SENSITIVE FLAMES AND SOUND-SHADOWS. 47 



of theory to any test. The first physicist to recognize the value 

 of Young's optical papers was Arago, who at once adopted the 

 wave theory and started his friend Fresnel on a series of optical 

 researches that are now classic. In 1819 Fresnel gained a prize 

 from the French Academy for his work on diffraction of light. 

 Before the report was made to the Academy it was examined by 

 the mathematician Poisson, who criticised it by showing that, if 

 the wave theory were ac- 

 cepted, the shadow of a 

 small disk should have a 

 bright spot in the middle, 

 due to diffraction, the illu- 

 mination of which should 

 be the same as if no disk 

 had been interposed. Ara- 

 go at once tried the ex- 

 periment ; and what Pois- 

 son had urged to prove the 

 impossibility of Fresnel's 

 views was found to be a 

 startling proof of their 

 correctness. The experi- 

 ment is easily tested, re- 

 quiring no more expensive 

 apparatus than a mirror 

 outside of an opening in 

 a window, a small bullet 

 suspended by a thin wire, 

 and a piece of rough- 

 ened glass to receive the 



shadow. A pin-hole through a sheet of tin foil covering the win- 

 dow opening yields the required light from the mirror. The 

 acoustic analogue of this celebrated experiment was first accom- 

 plished a few years ago by Lord Rayleigh; it has been lately 

 often repeated by the writer and perhaps others. A disk of card- 

 board about a foot in diameter is put between a whistle and sensi- 

 tive flame, with careful adjustment of distance and sensitiveness. 

 In certain positions the flame is protected within the shadow of 

 the disk ; but, by moving the latter to and fro, one position is 

 found where it causes the flame to be violently agitated by the 

 meeting of waves diffracted at the edge of the circle. The diffract- 

 ive effect is the same as if the impervious disk were a lens con- 

 verging the sound-waves to a focus. 



The effect just described may be much intensified by construct- 

 ing an acoustic diffraction grating and using it in place of the 

 simple disk. The explanation of the principle on which such a 



T' ? 3? %< f 1 5 T * 6 

 Piq. 8. Hyperbolas produced by Interference op 



Waves. 



