16. The lighter shades are stopped out at an early stage of the process, and the _biting in_ is continued for the darker ones. After the plate is judged to be sufficiently corroded, it is cleansed, and an impression is taken on paper. The process is finished by burnishing the shades, to give them greater softness, and by touching up the defective parts with the graver. 17. This mode of engraving is well adapted to light subjects, sketches, landscapes, &c.; but, owing to the fineness of the ground, the plates wear out rapidly, and seldom yield, when of ordinary strength, more than six hundred impressions. The prints taken from such plates bear a strong resemblance to paintings in Indian ink, or to drawings in black-lead pencil. Aqua-tinta is the most precarious kind of engraving, and requires much attention on the part of the artist. It was invented by a Frenchman, named Leprince, who, for a time, kept the process a secret, and sold his impressions for original drawings. 18. _Steel engraving._ The process of engraving on steel plates differs but little in its details from that on copper plates; and the chief advantage derived from this method, arises from the hardness or toughness of the material, which renders it capable of yielding a greater number of impressions. 19. This mode of engraving was first practised, in England, by the calico-printers; but steel was first employed for bank-notes, and for common designs, by Jacob Perkins, of Newburyport, Massachusetts; and by him, in conjunction with Asa Spencer, of New-London, and Gideon Fairman, of Philadelphia, the use of steel in this application was generally introduced, not only in the United States, but also in Great Britain, some time before the year 1820. 20. The plates are prepared for the engraver from sheets of steel about one-sixth of an inch in thickness. A plate cut from a sheet of this kind is first softened by heating it with charcoal, and suffering it to cool gradually in the atmosphere. It is next _planished_, or hammered on a peculiar kind of anvil, to make it perfectly level, and afterwards ground on one side upon a grindstone. The operation is completed by polishing it with Scotch stone and charcoal. When steel was first substituted for copper, it was hardened before it was used in printing; but it is now used in its soft state, as it comes from the hands of the artist. [Illustration: COPPERPLATE PRINTER.] THE COPPERPLATE-PRINTER. 1. The copperplate-printer takes impressions on paper from engraved plates by means of a rolling press. This machine, together with some of the operations in its application, are well exhibited in the above picture.