Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/134

 diffraction. In his earliest memoir he propounded a theory similar to that of Young, which was spoiled like Young's theory by the assumption that the fringes depend on light reflected by the diffracting edge. Observing, however, that the blunt and sharp edges of a knife produce exactly the same. fringes, he became dissatisfied with this attempt, and on July 15th, 1816, presented to the Academy a supplement to his paper, in which, for the first time, diffraction-effects are referred to their true cause—namely, the mutual interference of the secondary waves emitted by those portions of the original wave-front which have not been obstructed by the diffracting Fresnel's method of calculation utilized the principles of both Huygens and Young; he summed the effects due to different portions of the same primary wave-front, with due regard to the differences of phase engendered in propagation.

The sketch presented to the Academy in 1816 was during the next two years developeil into an exhaustive memoir, which was submitted for the Academy's prize.

It so happened that the earliest memoir, which had been presented to the Academy in the autumn of 1815, had been referred to a Commission of which the reporter was François Arago (b. 1786, d. 1853); Arago was so much impressed that he sought the friendship of the author, of whom he was later a strenuous champion.

A champion was indeed needed when the larger memoir was submitted; for Laplace, Poisson, and Biot, who constituted a majority of the Commission to which it was referred, were all zealous supporters of the corpuscular theory. During the examination, however, Fresnel was vindicated in a somewhat curious way. He had calculated in the mernoir the diffraction-patterns of a straight edge, of a narrow opaque body bounded by parallel sides, and of a narrow opening bounded by parallel cdges, and had shown that the results agreed excellently with