130 !rHE CA.NUDIAN NATURALISI*. [Vol. 



Vl. 



consequent discovery of the dark lines, was reserved for the nine- 

 teenth century. Our fundamental knowledge of the dark lines 

 is due solely to Fraunhofer. Wollaston saw them, but did not 

 discover them. Brewster laboured long and well to perfect the 

 prismatic analysis of sunlight ; and his observations on the dark 

 bands produced by the absorption of interposed gases and vapours 

 laid important foundations for the grand superstructure which he 

 scarcely lived to see. Piazzi Smyth, by spectroscopic observation 

 performed on the Peak of TeneriiFe, added greatly to our know- 

 ledge of the dark lines produced in the solar spectrum by the 

 absorption of our own atmosphere. The prism became an instru- 

 ment for chemical qualitative analysis in the hands of Fox Tal- 

 bot and Herschel, who first showed how through it the old " blow- 

 pipe test," or generally the estimation of substances from the 

 colours which they give to flames, can be prosecuted with an accu- 

 racy and a discriminating power not to be attained when the 

 colour is judged by the unaided eye. But the application of this 

 test to solar and stellar chemistry had never, I believe, been sug- 

 gested, either directly or indirectly, by any other naturalist, when 

 Stokes taught it to me in Cambridge, at some time prior to the 

 summer of 1852. The observational and experimental founda- 

 tions on which he built were : — 



(1) The discovery by Fraunhofer of a coincidence between his 

 double dark line D of the solar spectrum and a double bright line 

 which he observed in the spectra of ordinary artificial flames. 



(2) A very rigorous experimental test of this coincidence by 

 Prof. W. H. Miller, which showed it to be accurate to an aston- 

 ishing degree of minuteness. 



(3) The fact that the yellow light given out when salt is thrown 

 on burning spirit consists almost solely of the two nearly identical 

 qualities which constitute that double bright line. 



(4) Observations made by Stokes himself, which showed the 

 bright line D to be absent in a candle-flame when the wick was 

 snufied clean, so as not to project into the luminous envelope, and 

 from an alcohol flame when the spirit was burned in a watch-glass. 

 And 



(5) Foucault's admirable discovery (^L'lnstitut, Feb. 7, 1849), 

 that the voltaic arc between charcoal points is '•' a medium which 

 emits the rays D on its own account, and at the same time absorbs 

 them when they come from another quarter." 



