The advance of photography : its history and modern applications (1911)

80 THE ADVANCE OF PHOTOGRAPHY by the sun's rays. . . . When an object is laid in the usual way on this paper, the portion exposed to the light speedily becomes tawny, passing more or less into a deep orange, according to the strength of the solution and the intensity of the light. The portion covered by the object retains the original bright yellow tint which it had before exposure, and the object is thus represented yellow upon an orange ground, there being several gradations of shade or tint according to the greater or less degree of transparency in the different parts of the object. " In this state, of course, the drawing, though very beautiful, is evanescent. To fix it, all that is required is careful immersion in water, when it will be found that those portions of the salt which have not been acted on by the light are readily dissolved out, while those which have been exposed to the light are completely fixed on the paper. By this second process the object is obtained white upon an orange ground, and quite permanent. If exposed for many hours together to strong sunshine, the colour of the ground is apt to lose in depth, but not more so than most other colouring matters." It appears that Ponton made experiments similar to those of Talbot, in the first period of silver photography. Perhaps he also copied leaves (see p. 17). The copies which are produced in this manner on chromate of potash are, however, immeasurably fainter than the copies on silvered paper. They may be easily prepared by exposing under dried leaves, a drawing, or a negative in a printing frame, pieces of paper which have been soaked in a solution of chromate of potash for about a minute, and then hung up to dry in the dark or by lamplight. The chromic acid is then reduced to brown peroxide of chromium, but if the exposure lasts very long the reducing process goes further, and a green oxide of