xsiii.] ■ 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 luove in straight 

 lines, must be devoid of any iutluence upon each other. 



The Huygheuian or Undulatory theory of light was also 

 able to explain the same phenomena, but with one re- 

 markable dillereuce. If the undulatory theory be true, 

 light must move more slowly in a dense refracting medium 

 tlian in a rarer one ; but the Newtonian theory assumed 

 that the attraction of the dense medium caused tlie par- 

 ticles of light to move more rapidly than in the rarp 

 medium. On this point, then, thei'e 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 jilate of glass into two pieces, 

 and slightly inclining one piece so as to increase the 

 length of the path of a ray ])assing through it, experi- 

 menters were able to show that light does move more 

 slowly in glass than in air.' JMore recently Fizeau and 

 Foucault inde})endeiitly measured the velocity of light in 

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

 air.^ 



There are a number of other points at wliich experi- 

 ence decides against Newton, and in favour of Huyghens 

 and Young. Laplace pointed out that the attraction sup- 

 ]»osed to exiat 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.^ But 

 experience shows that the velocity of light is uniform, 

 and indiipendent 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 jdaudhle, and not true ; lor Fresnel 

 ascertained that the dimensions of the fringes are not what 

 they would be according to Newton's theory. 



Although the Science of Light preseiits us with the 



' Airy's Mathematical Tracts, yA edit. pp. 286 — i88. 



'^ Jiiiiiin, Couisdc I'hyd'jue, vol. iii. p. 572. 



3 Vouuy'a Lectures on Natural rhilosoijky (1845), vol. i. [). 361, 



