CHAP. VIL] PHOTOGRAPHY ON PAPER AND ON GLASS. 303 



collodion photography have been varied in a hundred ways : in 

 describing what is required in one of these we shall have elucidated 

 all the others. But we must repeat that here, as in the daguerreotype, 

 as in photography on paper and on albuminized glass, we omit all 

 details of the manipulation, although they are of the highest im- 

 portance, for they are frequently indispensable conditions of success. 

 As it is not our intention to make this even an abridged manual 

 of photography, but to make the physical principles of this widely- 

 practised art clear, we merely give the formula of normal collodion as 

 prepared before the addition of those substances which contribute to 

 the production of the sensitized coating, and which is as follows : 



Kectified sulphuric ether . . .- . 600 

 Pyroxyline . . . ' . . . . 12 



Alcohol at 40 , ' . " . . .. . 300 



The iodized liquid is an alcoholic solution of the iodides of potas- 

 sium, cadmium, and ammonium, and of the bromides of the same 

 metals. To this is added a fragment of iodine. The liquid formed of 

 the mixture of these two solutions is, like the albumen, allowed to- 

 coat a well-cleaned glass. Just before the coating is dry, the glass is 

 plunged in a bath of nitrate of silver. The formation of iodide and 

 bromide of silver which ensues produces a whitish, opaque film, which 

 is sensitive to light ; hence this operation is always performed in a 

 room which is glazed with yellow or red glass, the blue and violet 

 rays being those which are generally photographically active. The 

 glass is then placed in the slide of the camera, and it can now be 

 operated on that is, exposed to the action of the light. In a few 

 seconds, the impression is produced, and it only remains to submit 

 the proof to the operations of developing and fixing the image. The 

 former is accomplished by an acid solution of protosulphate of iron 

 or pyrogallic acid, and the latter with hyposulphite of soda or cyanide 

 of potassium. 



If a collodionized plate prepared as above be washed and be then 

 coated with albumen, it may be dried and exposed in the camera even 

 months after preparation. This is Taupe not's process with dry collodion. 



Having obtained the negative proof, we proceed, as before de- 

 scribed, for the positive proofs. 



