KINETIC OR MECHANICAL VIEW OF NATURE. 49 



birth to two great applications of his principle the 

 search, through the study of the spectra of distant stellar 

 sources of light, after the ingredients which are present 

 in those distant luminaries, and the search, through the 

 study of the flames of terrestrial substances, for new 

 spectral lines announcing yet undiscovered elements." l 

 Whilst in these two independent directions an enormous 

 amount of new knowledge has been accumulated, the 

 mechanical explanation through which Sir G. Stokes 

 anticipated these phenomena, and the further applications 

 of this principle by him, have done much to confirm the 

 conviction, that in looking upon light as a vibratory mode 

 of motion, we are on the road towards an adequate 

 description of these phenomena. 



1 To this principle we owe the 

 spectrum analysis of stellar at- 

 mospheres and the discovery of 

 new chemical elements, of which 

 no fewer than six have been iden- 

 tified by this method, beginning 

 with caesium and rubidium (found 

 by Kirchhoff and Bunsen in the 

 waters of some mineral springs). 

 The suggestion of Doppler, men- 

 tioned above (p. 10, note), has only 

 become fruitful through the inven- 

 tion of the spectroscope. Colour 

 differences originating through the 

 change of the frequency of vibra- 

 tions depending on cosmical veloc- 

 ities in the line of sight, could 

 not be discovered by the most 

 sensitive eye. In the spectrum, 

 however, shown by the spectro- 

 scope, " not only the colours of the 

 bright lines have been altered, but 

 their position in the spectrum 

 relatively to a fixed point of 

 reference as well. . . . The measure- 

 ment of the displacement of spectral 

 lines in consequence of the altered 

 refrangibility of the rays is the only 



VOL. n. 



method yet known which possesses 

 sufficient accuracy for determining 

 the motions of objects in the line 

 of sight. Thus far it has not been 

 possible to produce in the laboratory 

 velocities high enough to occasion 

 a perceptible displacement of the 

 lines" (Scheiner, loc. cit., p. 148). 

 And as Doppler's principle in 

 acoustics was proved directly by 

 Buys Ballot through the whistle on 

 moving railway trains, so it has 

 been proved directly in optics by 

 observing the displacement in the 

 lines of the solar spectrum, when 

 this is derived from the outer rays 

 of the sun's disc, the light-giving 

 parts moving in the line of sight 

 towards or away from the observer 

 in consequence of the rotation of 

 the sun round its axis. " The re- 

 sulting velocity of the surface of 

 the sun is found to agree very 

 closely with the results of direct 

 observations of the revolution of 

 the spots, thus practically furnish- 

 ing a proof of the correctness of 

 Doppler's principle" (ibid., p. 149). 



D 



