KINETIC OR MECHANICAL VIEW uF NAT! KK. 49 



birth to two great applications of lii.s principle — tlie 

 search, through the study of the spectra of distant stellar 

 sources of light, after the ingredients which are jtresent 

 in those distant luminaries, and the search, through the 

 study of the flames of terrestrial substances, for new 

 sj)ectral lines announcing yet undiscovered elements." ' 

 AMiilst in these two independent directions an enormous 

 amount of new knnwledge has been accumulated, the 

 mechanical explanation througli which Sir (!. Stokes 

 anticipated these phenomena, and llie further applications 

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

 conviction, that in looking upon liglit as a vibratory mode 

 of motion, we are on the road towards an adequate 

 description of these phenomena. 



' 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 tliis method, lieginning 

 with cicsium 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 sjiectroscope. Colour 

 differences originating through the 

 change of the frecjuency 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 tiie 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 conseciuence of the altere<l 

 refraiigiljility of the rays is the only 



VOL. II. 



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, /oc. cit., p. 148). 

 And as Doppler's principle in 

 acoustics was proved directly bj- 

 Buys Ballot through the whistle on 

 moving railway trains, so it has 

 been proved directly in optics by 

 observing the displacetnent in the 

 lines of the solar spectrum, wlieu 

 this is deiived from the nuter rays 

 of the sun's disc, the light-giving 

 parts moving in the line of sight 

 towards or away from tiie observer 

 in consec|uence 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 ])roof of the correctness of 

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



D 



