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10 THE ADVANCE OF PHOTOGRAPHY
exposed it to the light beneath a drawing on paper. In this case the black lines of the drawing kept back the light ; and, accordingly, in these places the asphalt coating remained soluble ; under the white paper, on the contrary, it became insoluble. Therefore, when lavender oil was afterwards poured over the plate, the parts of the asphalt which had become insoluble adhered to the plate, whilst the soluble parts were dissolved and removed, and the plate in those places was laid bare. Thus a film of asphalt, in which the original drawing appeared as if engraved, was obtained on the plate.
If a corrosive acid is now poured on such a plate, it can only act on the metal in those places where it is not protected by the asphalt ; and in such places this metal plate was in fact eaten into. Thus an incised drawing upon a metal plate was produced by the corrosive action of the acid, and a plate was obtained which, after cleaning, could be used for printing like an engraved copper-plate. Copperplate prints of this kind have been found amongst the papers left by Niepce. These prints he called " heliographs," and he showed them to his friends as far back as 1826. This method, in an improved form, has been much used, especially in the printing of paper money, when it is requisite to produce a number of engraved plates which are all to be absolutely alike, so that one piece of paper money ma}' perfectly correspond to another, and may therefore be distinguished from counterfeits.
Daguerre' s Experiments. — Niepce *s impressions were undoubtedly very imperfect, and therefore remained unnoticed. He himself gave them up, and again entered upon a series of experiments to fix the charming images of the camera obscura. In 1829 Daguerre joined him ; and both carried on experiments in common until 1833, when Niepce died without having obtained the reward of his long-continued efforts. Daguerre went on with the experiments ; and he would not, perhaps, have got further