=Metal Engraving--The Invention.=--The engraving of metal plates for pictorial reproduction was a direct development of ornamental engraving. The Italian Niello work of the fourteenth and fifteenth centuries was chiefly applied to the embellishment of metal ornaments and utensils with elaborate engravings. To intensify their effect, the designs were filled in with a black pigment known as _Niello_, L. _Nigellus_--Black. Hence the name by which the process was generally known. Niello work was practised chiefly by gold and silversmiths, and it is recorded that one of these, Finiguerra by name, was filling up the lines of the engraving with black composition in the usual way when he accidentally spilled some hot wax over the plate. It rapidly cooled and hardened, and on scaling off bore a distinct black impression of the engraving. Quick to perceive the importance of his discovery, Finiguerra promoted a few experiments which ultimately led to a full realisation of his hopes. There is yet another account of the metamorphosis of metal engraving which, if true, reflects much more credit upon Finiguerra than the accidental discovery already described. To obtain a _proof_ of their work, the Florentine metal-workers covered the ornamentation with some fine plastic material. It was then a simple matter to convert the impression into a mould, which they filled with melted sulphur. The casts, when hard, formed exact replicas of the engravings, and afterwards, when the incised lines were filled with a black pigment, probably Niello, they presented an effective record of the original work. It is not by any means improbable that Finiguerra made his discovery when making such a cast. It is a noteworthy fact that the idea of producing impressions from engraved metal plates was not, as might readily be imagined, a development of wood engraving or of the then well-known method of printing from engraved wood blocks. It was a fortuitous discovery, and probably the direct result of an accident. The true importance of this transition, _i.e._ Niello work to engraving as a reproductive art, is seldom fully appreciated. It was a momentous change, bristling with possibilities, which subsequent developments amply proved. The time was peculiarly propitious. The beneficent influence of the Renaissance was at its flood, and a feverish spirit of progress swept over Europe. The imitative instinct inherent in mankind reasserted itself with an irresistible intensity, and new forms of pictorial expression were eagerly sought after. The art of engraving provided a medium for the extension of the artist's fame and the popularising of his creations. It rapidly gained favour, and its ultimate development and expansion fully justified the interest it aroused.