16 Prof. Kirchhoff on the Relation between the Radiating and 



moreover, in the case of salts which produce flames having such 

 maxima, it is the metal that determines the nature of the spectrum. 

 Imagine a body of very high temperature, in whose spectrum the 

 double line D does not appear, surrounded by a gaseous atmo- 

 sphere of somewhat lower temperature. If sodium be present in 

 the latter, the spectrum of the whole system so constituted will 

 contain the double line D . From the occurrence of these lines, the 

 presence of sodium in the atmosphere may therefore be concluded. 

 Now the sun is undoubtedly a body of this description*; and 

 therefore, from the occurrence of the lines D in the solar spectrum, 

 the presence of sodium in the sun's atmosphere may be concluded. 

 An objection may perhaps be urged against the justice of this 

 conclusion. " The cause of the line D is," it may be said, " to 

 be sought for in the atmosphere of the earth." This objection 

 may, however, be disposed of on the following grounds : — 



(1) The necessary quantity of sodium in the gaseous form 

 can hardly be present in our atmosphere, and the gaseous form 

 is necessary to produce the effect in question. 



(2) If the line D depended on our atmosphere, it would 

 become more strongly marked when the sun approached the 

 horizon. I have, however, never observed any such change in 

 the distinctness of these lines, though in the case of some of the 

 neighbouring lines, such changes are very conspicuous. 



(3) If the line D were not caused by the physical constitu- 

 tion of the sun itself, it would exist in the spectra of all the 

 fixed stars of sufficient brightness ; but according to Fraunhofer 

 and Brewster, it is wanting in the spectra of some of the fixed 

 stars though present in others. 



The precise coincidence of the sodium lines with the D lines of 

 Fraunhofer may be most satisfactorily proved by suffering the 

 sun's rays to fall on the slit of the apparatus through a sodium- 

 flame. The effect of the flame is exhibited in the increased 

 distinctness, darkness, and breadth of the lines D. At the first 

 glance it may appear somewhat strange that the sodium in a 

 small flame should perceptibly increase the effect of the sodium 

 present in the immense mass of the sun's atmosphere. Our 

 surprise at this will, however, be diminished when we consider 

 that the brightness of the lines D of the solar spectrum is deter- 

 mined by the temperature of the solar atmosphere, and especially 

 of its outer portion, and that the temperature of this is certainly 

 much greater than that of a gas-lamp. If a sodium-flame be 

 imagined whose thickness may be regarded as infinite with 

 respect to its power of absorbing the rays that correspond to the 



* Whether the central mass of the sun, from which the light principally 

 proceeds, is solid, liquid, or gaseous, may, as far as we are here concerned, 

 be regarded as an open question. 



