dry, this is exposed under a negative, with the back in contact, according to the now recognized principle of obtaining half-tone. After exposure, the unaltered matter was removed by means of turpentine or other similar solvent, leaving an image with perfect gradation, in a material analogous to printing ink. The picture thus obtained was then mounted on white or tinted paper. The chief drawbacks to this process were: first, the impure lights which resulted from the tracing paper on which it was necessary to produce the prints to secure sufficient transparency, in printing through the paper. This has, however, been overcome by adopting a transferring process in which the image is removed from tracing paper to any ground which may be chosen. The second difficulty was the long exposure, which was about three times as protracted as that necessary for silver printing on albumenized paper. Many of the results we have seen are, however, very excellent.
Later in the same year, Mr. Blair[1] expressed a conviction as to the disadvantages of any method of producing prints on waxed, oiled, or varnished paper, and proposed a method on plain paper, in which he endeavored to compensate for the tendency to lose half-tone when the prepared surface was presented direct to the light, by the mode of preparing the paper. It was at first coated with gelatine; then, when dry, with albumen and syrup (containing a little transparent color, to give a delicate half-tint to the paper). Subsequently the surface was coated with carbon powder, which was made to adhere by moistening the back of the paper. Finally, when required for
- ↑ "Photographic Notes," vol. viii.
stone consisted of copal varnish, raw linseed oil, bichromate of potash, Brunswick black, mastic varnish, and turpentine, ground up together.
