as Photographic Agents. 205 



By attention to the following directions, simple in their 

 character, but arrived at by a long series of inquiries, any one 

 may prepare photographic papers, on which the hydriodic 

 solutions shall act with perfect uniformity. 



10. Soak the paper for a few minutes in a muriated wash, 

 removing with a soft brush any air-bubbles which may form 

 on it. The superfluous moisture must be wiped off with very 

 clean cotton cloths, and the papers dried at common tem- 

 peratures. When dry, the paper must be pinned out on a 

 board, and the silver solution spread over it boldly but lightly, 

 with a very soft sponge-brush. It is to be instantly exposed 

 to sunshine, and, if practicable, carried into the open air; as 

 the more speedily evaporation proceeds, the less does the silver 

 penetrate the paper, and the more delicate it is. The first 

 surface is very irregular, being made up of blue streaks, which 

 are parts on which a true chloride is formed; and of brown 

 ones, which appear to be the chloride of silver combined 

 with a portion of undecomposed nitrate. As soon as the sur- 

 face appears dry the silver solution must be again applied as 

 before, and the exposure repeated. It must now be exposed 

 until a fine chocolate-brown colour is produced equally on 

 all parts of the surface, and then, until required for use, care- 

 fully preserved from the further influence of light. 



11. In darkening these papers, the greatest possible atten- 

 tion must be paid to the quantity of light to which they are 

 submitted, everything depending on the rapidity of the black- 

 ening process. The morning sun should be chosen, it being 

 very evident that sonje portion of the violet rays are absorbed 

 by the atmosphere after the sun has passed the meridian, 

 which permeated it freely before he had arrived at that point. 



A perfectly cloudless sky is of great advantage. The in- 

 jurious consequence of a cloud obscuring the sun during the 

 last darkening process, is the formation of a surface which 

 has the appearance of being washed with a dirty brush. This 

 is with difficulty removed by the hydriodates, and the re- 

 sulting pictures want that clearness which constitutes their 

 beauty. Papers darkened by the diffused light of a cloudy 

 day are scarcely, if at all acted on by these salts. 



12. The kind of paper on which the silver is spread, is an 

 object of much importance. A paper known to the trade as 

 satin-post, double-glazed, bearing the mark of J. Whatman, 

 Turkey Mill, is decidedly superior to every other kind I have 

 tried. 



The demy printing papers are many of them bleached by 

 chlorine, after an artificial substance has been given them by 

 lime. These reverse the photographic process, and the parts 



