276 On the Sensibility to Light of Bromide of Silver. 



in the indigo, from there their sensibility decreased till the light 

 blue, became weak at F, then increased again, and in the yellow 

 was found to be almost as efficient as in the indigo. Thus a me- 

 thod was attained for preparing bromide- of-silver plates that are 

 acted upon almost as strongly by a colour hitherto held to be 

 chemically inert, namely yellow, as they are by indigo, which 

 hitherto has been held to be the colour possessing the strongest 

 chemical energy. 



After this experiment I was justified in hoping that some 

 other bromide-fixing substance endowed with a powerful absorp- 

 tion of the red would in like manner heighten the sensibility of 

 bromide of silver for red. Such a body I met with among the 

 green aniline products. The body in question absorbed in a 

 marked manner the red rays in the middle between D and C ; 

 when further concentrated this absorption became extended 

 towards D, while the yellow, green, and blue were transmitted 

 almost unimpaired. In point of fact a collodion coloured with 

 this green proved sensitive to light as far as into the red. 



The sensibility fell off from the indigo to the yellow, then in- 

 creased again ; and at the identical place where the above-men- 

 tioned absorption-band was visible a powerful action in the red 

 was manifested. 



From these experiments I think I am pretty well justified in 

 inferring that we are in a position to render bromide of silver 

 sensitive for any colour we choose — that is to say, to heighten for 

 particular colours the sensibility it was originally endowed with. 

 To effect this, all that is required is to add thereto some sub- 

 stance which promotes the chemical decomposition of the bro- 

 mide of silver and absorbs the particular colour in question but 

 not the others. Perhaps we may even arrive at this, namely 

 photographing the ultra-red as we have already photographed 

 the ultra-violet. The photographic inefficiency of certain colours, 

 which has hitherto proved so great an obstacle, would in that 

 case be surmounted. The following experiment shows to what 

 extent this is practically substantiated. A photograph was taken 

 of a band of blue upon a yellow ground. Employing an ordi- 

 nary collodion plate with iodide of silver, I obtained thereupon a 

 white band upon a black ground. On a bromide-of-silver and 

 collodion plate, where the action of the blue and the yellow is 

 equally powerful, it was to be anticipated that no effect was to 

 be obtained. I therefore placed in front of the object-glass a 

 disk of yellow glass, which absorbed the blue light and trans- 

 mitted the yellow unimpaired ; and then, after an exposition of 

 suitable duration, I obtained in point of fact a dark band on a 

 light ground. 



The matter is not alone of practical but also of scientific in- 



