A Manual Of Photography

MISCELLANEOUS SALTS OF SILVER. 133 speaking of its use in the camera obscura:—“ Though repre- sentations may be got in this way, yet, so far as I have found, they have not the minute distinctness of those got by the method already mentioned (t c. by application). Owing to the inter- ference of the lens, the light does not act nearly so powerfully on the paper, as when it has to permeate merely a frame of glass. Tor all practical purposes, the method which Dr. Fyfe has given of preparing these papers is, perhaps, the best:—" The paper is first soaked in the phosphate of soda, and then dried, after which the nitrate is spread over one side by a brush ; the paper again dried, and afterwards again put through the salt by which any excess of silver is converted to phosphate. As thus prepared, it acquires a yellow tinge, which becomes black by exposure to light." It will be evident from these directions, that what was formerly said about the necessity of having the nitrate of silver in excess, is here, according to Dr. Fyfe, objectionable. It certainly does not appear to be so essential in this preparation, that anything but pure phosphate of silver should be used ; yet I cannot help fancying that a slight advan- tage is gained, even here, by allowing à little excess of nitrate. Dr. Fyfe has given a process for applying the phosphate of silver, mixed as a paint, on metal, glass, or paper. It, however lequires the skill of an artist to produce an even surface, and unless a uniform ground is given, the picture is deformed by waving lines of different shades. A method of precipitating argentine salts on smooth surfaces will be given in the following pages, by which means the most uniform face is procured and many beautiful effects produced. Papers prepared with other Salts of Silver.-With the ex- ception of the carbonate, tartrate, acetate, citrate, oxalate, and one or two others, the salts of silver, besides those already de- scribed, do not appear to be sensibly influenced by light. Many have been mentioned by authors as absolutely insensible to its influence ; but recent experiments have produced modifications of these salts, which are delicately sensitive to the solar ray. Amongst others, the chromate has been named, and certainly it has not yet been rendered sensitive to an exposure of some hours to daylight; but one experiment of mine has proved that the solar beam will, in a few days, produce a fine revival of metallic silver from its chromate; and another experiment with it has the most pleasing result of bringing within the ran-m of probabilities the production of photographic pictures in their natural colours. Lesearches having this object in view led to the discovery of