360
EXPOSITION AT VIENNA.
on the haloid salts of silver,—an action so subtle and imperceptible as to require the supplementary force of a reducing agent, or developer, and the ordinary silver prints being the result of the same agency without this supplementary action, making the reproduction of the impression dependent on the uncertain condition of weather,—the printing processes depend on a preliminary action of light on a film of gelatine, which, when impregnated with a chromate, has the fortunate faculty of being rendered insoluble by the influence of light, and the copies are then produced by a merely mechanical action. The gelatine film, charged with the bichromate, is exposed to light under a negative; and as the ray penetrates more readily through the shadows or transparent portions of the negative, these become indurated and repel the action of water. On this fact, employed in several ways, the different processes are based ; one employing the film in its unequally softened condition, in which the portions imbued with water repel the printing ink, and those parts which were protected by the denser portions of the negative become the lights of the print, the indurated portions receiving the ink in proportion to their induration. This, with various provisions for the adhesion of the gelatine film to the basis, is the Albert- type, of which some examples are contributed to the Exhibition by Herr Albert, of a size hitherto unknown in such perfection of workmanship, some of them being a metre in length, portraits in life-size, copies of pictures and drawings in graduated tints, in which the reproduction is simply incapable of being bettered for large prints.
This gelatine film, exposed to light, as indicated, and then subjected to the action of moderately warm water, has the unindurated portions washed away, and, on drying, becomes a horny pellicle, with the subject in relief, and may be separated from the support used in printing and kept between the leaves of a book and used again as often as required, being in effect insoluble and indestructible, except by application of a force not required in printing. In this state it is utilized by Mr. Woodbury in his photo-relief process (better known by the name of its inventor, as the Woodbury-type), by producing, under hydraulic pressure, a relief in soft metal, which, filled with a transparent gelatinous ink and put