No. 2.] BRITISH ASSOCUflO.^ MEEfiNCf. 131 



The conclusions, theoretical and practical, which Stokes taught 

 me, and which I gave regularly afterwards in my public lectures 

 in the University of Glasgow, were : — 



(1) That the double line D, whether bright or dark, is due to 

 vapour of sodium. 



(2) That the ultimate atom of sodium is susceptible of regular 

 elastic vibrations, like those of a tuning-fork or of stringed musical 

 instruments; that like an instrument with two strings tuned to 

 approximate unison, or an approximately circular elastic disc, it 

 has two fundamental notes or vibrations of approximately equal 

 pitch ; and that the periods of these vibrations are precisely the 

 periods of the two slightly different yellow lights constituting the 

 double bridit line D. 



(3) That when vapour of sodium is at a high enough tempera- 

 ture to become itself a source of light, each atom executes these 

 two fundamental vibrations simultaneously; and that therefore 

 the light proceeding from it is of the two qualities constituting 

 the double brio-ht line D. 



(4) That when vapour of sodium is present in space across 

 which light from another source is propagated, its atoms, accord- 

 ing to a well-known general principle of dynamics, are set to 

 vibrate in either or both of those fundamental modes, if some of 

 the incident light is of one or other of their periods, or some of 

 one and some of the other ; so that the energy of the waves of 

 those particular qualities of light is converted into thermal vibra- 

 tions of the medium, and dispersed in all directions, while light 

 of all other qualities, even though very nearly agreeing with them, 

 is transmitted with comparatively no loss. 



(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 had been 

 observed. 



(6) That other vapours than sodium are to be found in the 

 atmospheres of sun and stars by searching for substances produc- 

 ing in the spectra of artificial flames bright lines coinciding with 

 other dark lines of the solar and stellar spectra than the Fraun- 

 hofer line D. 



The last of these propositions I felt to be confirmed (it was, 

 perhaps, partly suggested) by a striking and beautiful experiment, 

 admirably adapted for lecture illustrations, due to Foucault, 



