xiii.] MODIFICATIONS SUGGESTED. 165 



and therefore the temperature is reduced, the longest lines, will be 

 the dimmest if they are produced by layers. 



The lines of A the hottest layer will be "brightest and 

 shortest. 



The lines of B the next cooler layerwill be less bright 

 and longer, and will also appear to rest on the spectrum of the 

 sun, as before explained, on account of the part of the layer 

 at B', although it is unrepresented at A, along the section X Y. 



And so on with c. 



Now, chiefly because the lines increased both in width and 

 number as we neared the photosphere, and also on account 

 of other considerations, Dr. Frankland and myself soon came 

 to the conclusion that in many particulars KirchhofFs theory 

 required modification. 



In 1869 we wrote as follows 1 : 



" We believe that the determination of the above-mentioned facts 

 leads us necessarily to several important modifications of the received 

 theory of the physical constitution of our central luminary, the 

 theory we owe to Kirchhoff, who based it upon his examination of 

 the solar spectrum. According to this hypothesis, the photosphere 

 itself is either solid or liquid, and it is surrounded by an atmo- 

 sphere composed of gases and the vapours of the substances incan- 

 descent in the photosphere. 



" We find, however, instead of this compound atmosphere, one 

 which gives us merely, or at all events mainly, the spectrum of 

 hydrogen (it is not, however, composed necessarily of hydrogen 

 alone, and this point is engaging our special attention) ; and the 

 tenuity of this incandescent atmosphere is such that it is extremely 

 improbable that any considerable atmosphere such as the corona 

 has been imagined to indicate, lies outside it ; a view strengthened 

 by the fact that the chromospheric bright lines present no appear- 

 ance of absorption, and that its physical conditions are not statical. 



" With regard to the photosphere itself, so far from being either 

 a solid surface or a liquid ocean, that it is cloudy or gaseous, or 



1 Proc. Roy. Soc. vol. xvii. p. 288. 



