v.] ANGSTROM'S INVESTIGATIONS. 53 



"5. That Fraunhofer 's double dark line, D, of solar and stellar 

 spectra, is due to the presence of vapour of sodium in atmospheres 

 surrounding the sun and those stars in whose spectra it has been 

 observed. 



"6. That other vapours than sodium are to be found in the at- 

 mosphere of sun and stars by searching for substances producing in 

 the spectra of artificial flames bright lines coinciding with other 

 dark lines of the solar and stellar spectra than the Fraunhofer 

 line D." 



Although Professor Stokes unfortunately did not publish his 

 theory I say unfortunately because valuable time has been 

 lost the world was not long in ignorance of a matter of such 

 general interest, for in 1853 the idea was published by the 

 celebrated Angstrom. 1 



In his memoir, illustrating the absorption of light, he made 

 use of a principle already propounded by v Euler (in his Theoria 

 lucis et caloris), that the particles of a body, in consequence of 

 resonance, absorb principally those ethereal undiilatory motions 

 which have previously been impressed upon them. He also 

 endeavoured to show that a body in a state of glowing heat emits 

 just the same kinds of light and heat which it absorbs under 

 the same circumstances. He further, following in this respect 

 the steps of Foucault, undertook a series of researches on the 

 electric arc just such researches as those Stokes had only 

 suggested and found that in many cases the Fraunhofer lines 

 were an inversion of bright lines which he observed in the 

 spectra of various metals. 2 Of these we shall have much to say 

 in the sequel. 



Early in 1858 Balfour Stewart independently discovered the 

 law which binds together radiation and absorption, establishing 

 it experimentally and also theoretically as an extension of 



1 " Optiska Undersokningar," Trans. Royal Academy of Stockholm, 1853. 

 Translated in Phil. Mag., Fourth Series, vol. ix. p. 327. 



2 See Phil. Mag., Fourth Series, vol. xxiv. pp. 2, 3: Monatsbericht, 1859, 

 p. 662. 



