A Manual Of Photography

SIR J. HERSCHEL’S CHRYSOTYPE. 57 analogous to those of the region of the spectrum above indicated.” Sir John Herschel then proceeds to show that whatever be the state of the iron in the double salts in question, its reduction by blue light to the state of protoxide is indicated by many other reagents. Thus, for example, if a slip of paper prepared with the ammonio-citrate of iron be exposed partially to sun- shine, and then washed with the bichromate of potash, the bichromate is deoxidised, and precipitated upon the sunned portion, just as it would be if directly exposed to the sun s rays. I have proved this fact with a great number of preparations of cobalt, nickel, bismuth, platinum, and other salts which have been thought hitherto to be insensible to solar agency ; but if they are partially sunned, and then washed with nitrate of silver, and put aside in the dark, the metallic silver is slowly reduced upon the sunned portion. In many instances days were required to produce the visible picture ; and in one case, paper, being washed with neutral chloride of platinum, was sunned, and then Washed in the dark with nitrate of silver : it was some weeks before the image made its appearance, but it was eventually per- fectly developed. This specimen has been kept for some years, and continues constantly to improve in clearness and definition. Section II.—Chrysotype. A process of an analogous character to that which lias just been described, and in which the chloride of gold is an agent, Must be next described: this was discovered at the same time as the cyanotype, and has been termed the chrysotype.. " In order to ascertain whether any portion of the iron in the double ammoniacal salt employed had really undergone deoxida- tion, and become reduced to the state of protoxide, as supposed, I had recourse to a solution of gold exactly neutralised by carbonate of soda. The proto-salts of iron, as is well known to chemists, precipitate gold in the metallic state. Hie effect proved exceedingly striking, issuing in a process nowise in- ferior in the almost magical beauty of its effect to the calotype process of Mr. Talbot, which in some respects it nearly resembles; with this advantage, as a matter of experimental exhibition, that the disclosure of the dormant image does not require to be per- formed in the dark, being not interfered with by moderate day- light. As the experiment will probably be repeated by others, I shall here describe it a6 initio. Paper is to be washed