SECT, xxiv.] PHOTOGEAPHY. 233 



occupied by the most refrangible rays and considerably be- 

 yond them, thus inducing a negative action in the most 

 refrangible part of the spectrum. 



In certain circumstances the red rays, instead of restoring 

 darkened photographic paper to its original whiteness, pro- 

 duce a deep red colour. When Sir John Herschel received 

 the spectrum on paper somewhat discoloured by exposure to 

 direct sunshine, instead of 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 foregoing 

 experiment. Sir John Herschel placed an engraving over a 

 paper blackened by exposure to sunshine, covering the whole 

 with a dark red-brown glass previously ascertained to ab- 

 sorb 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 colour of venous blood, and no other colour 

 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 assimi- 

 lating to their own colour the blackness already impressed 

 on photographic paper. 



That there is a deoxydating property in the more refran- 



