i;2 POPULAR LECTURES AND ADDRESSES. 



other ; so that the energy of the waves of those 

 particular qualities of light is converted into 

 thermal vibrations 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 Fraunhofcr'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 arc to be 

 found in the atmospheres of sun and stars by 

 scarching for substances producing in the spectra 

 of artificial flames bright lines coinciding with 

 other dark lines of the solar and stellar spectra 

 than the Fraunhofcr line D. 



The last of these propositions I felt to be con- 

 firmed (it was perhaps partly suggested) by a 

 striking and beautiful experiment admirably 

 adapted for lecture illustrations, due to Foucault, 

 which had been shown to me by M. Duboscque 

 Solcil, and the Abbe Moigno, in Paris in the 



