A Manual Of Photography

170 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. the early daguerreotypes and the photographic copies prepared with the chloride of silver. The following is the process adopted by me, which I consider far from perfect, but which affords us very delicate images :— A well-polished plate of copper is rubbed over with the nitrate of mercury, and then well washed to remove any nitrate of copper which may be formed; when quite dry, a little mercury taken up on soft leather or linen is well rubbed over it, and the surface worked to a perfect mirror. The sheet to be copied is placed smoothly over the mercurial surface, and a sheet or two of soft clean paper being placed upon it, is pressed into equal contact with the metal by a piece of glass, or flat board: in this state it is allowed to remain for an hour or two. The time may be considerably shortened by applying a very gentle heat for a few minutes to the under surface of the plate. The heat must on no account be so great as to volatilise the mercury. The next process is to place the plate of metal in a closed box, prepared for generating the vapour of mercury. The vapour is to be slowly evolved, and in a few seconds the picture will begin to appear ; the vapour of mercury attacks those parts which correspond to the white parts of the printed page or engraving, and gives a very faithful but some- what indistinct image. The plate is now removed from the mer- curial box, and placed into one containing iodine, to the vapour of which it is exposed for a short time : it will soon be very evident that the iodine vapour attacks those parts which are free from mercurial vapour, blackening them. Hence there results a perfectly black picture, contrasted with the grey ground formed by the mercurial vapour. The picture being formed by the vapours of mercury and iodine, is of course in the same state as a daguerreotype picture, and is readily destroyed by rubbing. From the depth to which I find the impression made in the metal, I confidently hope to be enabled to give to these singular and beautiful productions a considerable degree of permanence, so that they may be used by engravers for working on. It is a curious fact that the vapours of mercury and of iodine attack the plate differently ; and I believe it will be found that vapours have some distinct relation to the chemical or thermo- electrical state of the bodies upon which they are received. Moser has observed this, and attributes the phenomena to the colours of the rays, which he supposes to become latent in the vapour on its passing from the solid into the more subtile form. I do not, however, think this explanation will agree with the results of experiments. I feel convinced that we have to do with some thermic influence, and that it will eventually be found