THE USE OF HYPOTHESIS. 151 



then a ray of light, falling on the surface of a transparent 

 medium, will suffer an increase in its velocity of motion- 

 perpendicular to the surface, and the familiar law of sines 

 is the necessary consequence. This remarkable expla- 

 nation of the law of refraction had doubtless a very strong 

 effect in leading Newton to entertain the corpuscular 

 theory, and he appears to have thought that the analogy 

 between the propagation of the rays of light and the 

 motion of bodies was perfectly exact, whatever might be 

 the actual nature of lightP. It is highly remarkable, again, 

 that Newton was able to give, by his corpuscular theory, 

 a plausible explanation of the inflection of light as dis- 

 covered by Grimaldi. The theory would indeed have 

 been a very probable one could Newton's own law of 

 gravity have been applied ; but this was excluded, be- 

 cause the particles of light, in order that they may move 

 in straight lines, must be assumed devoid of any influence 

 upon each other. 



The Huyghenian or Undulatory theory of light was 

 also able to explain the same phenomena, but with one 

 remarkable difference. If the undulatory theory be true, 

 light must move more slowly in a dense refracting medium 

 than in a rarer one ; but the Newtonian theory assumed 

 that the attraction of the dense medium caused the par- 

 ticles of light to move more rapidly than in the rare medium. 

 On this point, then, there was a complete discrepancy 

 between the two theories, and observation was required 

 to show which theory was to be preferred. Now by 

 simply cutting a uniform plate of glass into two pieces, 

 and slightly inclining one piece so as to increase the 

 length of the path of a ray passing through it, experi- 

 menters have been able to show that the light does move 



P ' Principia/ bk. I. Sect. xiv. Prop. 96. Scholium, ' Opticks,' Prop. 

 VI. 3rd edit. p. 70. 



