A Manual Of Photography

TO GIVE THE SENSITIVE SURFACE TO THE PLATE. 243 fine straw yellow it is removed to the action of the accelerating agents, liquid or otherwise, as the case may he. The following rules will guide the experimenter in using the different liquids. If bromide of iodine he used as the accelerating agent, the plate should remain over the iodine until it is of a pure yellow tint, and over the bromine till of a deep rose colour. By observing the time of exposure necessary to render a plate sensitive, any number of plates may be prepared exactly alike, provided that the same quantity of the solution, always of an uniform strength, be put into the pan. By using a much weaker solution a longer exposure is then necessary, but the plate becomes more evenly covered, and there is less danger of having too much or too little of the accelerator upon it. The same remark may apply to other accelerating solutions. If Bedman's solution, or the Hungarian liquid, a pale yellow and light rose will be found most sensitive. As a general rule, if the yellow colour produced by the iodine be pale, the red should be pale also ; if deep, the red must incline to violet. When several plates are to be pre- pared at one time, the same solution will serve for all; but it seldom answers to preserve the mixture for any time ; and its use, after keeping, is one great cause of the failures which so annoy amateurs. The bromine contained in these solutions is very subtile, and escapes, leaving little else but iodine remaining, which will, after some little time, give a red colour to the plate, without rendering it sensitive, entirely disappointing the expec- tations of the operator. Eau bromée, or bromine water, which is very easily prepared, is extensively used on the Continent, and is simple in its use. If a certain quantity of an uniform solution be placed in the pan, for each plate prepared one observation will suffice to determine the time of exposure ; if not, the colour must guide the operator, varying according to the degree of colour obtained over the iodine : thus, if the first colour obtained be a light yellow, the plate should attain a full golden tint over tlie iodine, and may then be retained over the bromine until it acquires a rose colour. If iodized of a golden yellow, then, in the second operation, it is taken to a pale rose, and in the third to a deep rose. If in the first of a full red, in the second to a deep red, and lastly to a grey ; if the first to a deep red, in the second to a light blue, and in the third, to a white, or nearly the absence of all colour. Experience, however, must invariably guide the operator, as scarcely any two solutions, though professedly the same in character, possess the same properties. In a pamphlet published by M. Fizeau, bromine-water is recommended to be prepared as follows :—“ To prepare a solution