202 PHOTOGRAPHY. SECT. XXIV. 



whiteness, a red border was formed extending from the 

 space occupied by the orange, and nearly covering that 

 on which the red fell. When, instead of exposing the 

 paper in the first instance to direct sunshine, it was 

 blackened by the violet rays of a prismatic spectrum, or 

 by a sunbeam that had undergone the absorptive action 

 of a solution of ammonia-sulphate of copper, the red 

 rays of the condensed spectrum produced on it, not 

 whiteness, but a full and fiery red which occupied the 

 whole space on which any of the visible red rays had 

 fallen, and this red remained unchanged, however long 

 the paper remained exposed to the least refrangible rays. 



Sunlight transmitted through red glass produces the 

 same effect as the red rays of the spectrum in the fore- 

 going experiment. Sir John Herschel placed an en- 

 graving over a paper blackened by exposure to sunshine, 

 covering the whole with a dark red-brown glass previ- 

 ously ascertained to absorb every ray beyond the orange : 

 in this way a photographic copy was obtained in which 

 the shades were black, as in the original engraving, but 

 the lights, instead of being white, were of the red color 

 of venous blood, and no other color could be obtained by 

 exposure to light, however long. Sir John ascertained 

 that every part of the spectrum impressed by the more 

 refrangible rays is equally reddened, or nearly so, by the 

 subsequent action of the less refrangible ; thus the red 

 rays have the very remarkable property of assimilating 

 to their own color the blackness already impressed on 

 photographic paper. 



That there is a deoxy dating property in the more re- 

 frangible rays, and an oxydating action in the less re- 

 frangible part of the spectrum, is manifest from the 

 blackening of one and the bleaching effect of the other ; 

 but the peculiar action of the red rays in the experi- 

 ments mentioned, shows that some other principle exists 

 different from contrariety of action. These opposite 

 qualities are balanced or neutralized in the region of the 

 mean yellow ray. But although this is the general 

 character of the photographic spectrum, under certain 

 circumstances even the red rays have a deoxydating 

 power, while the blue and scarlet exert a contrary influ- 

 ence ; but these are rare exceptions. - 



