1888.] 



on Diffraction of Sound. 



193 



ment is rather a difficult one before an audience, because everything 

 turns on getting the exact adjustment of distances relatively to the wave- 

 length. At present the sound is passing through this comparatively 

 large hole in the glass screen, and is producing, as you see, scarcely 

 any effect upon the flame situated opposite to its centre. But if 

 ^Fig. 2) I diminish the size of the hole by holding this circle of zinc 

 (perforated with a hole 14 cm. in diameter) in front of it, it is seen 

 that, although the hole is smaller, we get a far greater effect. That 



Source 

 O 



Fig. 2. 



Burner 

 — O 



is a fundamental phenomenon in diffraction. Now I reopen the larger 

 hole, and the flame becomes quiet. So that it is evident that in this 

 case the sound produces a greater effect in passing through a small 

 hole than in passing through a larger one. The experiment may be 

 made in another way, by obstructing the central in place of the mar- 

 ginal part of the aperture in the glass. When I hold this unperforated 

 disc of zinc (14 cm. in diameter) centrically in front, we get a greater 

 effect than when the sound is allowed to pass through both parts of 

 the aperture. The flame is now flaring vigorously under the action 

 of the sonorous waves passing the marginal part of the aperture, 

 whereas it will scarcely flare at all under the action of waves passing 

 through both the marginal and the central hole. 



This is a point which I should like to dwell upon a little, for it 

 lies at the root of the whole matter. The principle upon which it 

 depends is one that was first formulated by Huygens, one of the 

 leading names in the development of the undulatory theory of light. 

 In this diagram (Fig. 3) is represented in section the different parts 

 of the obstacle. C represents the source of sound, B represents the 



