Copper-plate printing is, in all points, the reverse of typographic printing. The engraved lines, cut below the surface, are filled with ink in a compact body, and not in a thin film, liable to spread under pressure, as it may on a type or on a wood-cut; the ink from a copper-plate is pressed in such a way that it re-appears on the paper in a low relief—it is not squeezed on and flatted out, but stands up with sharper line and shows a greater depth of color. The slenderness of the incised lines, the fineness and hardness of the metal, and the peculiar method by which the ink is laid on the plate and fixed to the paper, give to prints from engravings on steel or [p020] on copper a sharpness of line, a brilliancy of color, a delicacy of tone, and a receding in perspective, which have always won for this branch of printing the preference of artists. Yet it is a slow and expensive process. A steel-plate engraver may be engaged for many months upon a large plate, from which but forty perfect impressions can be taken in a day. On ordinary work on a large plate, three hundred impressions per day is the average performance of a copper-plate press. Steel and copper-plate printing is largely used for bank-notes, portraits, fine book illustrations, revenue and postage stamps, and sometimes for commercial formularies, but it is in every way unfitted for the printing of books. It has not been much improved since its invention. Steel plates may be duplicated by means of electrotyping, or by the process of transfer to soft steel, but these duplicates cannot be made so cheaply as typographic stereotype plates, nor so promptly as transfers by lithography. The inking and cleansing of the plate, always dirty and disagreeable work, has hitherto been done only by hand. All the manipulations of copper-plate work are slow and difficult: they present many obstacles to the use of labor-saving machinery.