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 
Fig. 8.—Hyperbolas produced by Interference of Waves. the wave theory were accepted, the shadow of a small disk should have a bright spot in the middle, due to diffraction, the illumination of which should be the same as if no disk had been interposed. Arago at once tried the experiment; and what Poisson had urged to prove the impossibility of Fresnel's views was found to be a startling proof of their correctness. The experiment is easily tested, requiring 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 roughened glass to receive the
shadow. A pin-hole through a sheet of tin foil covering the window opening yields the required light from the mirror. The acoustic analogue of this celebrated experiment was first accomplished 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 sensitive 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 diffractive effect is the same as if the impervious disk were a lens converging the sound-waves to a focus.
The effect just described may be much intensified by constructing an acoustic diffraction grating and using it in place of the simple disk. The explanation of the principle on which such a
