i 9 4 THE POPULAR SCIENCE MONTHLY. 



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 characterizing 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 instance, with the line or rather lines D 

 in the sun's spectrum. The conclusion was obvious. There was sodi- 

 um 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 labor. 



But why is the bright line of a sodium-flame represented by a black 

 one in the spectrum of the sun ! To Angstrom is due the theory that a 

 vapor or gas can absorb luminous rays of the same refrangibility only 

 which it emits when highly heated ; while Balfour Stewart independ- 

 ently discovered the same law with reference to radiant heat. 



This is the basis of Kirchhoff's theory of the origin of Fraunhofer's 

 lines. In the atmosphere of the sun the vapors of various metals are 

 present, each of which would give its characteristic lines, but within 

 this atmospheric envelope is the still more intensely heated nucleus of 

 the sim, which emits a brilliant continuous spectrum, containing rays 

 of all degrees of refrangibility. When the light of this intensely- 

 heated nucleus is transmitted through the surrounding atmosphere, 

 the bright lines which would be produced by this atmosphere are seen 

 as dark ones. 



Kirchhoff and Bunsen thus proved the existence in the sun of hy- 

 drogen, sodium, magnesium, calcium, iron, nickel, chromium, manga- 

 nese, titanium, and cobalt; since which Angstrom, Thalen, and Lockyer 

 have considerably increased the list. But it is not merely the chem- 

 istry of the heavenly bodies on which light is thrown by the spec- 

 troscope ; their physical structure and evolutional history are also 

 illuminated by this wonderful instrument of research. It used to be 

 supposed that the sun was a dark body enveloped in a luminous atmos- 

 phere. The reverse now appears to be the truth. The body of the 

 sun, or photosphere, is intensely brilliant ; round it lies the solar 

 atmosphere of comparatively cool gases, which cause the dark lines in 

 the spectrum ; thirdly, a chromosphere a sphere principally of hydro- 

 gen, jets of which are said sometimes to reach to a height of 100,000 

 miles or more, into the outer coating or corona, the nature of which is 

 still very doubtful. 



Formerly the red flames which represent the higher regions of the 

 chromosphere coidd be seen only on the rare occasions of a total solar 

 eclipse. Janssen and Lockyer, by the application of the spectroscope, 

 have enabled us to study this region of the sun at all times. It is, 



