ELECTROTYPE, PHOTOGRAPHIC. Much thought and labor have been expended in producing a relief-plate to take the place of wood-engraving, and various methods of etching on metal by the aid of photography have been brought to light. The earlier of these never were successful, because after the acid has eaten or etched below the surface protected by the asphaltum, there is nothing to prevent it from undermining the lines, as the acid will eat in one direction as well as another, thus weakening them to such an extent that they often break down in printing. With the gelatine process, the gelatine must be of a thickness compatible with the depth desired A gelatine of this thickness will become nearly, if not quite, insoluble before it is dry, through the action of bichro mate alone. Also the color of the gelatine, after the hichromate has been added, is such as to prevent the action of light from penetrating to the proper depth in the time during which it can be exposed. These are the almost insurmountable reasons why a relief-plate in gelatine has not been obtained till the advent of a new process, called photo-electrotype. W. II. Mumler, of Boston, Mass., has now succeeded in 'over coming these obstacles. After printing upon his gelatine, through a negative, the neces
sary time to secure all the details, the parts unaffected are dissolved away to a slight depth. The interstices are then filled with a black paste, when it is again exposed to light; the soluble parts that were protected by the opacity of the negative in the first printing are now protected from the action of light by the black paste that covers them. The second exposure may be continued for a length of time sufficient to allow the light to penetrate its entire depth; and the action of light being to render the gelatine insoluble, it can readily be seen that the protected parts can be dissolved away to the depth to which the light has penetrated. The gelatine relief is then placed in a drying closet for a few hours, when it becomes as hard as horn. From this an electrotype is taken in precisely the same manner as from a wood-cut. It is then mounted on mahogany blocks, type high, when it is ready for the press. The result is an electro type plate with a surface as smooth as polished plate-glass, and a depth far exceeding that of ordinary wood-cuts. See PHOTOGRAPHIC Els.;012AVIls"G.