314 BELL SYSTEM TECHNICAL JOURNAL 



gradual and cyclic way between zero as one extreme and double the 

 amplitude of the unintercepted wave-train as the other. As the 

 field-point is displaced along the axis towards or away from the aper- 

 ture, as the aperture itself is expanded or contracted, doubled agitation 

 succeeds upon quiescence and quiescence upon agitation; and the 

 opening, far from serving as a window to let a segment of the oncoming 

 wave-train pass unaltered by, acts as an agency for producing a curious 

 pattern of varying amplitudes over the region before it. 



Now these are precisely the conditions under which, as I remarked 

 before, one can arrange a test of the wave-theory of sound or light; 

 for here we have the amplitude varying from point to point, in a pattern 

 depending in detail upon the wave-length. Experience of light reveals 

 just such a pattern; when parallel light is shed normally upon a screen 

 pierced with a small and accurately rounded hole, the illumination in 

 the axis of the hole passes alternately through maxima and minima 

 as the observer recedes along it. Fresnel was led in a curious way to 

 discover the minima. The French Academy having offered a com- 

 petitive prize for a study of diffraction — an action instigated, it appears, 

 by adherents of the corpuscular theory of light, who expected that a 

 thorough knowledge of the phenomena of diffraction would demolish 

 the support which they were vaguely supposed to provide for the wave- 

 theory — Fresnel conducted a research and submitted a memoir which 

 ranks among the classics of physical science. It went for judgment to 

 an illustrious committee of five,^ one of whom, the very eminent 

 mathematician and physicist Poisson — who had been an upholder of 

 the corpuscular theory — promptly deduced the law of the maxima and 

 minima along the axis from Fresnel's conception of the wavelets. He 

 imparted this prediction to the author of the memoir; and in a note 

 appended to the published version, Fresnel has left it on record that he 

 looked for a minimum and found it "like an inkspot" in the centre of 

 the field before the hole. 



Equation (81) shows further that the amplitude at any point upon 

 the axis must vary to and fro between the same two extremes — zero, 

 and double the amplitude of the unhindered waves — as the hole ex- 

 pands or shrinks. Wood has described how this may be observed 

 with an iris diaphragm. For an observer stationed at a fixed point 

 upon the axis at a distance xo from the hole, the amplitude falls to zero 

 whenever the radius of the circle has one of the values determined by 

 the condition 



mR^/lXf, = even integer multiple of t, (82) 



^ Arago, Biot, Gay-Lussac, Laplace and Poisson. It would be hard to assemble 

 a more distinguished group at any time or place. 



