ADDKESS. XCt 



attributed to the atom occupation of space with infinite hardness and strength 

 (incredible in any finite body) ; but we must realize it as a piece of matter 

 of measurable dimensions, with shape, motion, and laws of action, intelligible 

 subjects of scientific investigation. 



The prismatic analysis of light discovered by Newton was estimated by 

 himself as being " the oddest, if not the most considerable, detection which 

 " hath hitherto been made in the operations of nature." 



Had he not been deflected from the subject, he could not have failed 

 to obtain a pure spectrum ; but this, with the inevitably consequent 

 discovery of the dark lines, was reserved for the nineteenth century. 

 Our fundamental knowledge of the dark lines is due solely to Praun- 

 hofer. Wollastou saw them, but did not discover them. Ijrewster laboured 

 long and well to perfect the prismatic analysis of sunlight ; and his observa- 

 tions on the dark bands produced by the absorption of interposed gases and 

 vapours laid important foundatioiis for the grand superstructure which he 

 scarcely lived to see. Piazzi Smyth, by spectroscopic observation performed 

 on the Peak of Tenei-iffe, added greatly to our knowledge of the dark lines 

 produced in the solar spectrum by the absorption of our own atmosphere. 

 The prism became an instrirment for chemical qualitative analysis in the 

 hands of Fox Talbot and Herschel, who first showed how, throirgh it, the 

 old "blowpipe test" or generally the estimation of substances from the 

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

 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 suggested, either directly or indirectly, 

 by any other naturalist, when Stokes taught it to me in Cambridge at some 

 time prior to the summer of 1S52. The observational and experimental 

 foundations on which he built were : — 



(1 ) The discovery by Praunhofer 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 astonishing 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 con- 

 stitute 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 snuffed 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) Poucault's admirable discovery (L'Institut, Peb. 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." 



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 vi- 

 brations, like those of a tuning-fork or of stringed musical instruments ; that 

 like an instrument with two strings tuned to approximate unison, or an ap- 

 proximately circular clastic disk, it has two fundamental notes or vibrations 



