1888.] 



on Diffraction of Sound. 



197 



Although we have thus imitated the optical experiment, I must 

 not leave you under the idea that we are working under the same 

 conditions that prevail in optics. You see the diameter of my disc 

 is 15 inches, and the length of my sound-wave is about half an inch. 



Fig. 5. 



My disc is therefore about 30 wave-lengths in diameter, whereas the 

 diameter of a disc representing 30 wave-lengths of light would be only 

 about ToVo" iiich. Still the conditions are sufficiently alike to get 

 corresponding effects, and to obtain this bright point in the centre of 

 the shadow conspicuously developed. 



I will now make an experiment illustrating still further the prin- 

 ciple of Huygens' zones, which I have already roughly sketched. I 

 indicated that the effect of contiguous zones was equal and opposite, 

 so that the effect of each of the odd zones is one thing, and of the 

 even zones the opposite thing. If we can succeed in so preparing a 

 screen as to fit the system of zones, allowing the one set to pass, and 

 at the same time intercepting the other set, then we shall get a great 

 effect at the central point, because we shall have removed those parts 

 which, if they remained, would have neutralised the remaining parts. 



