114 The Lumini/erous Medium, 



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,f 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 

 screen. 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 developed into an exhaustive memoir, J 

 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 Francois 

 Arago (&. 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 memoir the diffraction- 

 patterns of a straight edge, of a narrow opaque body bounded 

 by parallel sides, and of a narrow opening bounded by parallel 

 edges, and had shown that the results agreed excellently with 



* Annales de Chimie (2), i (1816), p 239 ; (Euvres, i, p. 89. 



t (Euvres, i, p. 129. 



I Mem. de 1'Acad., v (1826), p. 339 ; (Euvres, i, p. 247. 



