A Manual Of Photography

78 HISTORY OF PHOTOGRAPHY. but it must then be kept disturbed, by rapidly but lightly brush- ing it up, otherwise numerous black specks will form and destroy the photograph. Great care should be taken that the iron solution does not touch the back of the picture, which it will inevitably stain, and, the picture being a negative one, be rendered useless as a copy. A slight degree of heat will assist the development of the image where the time of exposure has been too short. The picture should be carefully washed to take off any super- ficial blackness, and may then be permanently fixed by being soaked in water to which a small quantity of ammonia, or, better still, hyposulphite of soda, lias been added. The paper must again be well soaked in clean water, to clear it from the soluble salts, and may then be dried and pressed. Exact copies of prints, feathers, leaves, Ac., may be taken on the succinated paper by exposing them to the light in the copying-frame, until the margin of the prepared paper, which should be left uncovered, begins to change colour very slightly. If the object to be copied is thick, the surface must be allowed to assume a darker tint, or the light will not have penetrated to the paper. Positive copies of the camera negatives are procured in the same manner as the copies of the prints, Ac., just described. Instead, however, of using the iron solution, the paper must be exposed to the light, in the frame, a sufficient time to obtain perfect copies. The progress of the picture may be observed by turning up the corner of the paper, and, if not sufficiently done, replacing it exactly in the same position. They should be fixed with hyposulphite, as before directed. At the meeting of the British Association at York in 1844, I showed, by a series of photographs, that the protosulphate of iron was most effective in developing any photographic images, on whatever argentiferous preparation they may have been re- ceived. Every subsequent result has shown that with proper care it is the most energetic agent for developing witli which we are acquainted. The difficulty of obtaining, and of preserving, the salt free of any peroxide, or a basic salt which falls as a brownish-yellow powder, has been the principal cause why it has not been so generally employed as the gallic acid: this can bo insured by adding a few drops of sulphuric acid to the solution of the protosulphate of iron, and some iron filings. Mr. Robert Ellis has recommended the use of the protonitrate of iron as a developing agent.