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Rh removed, but those parts representing the lighter tones of the picture, having become insoluble on the upper surface only, the layer underneath is readily dissolved, and the whole film in such parts is thus removed by the solvent. An imperfect image, possessing only deep blacks and masses of white without gradation of half-tone, is the result.

This was the great difficulty of carbon printing in the early experiments which succeeded the discovery by M. Poiteven, in 1855, of the principles upon which it is based, although the cause was not at first fully understood. After two or three years of comparatively unsuccessful effort, it was discovered, that in order to succeed in producing gradations of half-tone in such pictures, it was necessary to wash away the unaltered and still soluble matter at the side of the film opposite to that exposed to light, in order to preserve intact every portion of the film which had been rendered insoluble, and so leave a film varying in thickness according to the depth to which light had penetrated; this depth being governed by the varying degrees of transparency of the different parts of the negative. This varying thickness of a colored translucent film upon a white ground, accordingly rendered the gradations of the picture, a thick layer representing deep blacks, and thinner layers, various gradations of half-tint.

This principle recognized, it became possible to produce carbon prints, which were true transcripts of a photographic negative; but a mode of rendering the principle practically useful was wanting. The first idea was to place the back of the prepared paper in contact with the negative, so that the light should, after traversing the paper, act upon that surface of the sensitive layer which was in contact with the paper, rendering it insoluble in varying degrees. The soluble portion was then