THE CONSTITUTION OF THE SUN, 49 



of Fraunhofer's lines. In the atmosphere of the sun 

 the vapours of various metals are present, each of which 

 would give its characteristic lines, but within this at- 

 mospheric envelope is the still more intensely heated 

 nucleus of the sun, 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 atmo- 

 sphere are seen as dark ones. 



Kirchhoff and Bunsen thus proved the existence in 

 the sun of hydrogen, sodium, magnesium, calcium, iron, 

 nickel, chromium, manganese, titanium, and cobalt ; 

 since which Angstrom, Thalen, and Lockyer have con- 

 siderably increased the list. 



But it is not merely the chemistry of the heavenly 

 bodies on which light is thrown by the spectroscope ; 

 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 atmosphere. 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, the 

 chromosphere, a sphere principally of hydrogen, 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 could be seen only on the 

 rare occasions of a total solar eclipse. Janssen and 



E 



