A Manual Of Photography

BROMIDE OF SILVER AND MERCURIAL VAPOUR. 85 u "We find it is better to add to the proto-sulphate of iron a little acetic or sulphuric acid: this will be found to prevent the darkening of the lights of tire picture to a great extent, and it will be found better not to prepare the paper long liefere it is required for use, this being one reason why the picture often becomes dusky on application of the proto-sulphate. “Reasoning upon the principle that the action of light is to reduce the salts of silver in the paper to the metallic state, and that any substance which would reduce silver would also quicken the action of light, we were led to the following experiment:— The protochloride of tin possesses the property of reducing the salts both of silver and of gold: a paper was prepared with the bromide of silver, and previously to exposing it to light it was washed over with a very weak solution of the chloride of tin; the action of light upon the paper was exceedingly energetic; it was almost instantaneously blackened, and a copy of a print was obtained in a few seconds.” The use of fluorides has been recently introduced as a novelty by some French photographers, but reference to te author’s Researches on Light, published in 1844, will dis- tinctly show that he was the first to employ these salts, which were, however, first suggested by Sir John Ilerschel. Section VI.— Bromide of Silver and Mercurial Vapour. In my first publication on this subject, in Griffin’s Scientific Miscellany, I introduced the following process, which, although it lias never yet been properly worked out, involves many points of interest:—Some extremely curious results, which are omitted from their not having any practical bearing, led me to examine the effect of the mercurial vapour on the pure precipitated iodides and bromides. I was long perplexed with some exceed- ingly anomalous results, but being satisfied from particular expe- riments that these researches promised to lead to the discovery of a most sensitive preparation, 1 persevered in them. Without stopping to trace the progress of the inquiry, 1 may at once state, that I have the satisfaction of being enabled to add to the present treatise an account of a process which serves to prepare papers that are much more sensitive than Daguerre's iodidated plates. The exquisite delicacy of these new photographie papers may be imagined, when I state that in five seconds in the camera obscura, I have, during sunshine, obtained perfect pictures ; and that, when the sky is overcast, one minute is quite sufficient to