KINETIC OR MECHANICAL VIEW OF NATURE. 11 



firmed, that light takes time to travel from one point 

 in space to another. Wherever time is involved in a 

 phenomenon, motion of something is suggested, and 

 this something, as well as the nature of its motion, 

 become subjects of speculation. At the beginning of 5. 



. . . Undulatory 



the nineteenth century two distinct theories existed and emission 



theories. 



regarding these matters. Both had succeeded in ex- 

 plaining and calculating satisfactorily a large number 

 of the phenomena of light as exhibited by mirrors and 

 lenses, as well as in optical instruments and crystals. 

 One of these theories, the so-called emission, emanation, 

 or corpuscular theory of light, held that luminous 

 bodies send out minute particles which travel in 

 straight lines, and, impinging upon the eye, create the 

 sensation of light. The rival hypothesis, the undul- 

 atory or vibratory theory, held light to consist in 

 the periodic wave-motion of a substance called ether, 

 which was supposed to exist everywhere, filling all 

 space and interpenetrating all ponderable matter. Both e. 

 theories are kinetic or mechanical theories, and for their theories 



kinetic. 



development require the analysis of certain modes of 

 motion. Both had to formulate their respective 

 notions as to the something that moved. Both could 

 point to analogies in other domains of natural science. 

 There existed at that time similar corpuscular ex- 

 planations of the phenomena of heat, of electricity 



velocity they move, how distant William Huggins (1868), Fox-Tal- 



they are from us, and much else bot, and others. That Doppler's 



besides," a prediction which, since principle is really none other than 



the invention of spectrum analysis ' Romer's was remarked by P. G. 



and various controversies connected . Tait in 'Light' (2nd ed., p. 220). 



with the subject, has been brilliantly See also Rosenberger, ' Gesch. d. 



verified by the discoveries of Sir Physik,' vol. iii. p. 708 sqq. 



