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

266 THE ADVANCE OF PHOTOGRAPHY vibration. The camera carriage attached to such a stand is so arranged that the camera can be used direct for copying, or can be turned into a position at right angles to this direct position when it is necessary to use a prism. Such a stand without the camera is shown in fig. 108. Half -Tone Process. — We will now try to trace the steps by which a process-block for a half-tone illustration is prepared. It will be necessary first of all to explain what is meant by a half-tone illustration. In making a block from a photograph or other picture in which there is a wellmarked gradation of light and shade, it is necessary to break up the half-tones and lights into distinct parts by dots or stipple, so as to be able to reproduce them in the press, and the best means of doing this furnished food for thought for many workers until the line screens were invented. It will be instructive to consider the attempts made in this direction, and it will be well to do so by referring to them in their historical order. The idea appears to have originated with Fizeau 1842, but it was Fox Talbot who was the real father of the process. He used what he called a photographic veil, which was composed of two or more thicknesses of crape or gauze. His work on photographic engraving appeared in 1852. In 1854 Pretsch made use of the reticulation of the gelatine for the same purpose. Berchtold, between the years 1854 and 1859, was the first to make use of crossed rulings. His method was somewhat analogous to that afterwards adopted by Meisenbach, in that he used a single line-screen with equal parallel spaces and lines, but in his case the ruling was applied directly to the print. In 1860 Asser used starch, and in 1865 J. W. Swan used gelatine mixed with charcoal or other fine powder of a chemically inert nature, and in the same year specifications were also published by