KINETIC OR MECHANICAL VI KW OF NATfl'.K. 47 



in giving a mechanical explanation of the dark and bright 



lines of the spectrum, upon which Kirchhoff and liunseu [^',^1'/'*** 



founded spectrum analysis about the year 1860. ',. 



Wollaston ^ had in 1802, on examining the solar "^^ 

 spectrum (the succession of rainbow coloui-s expanded on 

 a white screen placed behind a prism uf white glass 

 through which a narrow beam of sunlight is made t(j pasij), 

 noticed that with a sufficient enlargement black lines in 

 great number could be detected. Fraunhofer,'- in ^lunich, 

 made a special study of them, named them by letters of 

 the alphabet, and compared the solar spectrum with the 

 spectra of artificial terrestrial sources where light is 

 created by combustion or incandescence. He found that 

 these spectra differed, the peculiar colour exhibited by 

 various flames being defined in the spectra by special 

 bright lines of different colours. Thus notably the two 

 dark lines called by him D in the solar spectrum were 

 replaced in the spectrum of a flame in which a volatile 

 salt of sodium was present, by two bright lines : Brewster 

 found the same coincidence of others of Fraunhofer's 

 lines with the bright lines of a flame in which nitre was 

 volatilised. Very similar and very accurate observations 

 of A. Millar as to the identity of the dark Hues D in the 

 solar spectrimi with the two bright lines of the sodium 

 flame were explained by Sir G. Stokes about the year ss. 



^ ' Sir G. 



1850 by the following theoretical reasoning: The sodium stokes. 



1 "A method of examining re- after him in his investigations of 



fractive and dispersive powers by the "refractive and dispersive 



prismatic reflection" ('Trans, of i)o\vers of various kinds of gbss" 



the Royal Society,' 1802). tor the purpose of improving the 



3 Fraunhofer, whose epitaph, achromatic telescoiie (' Denkschrif- 



" approximavit sidera," describes ten der Miinchener Akadcmie,' vol. 



beautifully his life-work, was led to i., 1814-15). 

 the discovery of the lines named 



