636 THE PRINCIPLES OF SCIENCE. [CHAP. 



played a more complete comprehension of the same ana- 

 logy. It remained for Newton to create the theory of un- 

 dulatory motion in showing by mathematical deductive 

 reasoning that the particles of an elastic fluid by vibrating 

 backwards and forwards, might carry a pulse or wave moving 

 from the source of disturbance, while the disturbed particles 

 return to their place of rest. He was even able to make a 

 first approximation by theoretical calculation to the velocity 

 of sound-waves in the atmosphere. His theory of sound 

 formed a hardly less important epoch in science than his far 

 more celebrated theory of gravitation. It opened the way to 

 all the subsequent applications of mechanical principles to 

 th'e insensible motion of molecules. He seems to have been, 

 too, upon the brink of another application of the same 

 principles which would have advanced science by a century 

 of progress, and made him the undisputed founder of all the 

 theories of matter. He expressed opinions at various times 

 that light might be due to undulatory movements of a 

 medium occupying space, and in one intensely interesting 

 sentence remarks 1 that colours are probably vibrations of 

 different lengths, " much after the manner that, in the sense 

 of hearing, nature makes use of aerial vibrations of several 

 bignesses to generate sounds of divers tones, for the analogy 

 of nature is to be observed." He correctly foresaw that 

 red and yellow light would consist of the longer undulations, 

 and blue and violet of the shorter, while white light would 

 be composed of an indiscriminate mixture of waves of 

 various lengths. Newton almost overcame the strongest 

 apparent difficulty of the undulatory theory of light, 

 namely, the propagation of light in straight lines. For he 

 observed that though waves of sound bend round an ob- 

 stacle to some extent, they do not do so in the same degree 

 as water-waves. 2 He had but to extend the analogy 

 proportionally to light-waves, and not" only would the 

 difficulty have vanished, but the true theory of diffraction 

 would have been open to him. Unfortunately he had a 

 preconceived theory that rays of light are bent from and 

 not towards the shadow of a body, a theory which for once 

 he did not sufficiently compare with observation to detect 



1 Birch, History of the Royal Society, voL iii. p. 262, quoted by 

 Young, Works, vol. i. p. 246. 



2 Opticks, Query 28, 3rd edit. p. 337. 



