THE USE OF HYPOTHESIS. 521 



covered by Grimaldi. The theory would indeed have been 

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

 have applied ; but this was out of the question, because the 

 particles of light, in order that they may move in straight 

 lines, must be 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 re- 

 markable 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 complete discre- 

 pancy between the 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 were able to show that light does move more 

 slowly in glass than in air. 1 More recently Fizeau and 

 Foucault independently measured the velocity of light in 

 air and in water, and found that the velocity is greater in 

 air. 2 



There are a number of other points at which experi- 

 ence decides against Newton, and in favour of Huyghens 

 and Young. Laplace pointed out that the attraction sup- 

 posed to exist between matter and the corpuscular parti- 

 cles of light would cause the velocity of light to vary 

 with the size of the emitting body, so that if a star were 

 250 times as great in diameter as our sun, its attraction 

 would prevent the emanation of light altogether. 3 But 

 experience shows that the velocity of light is uniform, 

 and independent of the magnitude of the emitting body, as 

 it should be according to the undulatory theory. Lastly, 

 Newton's explanation of diffraction or inflection fringes 

 of colours was only plausible, and not true; for Fresnel 

 ascertained that the dimensions of the fringes are not what 

 they would be according to Newton's theory. 



Although the Science of Light presents us with the 



1 Airy's Mathematical Tracts, 3rd edit. pp. 286 288. 



2 Jamin, Cours de Physique, vol. iii. p. 372. 



3 Young's Lectures on Natural Philosophy (1845), vol. i. p. 361. 



