A Manual Of Photography

30 HISTORY OF PHOTOGRAPHY. or less porous, it requires more or less of the albuminous coating; it is best to employ a very close-grained porcelain, which requires but little white of egg. the prepared plate may be made sensi- tive to light in the same way in which a sheet of paper is rendered sensitive; and we generally find the same methods ap- plicable for photographic pictures on paper, applicable to those on porcelain plates, and one of the processes employed by the patentee is nearly the same as that patented by Mi. talbot in 1841. The prepared plate is dipped into a solution of nitrate of silver, made by dissolving twenty-five grains of nitrate in one ounce of water or the solution is spread over the plate uni- formly with a brush; the plate is then dried, afterwards dipped into a solution of iodide of potassium, of the strength of about twenty-five grains of iodide to one ounce oi water, again dried, and the surface rubbed clean and smooth with cotton., The plate is now of a pale-yellow colour, owing to the formation on its surface of iodide of silver. The plate, prepared as above directed, may be kept in this state until required, when it is to be rendered sensitive to light by washing it over witii a solution of gallo-nitrate of silver, then placed in the camera ; and the image obtained is to be rendered visible, arid sufficiently strengthened, by another washing of tire same liquid, aided by gentle warmth. The negative picture thus obtained is lived by washing it with water, then with bromide of potassium, or, what is still better, hyposulphite of soda, and again several times in water, the plate of porcelain being semi-transparent, positive pictures can be obtained from the above-mentioned negative ones by copying them in a copying-frame. The picture obtained on porcelain can be altered or modified in appearance by the application of a strong heat, a process not applicable to pictures taken on paper. With respect to this part of their invention, the patentees claim :—“ The obtaining by means of a camera, or copying-frame, photographic images or pictures upon slabs or plates of porcelain.” The second part relates to the process which has been discovered and improved unon bv Mr. Malone, who is associated with Mr. Pox Talbot in the patent "The patentees' improvement is a method of ob- taining more complete fixation of photographic pictures on paper. For this purpose, the print, after undergoing the usual fixing process, is dipped into a boiling solution of strong caustic potash, which changes the colour of the print, and usually, after a certain time, acquires something of a greenish tint, which indicates that the process is terminated. The picture is then well washed and dried, and if the tint ac- quired by it is not pleasing to the eye, a slight exposure to the vapours of sulphuretted hydrogen will restore to it an agreeable