SECT. XXIV. EFFECT OF THE RED BAYS. 211 



perceptible about the region of the orange and yellow. After 

 this a bleaching action begins over the part occupied by the red 

 rays, which extends to the green. By longer exposure an oval 

 spot begins again to darken about the centre of the bleached 

 space ; but, if the paper receive another wash of the hyclriodate 

 of potash, the bleaching action extends up from the green, over 

 the region occupied by the most refrangible rays and considerably 

 beyond 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, produce 

 a deep red colour. When Sir John Herschel received the spec- 

 trum on paper somewhat discoloured by exposure to direct 

 sunshine, instead of whiteness, a red border was formed extend- 

 ing 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 pro- 

 duced 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 experi- 

 ment. 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 absorb every ray 

 beyond the orange : in this way a photographic copy was ob- 

 tained 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 remark- 

 able property of assimilating to their own colour the blackness 

 already impressed on photographic paper. 



That there is a deoxydating property in the more refrangible 



