362
EXPOSITION AT VIENNA.
illustrations, with the exception of such illustrations as require a considerable extent of white ungraduated surface, on which the gelatinous ink will leave marks,—the result of its imperfect expulsion from the two plane surfaces in contact. In reproduction of drawings, therefore, and wherever broad, white masses are included, the Albert-type and its analogous process, the Heliotype, have a decided advantage, as well as in the almost unlimited size of which they may be produced. The Heliotype differs in several important particulars from the Albert-type, or its close relative, Lichtdruck. The latter have a gelatine, or gelatine and albumen, film supported on a plate of glass ; the former employs a film entirely detached, and only temporarily laid on a metallic or other basis while being printed from. The sheet of gelatine which is employed for the printing material in the Albert-type receives the image on its outer surface, which is necessarily more liable to accidental imperfections than the under surface or that which is formed by the glass on which the gelatine is spread, which is employed in the Heliotype ; and the flexibility of the detached film is of the highest value in receiving the impressions from the negative, insuring perfect contact between the negative and sensitive film, scarcely to be obtained when the latter is on a rigid material.
This, and the analogous processes, are in comparative infancy yet; but we have already results produced by them, which, in certain directions, are hardly capable of very material improvement; and, when the conditions of certainty and excellence are absolutely determined, we may expect to see the splendid results of photography made more accessible than those of any other form of pictorial reproduction.
The accidental employment of a granular pigment in the gelatine used for making the relief in the Woodbury process led to the discovery that in this way a grain maybe produced similar to that of a mezzotint engraving; and this was ingeniously developed by Mr. Woodbury into an admirable substitute for that kind of engraving. The film of gelatine, which was exposed under the negative, was prepared with a granular substance of various degrees of fineness, so arranged that the coarser particles are on the side to be placed opposite the negative, and the finest next to it; which is readily