264 PHYSICAL SCIENCE 



will have those constituent waves absorbed 

 which are tuned to the characteristic periods of 

 the absorbing systems. Substances, that is to 

 say, absorb the particular kinds of radiation 

 which they would themselves emit when hot. 



Applying these principles to the Fraunhofer 

 lines, Stokes held that when coincidences existed 

 between their positions and those of the bright 

 lines obtained by examining with a prism the 

 light of incandescent vapours, the coincidence 

 was to be interpreted by the supposition that 

 similar vapours were present in the atmosphere 

 of the sun, and absorbed the light coming from 

 the hotter regions below them. 



In i860 Bunsen and Kirchhoff, without know- 

 ing that Foucault had anticipated them in 1849, 

 devised and carried out an experiment on the 

 artificial production of Fraunhofer lines. They 

 passed the light of an electric arc, which gave 

 a perfectly continuous spectrum with no such 

 lines as those in the solar light, through the 

 vapour of sodium volatilised in the comparatively 

 cool region of a spirit lamp flame. They had the 

 joy of seeing a black absorption line, coincident 

 with the bright line given by hot sodium vapour, 

 crossing the continuous spectrum of the arc, just 

 as the black line, called by Fraunhofer the line D, 

 crosses the spectrum of the sun. The possibility 

 of determining the chemical constitution of the 

 heavenly bodies had opened before the eyes 

 of man. 



Hitherto the sun had been studied chiefly 

 in relation to the earth and the general solar 

 system, while little else was known about the 

 stars than their apparent relative positions on 



