Page:The Story of the House of Cassell (book).djvu/144

 The half-tone process developed slowly. Its first methods and results were coarse. The "screen" (the device employed for breaking up the surface of the drawing by criss-cross lines for rephotographing, which is the earlier stage of process work) was obtained by placing netting over the drawing. In later years sheets of closely ruled cross-lines were photographed and the resulting negatives used as screens. But ultimately Mr. Levy, of Philadelphia, invented a machine for ruling sheets of glass with a diamond. The machine worked automatically, and would rule screens containing three hundred lines to the inch. These diamond-ruled screens gave such a clear result that soon they revolutionized the process trade.

The cross-lined screen came to be used also for photogravure. This form of reproduction was introduced by a firm at Lancaster under the name of Rembrandt photogravure, which is an application of the lined screen to the production of an intaglio plate. The most important part of the business, however, was the machine which printed the plates. Prior to this invention photogravure plates were printed by hand like etchings, and only a very few copies per hour could be turned out, but the machine now introduced printed the plates at a rate of hundreds per hour, and thus photogravure, with its always acceptable rich brown shadings, became popular for frontispiece and other pictures in the better class of magazines and books. Of this new form of illustration the House, under Mr. Bale's guidance, took full advantage.

The last great development of process work was the invention of colour printing from blocks instead of from drawings on stone—lithography. Colour in magazine and cheap-book illustration had been always difficult to provide. Lithography had been its chief source of supply, but it was not possible with fewer than nine or ten printings to produce any sense of fullness of colour. The large colour supplements to Christmas numbers had had as many as twenty to twenty-five printings, each from a different stone. The wood-block printing, introduced by