144 ANNUAL OF SCIENTIFIC DISCOVERY. 



the impression. The picture is then protected by a film of varnish-gum, or 

 albumen, to preserve it. 



This is the process of M. Salmon, of Chartrcs. 



Me Craw's new, cheap, and permanent process in Photography. The following 

 account of a HCAV process for producing cheap and permanent photographs 

 without employing either silver or gold salts, or the noxious hyposulphite 

 of soda, was presented to the British Association, 1858, by Mr. W. McCraw: 



" The labors of the committee appointed by the Photographic Society of 

 London to inquire into the cause of the fading of photographs, after a 

 elapse of two years, have only amounted to this : that photographs of a cer- 

 tain kind have all faded; and that some of those of the kind that have stood 

 best, have unaccountably faded the sad presumption being, that in time all 

 photographs produced in the usual way, by the means of chloride of silver, 

 and fixed (as it is called) by hyposulphite of soda, will perish. These con- 

 siderations, and the fact of a prize being offered by a French nobleman for 

 the discovery of a process for printing photographs in carbon, set me to ex- 

 periment in that direction. But my experiments with carbon and various 

 pigments led me to think that no material applied mechanically, or that 

 could not be made to take the shape of a dye or chemical solution, would 

 ever give results with the exquisite half-tints of the present beautiful but 

 perishable process. The photographic properties of bichromate of potass 

 were pointed out by Mungo Poutou twenty years ago, giving photographs of 

 a pale, tawny color. A piece of paper is washed over with the saturated solu- 

 tion of the bichromate, and when dried in the dark is of a light yellow color, 

 and very sensitive to light. If a negative photograph, or a piece of lace or 

 a leaf, be placed over the prepared paper, and put in sunshine, in a few min- 

 utes a perfect impression of the object is obtained. The light darkens the 

 color of the bichromate, and renders it insoluble in water, while the yellow 

 color washes out from the parts protected from the light by the lace or leaf, 

 or negative photograph, as the case may be. But pictures of this kind have 

 little or no practical value; for although the lights are good enough, the deep 

 black shadows are only represented by a tawny shade. Some eighteen 

 months ago a process was patented for deepening these photographs by 

 treating them with gallic acid and a salt of iron, which went by the name of 

 * Sella's process.' I tried this process at the time according to the specifi- 

 cation of the patent, but failed to make one satisfactory specimen. They 

 wanted everything that a good photograph should have, pure lights, clear 

 half-tints, and deep shadoAvs, and as I found that others had not been more 

 successful, I abandoned my experiments. But in the course of further exper- 

 iments, a year afterwards, with carbon, I was struck with the fact, that a 

 drop of a solution of bichromate of potass allowed to fall on a piece of white 

 paper and afterwards dried and exposed to the sun, when washed with a solu- 

 tion of proto-sulphate of iron, and then with gallic acid, while the spot 

 became perfectly black, the surrounding white paper was unaffected by the 

 liquids. Knowing the photographic properties of the bichromate already 

 described, I believed that this might be the foundation of a good photo- 

 graphic process; and that if the bichromate could be kept from penetrating 

 the pores of the paper, by being kept on its surface, the defects of Sella's 

 process might be avoided. With this view, I began by filling the pores of 

 the paper with albumen, and then, to render it insoluble, immersing the paper 

 in ether. This, however, did not answer. But as it would be tedious to 



