ANALOGY. 295 



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 

 the insensible motion of molecules. He seemed to have 

 been frequently, too, upon the brink of another appli- 

 cation of the same principles which would have advanced 

 science by at least 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 

 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 big- 

 nesses 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 undula- 

 tions, 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 one of the 

 strongest apparent difficulties 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 obstacle to some extent, they do not do so in the same 

 degree as water-waves 11 . He had but to extend the ana- 

 logy proportionally to light-waves, and not only would 

 the difficulty have vanished, but the true theory of dif- 

 fraction 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 ob- 

 servation to detect its falsity. I am not aware, too, that 



g Birch, 'History of the Royal Society,' vol. iii. p. 262, quoted by 

 Young, ' Works,' vol. i. p. 146. 



h 'Opticks,' Query 28, 3rd edit. p. 337. 



