EPOCH OF YOUNG AND FRESNEL. 435 



of these persons, ground enough for adopting the 

 doctrine; and it had also the advantage of being 

 more ready of conception, for though the propa- 

 gation of a wave is not very difficult to conceive, 

 at least by a mathematician, the motion of a par- 

 ticle is still easier. 



On the other hand, the undulation theory was 

 maintained by no less a person than Euler; and 

 the war between the two opinions was carried on 

 with great earnestness. The arguments on one side 

 and on the other soon became trite and familiar, 

 for no person explained any new class of facts by 

 either theory. Thus it was urged by Euler against 

 the system of emission 12 , that the perpetual ema- 

 nation of light from the sun must have diminished 

 his mass ; that the stream of matter thus con- 

 stantly flowing must affect the motions of the planets 

 and comets ; that the rays must disturb each 

 other; that the passage of light through transpa- 

 rent bodies is, on this system, inconceivable: all 

 such arguments were answered by representations 

 of the exceeding minuteness and velocity of the 

 matter of light. On the other hand, there was 

 urged against the theory of waves, the favourite 

 Newtonian argument, that on this theory the light 

 passing through an aperture ought to be diffused, 

 as sound is. It is curious that Euler does not make 

 to this argument the reply which Huyghens had 

 made before The fact really was, that he was not 

 18 Fischer, iv. 449. 



FF2 



