234 EEFRANGIBILITY. [SECT. xxiv. 



gible rays, and an oxy dating action in the less refrangible 

 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 experiments 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 influence ; but 

 these are rare exceptions. 



The photographic action of the two portions of the solar 

 spectrum being so different, Sir John Herschel tried the 

 effect of their united action by superposing the less refran- 

 gible part of the spectrum over the more refrangible portion 

 by means of two prisms ; and he thus discovered that two 

 rays of different refrangibility, and therefore of different 

 lengths of undulation, acting simultaneously, produce an 

 effect which neither, acting separately, can do. 



Some circumstances that occurred during the analysis of 

 the chemical spectrum seem to indicate an absorptive action 

 in the sun's atmosphere. The spectral image impressed on 

 paper prepared with nitrate of silver and Rochelle salt com- 

 menced at, or very little below, the mean yellow ray, of a 

 delicate lead colour ; and when the action was arrested such 

 was the character of the whole photographic spectrum. But, 

 when the light of the solar spectrum was allowed to continue 

 its action, there was observed to come on suddenly a new and 

 much more intense impression of darkness, confined in length 

 to the blue and violet rays ; and, what is most remarkable, 

 confined also in breadth to the middle of the sun's image, so 

 far at least as to leave a border of the lead-coloured spectrum 

 traceable, not only round the clear and well-defined convexity 

 of the dark interior spectrum at the less refrangible end, but 

 also laterally along both its edges; and this border was the 

 more easily traced, and less liable to be mistaken, from its 



