PHOTOGRAPHIC ENGRAVING. Several ingenious attempts have been made to pre pare engraved plates by photogenic action; the earliest of these dates as far back as 1827, which was six years previous to the introduction of the daguerreotype process, and was the invention of 1I. Nicephore Niepee of Paris, who first discovered that thin plates of bitumen were curiously affected by light; he therefore coated metal plates with a thin layer of bitumen, of the kind called Jews' pitch, and placed them in a camera obscure, so arranged that he could insure their exposure to the same imagefor several hours. The plate was then submitted to the action of oil of spike, which readily dissolved those por tions not acted upon by the light, but exerted little action upon the remainder; the metal exposed by the solution of the bitumen was then acted upon by acid, which produced a complete etching-plate, the picture part being protected by its bituminous varnish from the action of the acid. About ten years after, M. Fizeau invented another process; he took a daguerreotype picture, and acted upon it with a mixture of nitric, nitrous, and hydrochloric acids, which, without affecting the silver where the metal was free from the photographic action, quickly attacked the dark portions of the picture in greater or less degree to their intensity, and thus etched the picture in the plate. The action at first produced only a slight erosion, because a coating of chloride of silver was formed; but upon this being removed by a solution of ammonia, it was repeated until a greater depth was gained. This, at the utmost, was not very great, and never sufficient to print from; but he most ingeniously met this difficulty by the plate with a drying oil, which was carefully wiped from the surface, and left to dry in the engraved parts; afterwards, he electrotyped the level surface with gpld until the neces sary depth was obtained, after which the plate was boiled in a of caustic potash, which remoted the varnish. The plate in this state required a little of the engraver's art to touch it up, and remedy some defects inherent in the process; and then, to pre vent injury to the soft metals—silver and gold—employed, an electrotype plate was taken for the printer's use. But these processes, notwithstanding their extreme ingenuity. never thoroughly succeeded, and have been abandoned for time more satisfactory inven tions of Dr. W. 1I. Fox Talbot; these were patented in 1852 (No. 179) and 1858 (No. 875). By his first plan, a steel plate, such as is prepared for engravers, is first dipped into a solution- containing acetic and sulphuric acids; it is then coated with a mixture contain ing a solution of tine gelatine and bichromate of potash. This is impressed with the image of a photographic negative by exposure in the copying-frame, and washed. The film of gelatine is previously yellow, but the action of the light through the light parts of the photograph change it dark brown, but the remainder is unaffected; conse quently, a picture is produced of a light yellow color on a brown ground. The action of
the light is to reduce the bichromate of potash, and, consequently, to render the gelatine combined with insoluble; whilst those portions which have been protected from the action of the light by the dark parts of the negative, are still readily soluble in water, and can be removed by soaking: the insoluble portion thus forms a raised picture, which is submitted to a solution containing bichloride of platina in certain proportions, with a little free acid and water, which etches out the exposed parts of the plate, and renders it fit for engraving from. In the same specification is added an ingenious method of giving to the whole picture the appearance of an engraving; it consists in spreading over the gelatinized plate, when nearly dry, a piece of very fine muslin, and evenly pressing it so as to leave an impression of the cross-lines of the textile material upon the surface. By his second specification, he alters the process so far as the washing is concerned, after obtaiff ing the picture on the gelatinized plate, and thus obviates some injuries to which it was thereby rendered liable. Instead of washing, the gelatinized surface is thinly but very evenly covered with finely powdered copal or other resin, and the under side of the plate exposed to sufficient heat to melt the resin, so as to form a thin varnish over the whole. The etching fluid is then poured on, and, notwithstanding the resin coating, it acts through to the metal, and cats in wherever time gelatine has not been rendered insoluble by the action of the bichromate of potash and the light. When sufficiently etched, it is washed in clean water, and the plate is freed from the resin and gelatine. Two modifi cations of this process are given in the specification, to which the reader is referred for fuller particulars. Dr. Talbot calls his process pholoslyphie engraving. The same proc esses, with some modifications, applied to zinc constitute and to stone .(q.v.), both of which are largely practiced; and they have been brought to such extraordinary perfection, especially by time late sir Henry James, director of the ordnance survey, and by an eminent firm in Brussels—Messrs. Simonau, Toovey, & Co.—that quite a new era is opened up in the art of engraving and printing.
These processes are particularly well adapted for copying maps and printed books, and sir 11. James has consequently .turned it to a most profitable account in producing reductions of the large plans of the ordnance survey to the proper sizes of maps; and he has also published perfect facsimiles of Doomsday Book and other important docu under PHOTOGRAPHY, and ELECTROTYPE, PHOTOGRAPHIC.