All these methods of illustrating books worked by hand have now been superseded by one or other of the wonderful processes made possible by the invention of photography. Some of these are expensive, but generally they are cheap. The most elaborate, and when well done the most wonderful, of these processes is that known as heliogravure. By means of this method reproductions of line engravings can be made so perfectly that detection is almost impossible. A metal plate is so treated by help of a photographic negative that the lines of the engraving are deposited in an insoluble form upon the plate, which is otherwise clean, then a thin film of metal is deposited on all the clean places by means of electrotyping, so that when the lines are dissolved out, they are in intaglio, just as they were in the original engraved plate. From this artificial plate prints can be made as if from an engraved plate. For the mezzotint another method is adopted, known as photogravure, and this is also a wonderful invention. A metal plate is slightly roughed--if it could be more roughed it would be better--and then a photographic relief in gelatine is put upon it and etched. The result is a plate resembling a fine grain mezzotint, but the prints made from it are always deficient in the blacks. To remedy this and other defects which at present seem to be inherent, an engraver generally goes over the plate with roulette and burnisher. The Photogravure has ruined the Mezzotint. The coating of the copper-plates with steel largely adds to their life. Then we come to the wood engravings, which are all perfectly imitated by the zinc block, made directly from the original drawing, and set on a wood block so as to range exactly with type in height. Wash drawings are closely copied by the half-tone process, which is also used with blocks that can be printed with type. The drawing to be copied is photographed through a glass screen very finely etched with minute lines crossing each other, so that the picture is ultimately represented by a series of little squares, black or white according to the tones of the original. But the whites are never quite satisfactory, the dots of the screen always show a little, so the dispossessed wood engraver has to be called in after all to touch them up with a graver. In America, where the half-tone process has reached a very high degree of excellence, the names of these helpful engravers are frequently added. A curious “shot-silk” effect is often seen in half-tone illustrations where the lines of the screen fall at a particular angle with the lines on the original. The same peculiarity sometimes shows in the ruled sky lines in the small nineteenth century line engravings.