A Manual Of Photography

102 HISTORY OF PHOTOGRAPHY. action of the hyposulphites on this compound of silver being comparatively slow and feeble. “ When the glass is coated with bromide of silver, the action, per se, is very slow, and the discoloration ultimately produced far short of blackness ; but when moistened with nitrate of silver, sp. gr. 11, it is still more rapid than with the iodide, turning quite black in the course of a very few seconds' exposure to sunshine. Plates of glass thus coated may be easily preserved for the use of the camera, and have tire advantage of being ready at a moment's notice, requiring nothing but a wash over with the nitrate of silver, which may be delayed until the image is actually thrown on the plate, and adjusted to the correct focus with all deliberation. The sensitive wash being then applied with a soft flat camel-hair brush, the box may be closed and the picture impressed, after which it only requires to be thrown into water, and dried in the dark, to be rendered comparatively in- sensible, and may be finally fixed with hyposulphite of soda, which must be applied hot, its solvent power on the bromide being even less than on the iodide." Sir .lohn Herschel suggested a trial of the fluoride of silver upon glass, which, he says, if proved to be decomposable by light, might possibly effect an etching on the glass, by the cor- roding property of the hydrofluoric acid. The metallic fluorides have been found to be decomposable, and a very sensitive process on paper, called the fluorotype, will be described in the chapter on Miscellaneous Processes. I am not aware that any experiments have been made directly upon glass, but it is certainly worthy of a careful trial. Herschel has remarked that we cannot allow the wash of nitrate to dry upon the coating of the chloride or iodide of silver. If, however, we dip a glass which has one film of chloride upon it into a solution of common salt, and then spread upon it some nitrate of silver, we may very materially thicken the coating, and thus produce more intense effects. Mr. Towson employed glass plates prepared in this manner with much success. The mode adopted by that gentleman was to have a box the exact size of the glass plate, in the bottom of which was a small hole; the glass was placed over the bottom, and the mixed solution, just strong enough to be milky, of the salt and silver poured in. As the fluid finds its way slowly around the edges of the glass, it filters out ; the peculiar surface action of the solid glass plate, probably a modified form of cohesive force, separating the fine precipitate, which is left behind on the surface of the plate. By this means the operation of coating the glass is much quickened. Another method by which films of any of the salts of silver can