THEORY OF ETHER 161 



which the intensity of the two waves of light is determined, 

 will vary for different media. In the same medium the speed 

 is unchanged. In the new medium the speed may naturally 

 alter. It has been clearly proved by direct observations that 

 the speed of propagation does vary in different media ; but, 

 whatever the speed, light always travels rectilinearly. We 

 may, however, first of all apply the principle of interference to 

 reconcile the wave theory with the observed fact that an 

 opaque body effectively shields a portion of the ether from 

 disturbance which must be occurring in its neighbourhood. 

 Let w v represent a section of portion of a wave, starting 



Fig. 55. 



from the luminous point o, and let P be the point at which 

 the effect of this wave is considered. Join o P cutting wv in 

 a, then let 6, c, d, e,f, &c., represent points such that the linear 

 distances b P, c P, d P, &c., on one side the line a P, and like- 

 wise those, e P,/P, gv, &c., on the other side, each differ from 

 one another by the distance of half a wave-length. Then, if 

 we consider each of these points as the centre of a new dis- 

 turbance, it is obvious that each consecutive pair of the 

 secondary waves produced will arrive at p in opposite phases, 

 and, if equal, would neutralise each other. That is, the general 

 result of the portion a b will be opposite to that of the portion 

 b c, and c d opposite in its effect to d e, and so on for the other 



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