A Manual Of Photography

CHAPTER VI. PROCESSES BY THE AUTHOR AND OTHERS. Section I.—Mr. Ponton’s Process. (Bichromate of Potash.) Under the general term of the Chromatype, I would propose to include all those processes which involve the use of any of the salts of chromium. It was originally introduced to distinguish a particular process which I discovered, and published at the meeting of the British Association at Cork, in August 1843; but it appears very convenient to adopt the principle intro- duced by Sir John Herschel, of grouping the phenomena of photography under a general heading, derived from the most prominent chemical preparation employed. There are many preparations which are affected by light in a similar manner to the salts of silver. Several have been tried as photographic materials, but as yet without much success, with the exception of the bichromate of potash, which was first an- nounced as a useful photographic agent by Mr. Mungo Ponton, in the Edinburgh New Philosophical Journal; from which I quote Mr. Ponton's own account. "When paper is immersed in the bichromate of potash, it is powerfully and rapidly acted on by the sun’s rays. When an object is laid in the usual way on this paper, the portion exposed to the light speedily becomes tawny, passing more or less into a deep orange, according to the strength of the light. The por- tion covered by the object retains the original bright yellow tint which it had before exposure, and the object is thus represented yellow upon an orange ground, there being several gradations of shade, or tint, according to the greater or less degree of trans- parency in the different parts of the object. “ In this state, of course, the drawing, though very beautiful, is evanescent. To fix it, all that is required is careful immersion in water, when it will be found that those portions ot the salt which have not been acted on by the light are readily dissolved out, while those which have been exposed to the light are completely fixed on the paper. By the second process the object is obtained