S.CT. XXIV. REFRANGIBILITY. 203 



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 refrangible part of the spectrum over the more re- 

 frangible portion by means of two prisms, and he thus 

 discovered that two rays of different refrangibility, and 

 therefore of different lengths of undulation, acting simul- 

 taneously, produce an effect which neither acting sepa- 

 rately 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, commenced at or very little below the 

 mean yellow ray, of a delicate lead color, and when tha 

 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 remarka- 

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

 image, so far at least as to leave a border of the lead- 

 colored spectrum traceable, not only round the clear 

 and well-defined convexity of the dark interior spectrum 

 at the least 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 striking contrast 

 of color with the interior spectrum, the former being 

 lead gray, the latter an extremely rich deep velvety 

 brown. The less refrangible end of this interior brown 

 spectrum presented a sharply terminated and regularly 

 elliptical contour, the more refrangible a less decided 

 one. " It may seem too hazardous," Sir John continues, 

 " $o look for the cause of this very singular phenomenon 

 in a real difference between the chemical agencies of 

 those rays which issue from the central portion of the 

 sun's disc, and those which, emanating from its borders, 

 have undergone the absorptive action of a much greater 

 depth of its atmosphere ; and yet I confess myself some- 

 what at a loss what other cause to assign for it. It 

 must suffice, however, to have thrown out the hint, re- 



