KINETIC OR MECHANICAL VIEW OF NATURE. 47 



in giving a mechanical explanation of the dark and bright 32. 



The clue 



lines or the spectrum, upon which Kirchhoff and Bunsen fm>sh 



by the 



founded spectrum analysis about the year 180. 



Wollaston 1 had in 1802, on examining the solar depend8 - 

 spectrum (the succession of rainbow colours expanded on 

 a white screen placed behind a prism of white glass 

 through which a narrow beam of sunlight is made to pass), 

 noticed that with a sufficient enlargement black lines in 

 great number could be detected. Fraunhofer, 2 in Munich, 

 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. Miller as to the identity of the dark lines D in the 

 solar spectrum with the two bright lines of the sodium 

 flame were explained by Sir G. Stokes about the year 33. 

 1850 by the following theoretical reasoning: The sodium stokeV 



1 "A method of examining re- 

 fractive and dispersive powers by 

 prismatic reflection" ('Trans, of 

 the Royal Society,' 1802). 



2 Fraunhofer, whose epitaph, 

 " approzimavit eidera," describes 

 beautifully his life-work, was led to 

 the discovery of the lines named 



after him in his investigations of 

 the ' ' refractive and dispersive 

 powers of various kinds of glass" 

 for the purpose of improving the 

 achromatic telescope (' Denkschrif- 

 ten der iliinchener Akademie,' vol. 

 i., 1814-15). 



