192 



Lord Bayleigh. 



[Jan. 20, 



I am now going to move the reflector to and fro along the line 

 perpendicular to that joining the source and the burner, all the while 

 maintaining the adjustment, so that from the position of the source of 

 sound the image of the flame is seen in the centre of the mirror. 

 Seen from the source, it is still as central as before, but it has lost its 

 effect, and as I move it to and fro I produce cycles of effect and no 



Fig. 1. 



Burner 



effect. What is the cause of this? The question depends upon 

 something different from what I have been speaking of hitheito ; 

 and the explanation is, that we are here dealing with a diffraclion 

 phenomenon. The mirror is a small one, and the sound-waves which 

 it reflects are not big enough to act in the normal manner. We are 

 really dealing with the same sort of phenomena as arise in optics when 

 we use small pin-holes for the entrance of our light. It is not veiy 

 easy to make the experiment in the present form quite simjDle, because 

 the mirror would have to be withdraAVTi, all the while maintaining a 

 somewhat complicated adjustment. In order to raise the question of 

 diffraction in its simjDlest shape, we must have a direct course for tlie 

 sound between its origin and the place of observation, and interpose 

 in the path a screen perforated with such holes as we desire to try. 



The screen I propose to use is of glass. It is a practically 

 perfect obstacle for such sounds as we are dealing with ; but it is 

 perforated here with a hole (20 cm. diameter), rendered more evident 

 to those at a distance by means of a circle of paper pasted round it. 

 The edge of the hole corresponds to the inner circumference of the 

 paper. We shall thus be able to try the effect of different sized aper- 

 tures, all the other circumstances remaining unchanged. The experi- 



