A Manual Of Photography

132 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. light. The different degrees of darkness induced are fairly repre- sented in the margin (Fig. 18). As the different bromides give to photographic paper varieties which much resemble those enumerated under the muriates, I have thought it unnecessary to give an account of any of them. The paper prepared with the bromide of potassium is the kind I have adopted, after having tried upwards of two hundred combinations of silver with the other bromides. To prepare a highly sensitive paper of this kind, select some sheets of very superior glazed post, and wash it on one side only with bromide of potassium—forty grains to one ounce of distilled water, over which, when dry, pass a solution of one hundred grains of nitrate of silver in the same quantity of water. The paper must be dried as quickly as possible without exposing it to too much heat ; then again washed with the silver solution, and, when dry, carefully preserved for use. It will be perceived that I adopt a slightly different manipu- lation from that recommended by Mr. Talbot. Instead of wash- ing the paper with the solution of silver first, and applying the bromide or the muriate over this, and then the silver wash again, I use the alkaline salt first, and apply the metallic washes one on the other. I have been induced to this from observing that the photographic preparation penetrates less deeply into the paper than when laid on as originally prescribed, and, con- sequently, the sensibility of it is increased. It will be found that an addition of about one-twelfth of spirits of wine to the solution of silver will much increase the blackness of the paper when solarised; and I think we may safely say that the sen- sibility is also improved by it,—at all events it is not impaired. M. Biot has expressed his opinion that it is not possible to find any substance more sensitive to light than the bromide of silverthis is true to a certain extent, but in combination with deoxidizing agents other preparations will be named which have a decided superiority over the pure bromide of silver. Fluoride of Silver.—The use of tins salt appears to have been first suggested by Sir John Herschel; it forms the basis of a process by the author, to be described in the chapter on Special Processes. It has lately been claimed as a new photographic agent by the French, but the date of publication determines this question in the author's favour. Phosphate of silver.—Dr. Fyfe appears to have been the first to suggest the use of the phosphate of silver as a photo- graphic material, but I am obliged to confess it has not, in my hands, proved anything like so successful as, from Dr. Fyfe's description, it was in . is own. Indeed, lie himself observes, in