48 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



the coincidence between the double dark line D of the 

 solar spectrum and a double line which he observed in 

 the spectra of ordinary flames, while Stokes pointed out 

 to Sir W. Thomson, who taught it in his lectures, that 

 in both cases these lines were due to the presence of 

 sodium. To KirchhofF and Bunsen, however, is due the 

 independent conception and the credit of having first 

 systematically investigated the relation which exists 

 between Fraunhofer's lines and the bright lines in the 

 spectra of incandescent metals. In order to get some 

 fixed measure by which they might determine and 

 record the lines characterising any given substance, it 

 occurred to them that they might use for comparison 

 the spectrum of the sun. They accordingly arranged 

 their spectroscope so that one-half of the slit was lighted 

 by the sun, and the other by the luminous gases they 

 proposed to examine. It immediately struck them that 

 the bright lines in the one corresponded with the dark 

 lines in the other the bright line of sodium, for in- 

 stance, with the line or rather lines D-in the sun's 

 spectrum. The conclusion was obvious. There was 

 sodium in the sun. It must indeed have been a 

 glorious moment when that thought flashed across 

 them, and even by itself well worth all their labour. 



But why is the bright line of a sodium flame repre- 

 sented by a black one in the spectrum of the sun ? To 

 Angstrom is due the theory that a vapour or gas can 

 absorb luminous rays of the same refrangibility only 

 as those which it emits when highly heated ; while 

 Balfour Stewart independently discovered the same law 

 with reference to radiant heat. 



This is the basis of KirchhofF' s theory of the origin 



