Berlin and Paris, but in all cases the results obtained were very crude, on account of the coarseness and irregularity of the rulings.
In 1873 Mr Woodbury patented a process in which use was made of a screen for producing half-tone blocks ; this screen was obtained by photographic reductions of nets. In 1882 Meisenbach brought out his method of using a single line-screen, this screen being turned during the process of exposure into a position at right angles to its original position. The screen was kept for half the exposure in one position, and for the other half in the other position. This proved to be a very great advance upon any of the previous attempts, and in fact has formed the basis on which all modern ruled screens have been developed.
The great defect of the early cross -line methods was that they were all placed in contact with the object which was being photographed. As this was the case, it will be readily understood that the diffusion and diffraction effects, upon which the screens which are now applied at a short distance in front of the sensitive film rely for their success, were entirely absent. A good many of the more recent improvements in ruled screens have been due to the ingenuity of Max Levy.
The modern screen consists of a net-work of lines, the fineness of the lines varying in different screens. A system of equidistant parallel lines is ruled on each of two pieces of glass, these pieces of glass being afterwards cemented together in such a manner that the parallel lines cross one another.
The distance which the screen is placed from the film is very important, and is a question which must be