THE WAVE THEORY. 267 



of the sound of a bell being heard behind a hill which 

 conceals the bell; of the turning of corners by sound ; 

 and then, with conclusive force, he points to the case of a 

 planet coming between a fixed star and the eye, when 

 the star is completely blotted out by the interposition 

 of the opaque body. This, Newton urged, could not 

 possibly occur if light were propagated by waves through 

 a fluid medium, for such waves would infallibly stir the 

 fluid behind the lanet, and thus obliterate the shadow. 

 Young was firmly persuaded of the truth of the 

 undulatory theory. The number of riddles that he 

 had solved by means of it, the number of secrets he 

 had unlocked, the number of difficulties he had crushed, 

 rendered him steadfast in his belief; still, he never 

 fairly got over this objection of Newton's. It was 

 finally set aside by one of the most illustrious men that 

 ever adorned the history of science. A young French 

 officer of engineers, Augustin Fresnel, first really 

 grappled with the difficulty and overthrew it. The 

 principle of Huyghens, to which I have already re- 

 ferred, is, that every particle, in every wave, acts as 

 if it alone were a centre of wave motion. When you 

 throw a stone into the Serpentine, circular waves or 

 ripples are formed, which follow each other in succes- 

 sion, retreating farther and farther from the point of 

 disturbance. 1 Fix your attention on one of these cir- 

 cular waves. The form of the wave moves forward, 

 1 ' I prove it thus, take heed now 



By experience, for if that thou 



Threw in water now a stone 



Well wost thou it will make anone, 



A little roundell as a cercle, 



Peraventure as broad as a couercle, 



And right anone thou shalt see wele, 



That wheie cercle wil cause another whele, 



And that a third and so forth brother, 



Every cercle causing other.' CHAUCEE'S House of Fame. 



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