XCvi KEPORT 1871. 



of approximately equal pitch ; and that the periods of these vibrations are 

 precisely the periods of the t'wo slightly different yellow lights constituting 

 the double bright line D. 



(3) That when vapour of sodium is at a high enough temperature to be- 

 come 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 qunlitics constituting the double bright line D. 



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

 from another source is propagated, its atoms, according to a weU-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 Fraunhofers double dark line D of solar and stellar spectra is due 

 to the presence of vapour of sodium in atmospheres surroiinding the sun 

 and those stars in whose spectra it had been observed. 



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

 of sun and stars by searcliing 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. 



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

 partlj' suggested) by a striking and beautiful experiment admirably adapted 

 for lecture illustrations, due to Foucault, which had been shown to me by 

 M. Duboscque Soleil, and the Abbe Moigno, in Paris in the month of 

 October 1850. A prism and lenses were arranged to throw upon a screen 

 an approximately pure spectrum of a vertical electric are between charcoal 

 poles of a powerful battery, the lower one of which was hollowed like a cup. 

 When pieces of copper and pieces of zinc were separately thrown into the 

 cup, the spectrum exhibited, in perfectly definite positions, magnificent well- 

 marked bauds of difiercnt colours characteristic of the two metals. AVlien 

 a piece of brass, compounded of copper and zinc, was put into the cup, 

 the spectrum showed all the bands, each precisely in the place in which 

 it had been seen when one metal or the other had been used separately. 



It is much to be regretted that this great generalization was not pub- 

 lished to the world twenty years ago. I say this, not because it is to bo 



o 



regretted that Angstrom should have the credit of having in 1853 pub- 

 lished independently the statement that " an incandescent gas emits lumi- 

 " nous rays of the same refrangibility as those which it can absorb " ; or that 

 Balfour Stewart should have been unassisted by it when, coming to the 

 subject from a very different point of view, he made, in his extension of the 

 "Theory of Exchanges"*, the still wider generalization that the radiating 

 power of every kind of substanee is equal to its absorbing power for every 

 kind of ray ; or that KirchhofF also should have in 1859 independently dis- 

 covered the same proposition, and shown its application to solar and stellar 

 chemistry ; but because we might now be in possession of the inconceivable 

 riches of astronomical results which we expect fi-om the next ten years' 

 investigation by spectrum analysis, had Stokes given his theory to the 

 world when it first occurred to him. 



To Kirchhoff belongs, I believe, solely the great credit of having first 

 * Edin. Transactions, 1858-59. 



