1888.] on Diffraction of Sound. 195 



With tlie apertures used above, x'^ = 49 for n = 1; x"^ = 100, for 

 n = 2; so that 



Xa = 100, 



the measurements being in centimetres. This gives the suitable 

 distances, when X is known. In the present case A. = 1'2, a = 83.] 



Closely connected with this there is another very interesting 

 experiment, which can easily be tried, and which has also an im- 

 portant optical analogy. I mean the experiment of the shadow 

 thrown by a circular disc. If a very small source of light be taken 

 — such a source as would be produced by perforating a thin plate 

 in the shutter of the window of a dark room with a pin and causing 

 the rays of the sun to enter horizontally — and if we interpose in 

 the path of the light a small circular obstacle and then observe 

 the shadow thrown in the rear of that obstacle, a very remarkable 

 peculiarity manifests itself. It is found that in the centre of 

 the shadow of the obstacle, where the darkness might be expected 

 to be greatest, there is, on the contrary, no darkness at all, but a 

 bright spot, a spot as bright as if no obstacle intervened in the 

 course of the light. The history of this subject is curious. The - 

 fact was first observed by Delisle in the early part of the eighteenth 

 century, but the observation fell into oblivion. When Fresnel 

 began his important investigations, his memoir on diffraction was 

 communicated to the French Academy and was reported on by the 

 great mathematician Poisson. Poisson rwas not favourably impressed 

 by Fresnel's theoretical views. Like most mathematicians of the 

 day, he did not take kindly to the wave theory ; and in his report on 

 Fresnel's memoir, he made the objection that if the method were 

 applied, as Fresnel had not then done, to investigate what should 

 happen in the shadow of a circular obstacle, it brought out this para- 

 doxical result, that in the centre there would be a bright point. This 

 was regarded as a reductio ad ahsurdum of the theory. All the time, 

 as I have mentioned, the record of Delisle's observations was in exist- 

 ence. The remarks of Poisson were brought to the notice of Fresnel, 

 the experiment was tried, and the bright point was rediscovered, to 

 the gratification of Fresnel and the confirmation of his theoretical 

 view^s. I don't propose to attempt the optical experiment now, 

 but it can easily be tried in one's own laboratory. A long room 

 or passage must be darkened: a fourpenny bit may be used as 

 the obstacle, strung up by three hairs attached by sealing-wax. 

 When the shadow of the obstacle is received on a piece of ground 

 glass, and examined from behind with a magnifying lens, the 

 bright spot will be seen without much difficulty. But what 

 I propose to show you is the corresponding phenomenon in the 

 case of sound. Fresnel's reasoning is applicable, word for word, 

 to the phenomena we are considering just as much as to that which 

 he, or rather Poisson, had in view. The disc (Fig. 4), which I shall 

 haug up now between the source of sound and the flame, is of glass. 



