A Manual Of Photography

CHAPTER IX. ON THE PRODUCTION OF POSITIVE PHOTOGRAPHS BY THE USE OF THE HYDRIODIC SALTS. A VERY short time after the publication of Mr. Talbot’s processes, which I anxiously repeated with various modifications, I dis- covered a singular property in the hydriodate of potash of again whitening the pap« darkened by exposure and also,^that the bleaching process was very much accelerated by the influence o SBully in the year 1839, Lassaigne, Mr Talbot „Sir John Herschel, and Dr. Fyfe, appear to have fallen on the san å^this process, giving by one operation pictures with their lights correct, is of much interest, I gave it for a very con- Khle time my undivided attention. The most extraordinary Ihar^t« of the hydriodic salts is, that a very slight difference in the strength of the solutions, in the composition of the photo- graphie paper, or in the character of the incident light, produces totally opposite effects ; in one case the paper is rapidly whitened, in the other a deep blackness is produced almost as rapidly. Sometimes these opposing actions are in equilibrium and then the paper continues for a long time perfectly insensible I am inclined to hope these researches have reduced to cer- tainty their somewhat inconstant effects, and rendered this method of producing photographs one of the most easy, as it is the most beautiful. That the various positions I wish to establish may be completely understood, and to ensure the same results'in other hands, it will be necessary to enter into a somewhat detailed account of the various kinds of paper used and to give tolerably full directions for successfully using them either in the camera, or for drawings by application,- to examine attentively the effects of different organic and inorganic preparations on the paper, and to analyse the influence of the diUrent rays upon it. See also Part I. Chapter VI. Section VII. page 88. These particulars will be copied chiefly from my paper, On the Use of the Hydriodic Salts as Photographic Agents,