196 Lord Itayleigh [Jan. 20, 



It is about 15 inches in diameter. I believe the flame is flaring now 

 from being in the bright spot. If I make a small motion of the disc 

 I shall move the bright spot and the effect will disappear. I am 

 pushing the disc away now, and the flaring has stopped. The flame 



Fig. 4. 

 Disc 



rLAME 



Source 



is still in the shadow of the disc, but not at the centre. I bring the 

 disc back again, and when the flame comes into the centre it flares 

 again vigorously. That is the phenomenon which was discovered by 

 Delisle and confirmed by Arago and Fresnel, but mathematically it 

 was suggested by Poisson. 



Poisson's calculation related only to the very central point in the 

 axis of the disc. More recently the theory of this experiment has been 

 very thoroughly examined by a German mathematician, Lommel ; and 

 I have exhibited here one of the curves given by him embodying the 

 results of his calculations on the subject (Fig. 5). 



The abscissae, measured horizontally, represent distances drawn 

 outwards from the centre of the shadow ; the ordinates measure the 

 intensity of the light at the various points. The maximum intensity 

 O A is at the centre. A little way outwards at B the intensity falls 

 almost, but not quite, to zero. At C there is a revival of intensity, 

 indicating a bright ring ; and further out there is a succession of 

 subordinate fluctuations. The curve on the other side of A would 

 of course be similar. This curve corresponds to the distances and 

 proportions indicated, a is the distance between the source of sound 

 and the disc ; b is the distance between the disc and the flame, the 

 place where the intensity is observed. The numbers given are taken 

 from the notes of an experiment which went well. If we can get our 

 flame to the right point of sensitiveness we may succeed in bringing 

 into view not only the central spot, but the revived sound which 

 occurs after you have got away from the central point and have passed 

 through the ring of silence. There is the loud central point. If I 

 push the disc a little we enter the ring of silence B ;* a little further, 

 and the flame flares again, being now at C. 



* With the data given above the diameter of the silent ring is two- thirds of 

 an inch. 



