Page:TheAmericanCarbonManual.djvu/98

 into a chloride of copper. The prints so produced were of a chestnut brown.

We now come to another definitely-marked feature in the history of carbon printing. In all the efforts hitherto made, there was one signal defect,—the absence of perfect gradation of tone from light to dark. A certain granular grayness in patches represented half-tones in some of the processes; but the tendency was to abrupt steps from white to black, any definite approach to delicately-marked gradation being rarely obtained. This was believed to be owing to the nature of the materials employed: the notion prevailed that the finest mechanical subdivison of a pigment could not equal in delicacy the fineness of the deposit obtained by the reduction or precipitation of a metallic salt. We know now that this was not the cause of the difficulty, which was due to the mode rather than to the material, as will be seen from the explanation which follows. When the surface of a film of bichromated gelatine is exposed to light, all portions upon which light acts in the slightest degree, whether through half-tones or shadows, are rendered insoluble at that surface, the only difference being that the light penetrates deeper in the shadows, and therefore produces a thicker layer of insoluble matter. When the exposed print is placed in a solvent, the whites, having been protected from the action of light, are laid bare at once; and the water then penetrating laterally, dissolves the soluble layer underneath the thin insoluble film, which forms the half-tones; these being thus deprived of their contact with the paper, float away, leaving only the deep shadows, in which the light has penetrated quite through the film. The picture thus consists of masses of black and white, without true gradation. The only wonder is that any approach to half-tone at all was ever secured by such a mode of operating. Such gradation