2IO 



NATURE 



{June 28, 1888 



opposed ; that whatever may be the effect of the first zone, A p, 

 the exact opposite will be the effect of P Q, and so on. Thus, if 

 A P and p Q are both allowed to operate, while all beyond Q is 

 cut off, the waves will neutralize one another, and the effect 

 will be immensely less than if a P or PQ operated alone. And 

 that is what we saw just now. When I used the inner aperture 

 only, a comparatively loud sound acted upon the flame. When 

 I added to that inner aperture the additional aperture P Q," the 

 sound disappeared, showing that the effect of the latter was 

 equal and opposite to that of a p, and that the two neutralized 

 each other. 



[If A c = a, A B = b, A R = x, wave length = A, the value of 

 x for the external radius of the «lh zone is 



x 2 = n\ 



or, if a = 'b, 



Source 



a + b 

 ab 



x 1 = \n\a. 



Source 

 O 



Burner 



Fig. 1. 



With the apertures used above, x- = 49 for n = 1 ; x 1 — 10c 

 for n = 2 ; so that 



Ka = 100, 



the measurements being in centimetres. This gives the suit- 

 able distances, when X is known. In the present case \ = 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 disk. 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 



Burner 

 — O 



Fig. 2. 



Fig. 



be expected to be greatest, there is, on the contrary, no darkness 

 at all, but a bright spot, a spot as br'ght 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 was 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 absurdum of the theory. All 

 the time, as I have mentioned, the record of Delisle's observa- 



tions was in existence. 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 views. 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 psssage 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 corre- 

 sponding phenomenon in the case of sound. Fresnel's reasoning 

 is applicable, word for word, to the phenomena we are consider- 

 ing just as much as to that which he, or rather Poisson, had in 

 view. The disk (Fig. 4), which I shall hang up now between 

 the source of sound and the flame, is of glass. It is about 15 

 inches in diameter. I believe the flame is flaring now from being 



