film to light, and of the other to the solvents which should remove the unaltered material, seemed to present an insuperable difficulty. Exposure through the prepared paper was attended by two grave difficulties: the passage of the light through the paper, rendered yellow by saturation with the bichromate, was exceedingly protracted; and actinic force transmitted by the negative was to a great extent arrested before it reached the sensitive coating of gelatine and pigment, a brown tint (highly adiactinic) being formed in the texture of the paper. The intervening paper between the negative and sensitive layer is further objectionable on account of the loss of brilliancy incidental to its becoming more deeply brown where the shades of the print come, and consequently offering a proportionally greater obstruction to the light there than elsewhere. Besides, the finished picture, no matter how delicate the negative, bore marks of all the granulation or defects in texture of the paper through which the light passed. The ingenious device of M. Fargier, in which the sensitive coating was applied to a plate of glass, and, after exposure, transported from the glass, by the aid of a film of collodion, in order to wash away or develop on the opposite side of the film, was manifestly impractical on a commercial scale, especially for large pictures, owing to the difficulty of manipulating a thin film of gelatine attached to a thin film of collodion, floating in a vessel of water; and the difficulty, almost amounting to impossibility, of transferring such a film perfectly to paper. The “tissue” described in Mr. Swan's specification, together with the series of ingenious devices accompanying its use, have been the means of effectually overcoming the many practical difficulties above enumerated, and of making the production of carbon prints, of any size, in any
