194 



Loj'd Mayleigli 



[Jan. 20, 



flame, and A P Q is the screen. If we choose a point P on this screen, 

 so that the whole distance from B to C, reckoned through P, viz. 

 B P C, exceeds the shortest distance B A C by exactly half the wave- 

 length of the sound, then the circular area, whose radius is A P, is the 

 first zone. We take next another point, Q, so the whole distance B Q 

 exceeds the previous one by half a wave-length. Thus we get the 



Fig. 



second zone represented by P Q. In like manner, by taking different 

 points in succession such tliat the last distance taken exceeds the 

 previous one every time by half a wave-length, we may map out the 

 whole of the obstructing screen into a series of zones called Huygens' 

 zones. I have here a material embodiment of that notion, in which 

 the zones are actually cut out of a piece of zinc. It is easy to prove 

 that the effects of the parts of the wave traversing the alternate zones 

 are opposed, that whatever may be the effect of the first zone, A P, the 

 exact opposite will be the effect of P Q, and so on. Thus, if 

 A P and P Q are both allowed to operate, while all beyond Q is cut 

 off, the waves will neutralise one another, and the effect will be 

 immensely less than if A P or P Q operated alone. And that is what 

 you saw just now. When I used the inner aperture only, a com- 

 paratively loud sound acted upon the flame. When I added to that 

 inner aperture the additional aperture P Q, the sound disappeared, 

 showing that the effect of the latter was equal and opposite to that 

 of A P, and that the two neutralised each other. 



[If A C = «, A B = 5, A P = X, wave-length = X, the value of x 

 for the external radius of the nth zone is 



n\ 



or, if a = h. 



ah 

 nXa. 



