268 THOMAS YOUNGK 



but the motion of its individual particles, at any mo- 

 ment, is simply a vibration up and down. Now each 

 oscillating particle of every moving wave, if left to itself, 

 would produce a series of waves, not so high, but in other 

 respects exactly similar to those produced by the stone. 

 The coalescence of all these small waves produces 

 another wave of exactly the same kind as that which 

 started them. The principle that every particle of a 

 wave acts independently of all other particles, while 

 the waves produced by all the particles afterwards 

 combine, is, as I have said, the great principle of 

 Huyghens. Taken in conjunction with the interference 

 of light, first established by Thomas Young, which 

 proved that when waves coalesce or combine, they may 

 either support each other or neutralise each other, the 

 neutralisation being either total or partial, according 

 as the opposition of the combining waves is complete or 

 incomplete taking, I say, the principle of interference 

 in conjunction with that of Huyghens, Fresnel proved 

 that although light does diverge behind an opaque 

 body, as Newton supposed that it would diverge, these 

 divergent waves completely efface each other, pro- 

 ducing the shadow -due to the tranquillity of the 

 medium which propagates the light. 



By reference to the waves of water, Young illus- 

 trates, in the most lucid manner, the interference of 

 the waves of light. He pictures two series of waves 

 generated at two points near each other in a lake, and 

 reaching a channel issuing from the lake. If the waves 

 arrive at the same moment, neither series will destroy 

 the other. If the elevations of both series coincide, 

 they will, by their joint action, produce in the channel 

 a series with higher elevations. But if the elevations of 

 one series correspond to the depressions of the other, 

 the ridges will exactly fill the furrows, smooth water 



