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 as was obtained appears to have been dependent upon the fact that a thin solution and an absorbent paper were employed, the bulk of the sensitive material being in absolute contact with the fibre of the paper; so that any portion, on which light had acted at all, was not readily removed from the paper. Certain it is, that the principle of washing away the unaltered material from the side opposite to that which was exposed to light is vital to the perfection of gradation in carbon printing. As the discovery of this principle has been the subject of a little misapprehension, it is interesting to trace it here to its first enunciation, and mark how its gradual recognition, and the discovery of practical means of applying it, have led to the perfection of carbon printing.

The first recognition which we find of this principle is in a paper on the use of linseed oil as a sensitive agent in photographic engraving, by M. L'Abbé Laborde, communicated to the French Photographic Society in July, 1858. In this paper, the Abbé announced his discovery that linseed oil which had been treated with litharge was sensitive to the action of light, becoming insoluble under its influence, in like manner to asphaltum. In the course of his experiments with this substance, he had discovered that the insolubility caused by the action of light commenced at the surface exposed, and gradually penetrated through the film in direct proportion to the intensity of the light. He acknowledged the loss of half-tone, although he did not suggest a mode of meeting the difficulty.

In November, 1858, Mr. J. C. Burnett, in a communication on carbon printing, pointed out the same fact still more clearly, and indicated the direction in which effort must be made in order to overcome the difficulty. After