KINETIC OR MECHANICAL VIEW OF NATURE. 25 



Sciences in 1817 issued for the competition on the j^iand 

 mathematical prize for 1 S 1 the subject of DilTraction, 

 " jiersuaded that a deeper investigation of these phenijmena, 

 which seemed opposed to their cherished doctrine, wcjidd 

 give occasion for new triumphs." ^ In this lliey were 

 doomed to disappointment. At the request of Arag(j and 

 Ampere, Fresnel entered for this competition, and his 

 * Meinoire sur la Diffraction ' was crowned the following 

 year. In it he viewed the subject from a much more 

 general point of view, examining the two rival systems — 

 that of emission and that of undulations — as to their 

 capacity for explaining the phenomena of diffraction. 

 The result seemed decisive in favour of the latter theory, 

 and the impression produced was all the greater because 

 Poisson," one of the judges and a believer in the emission 

 theory, drew certain apparently very paradoxical conse- 



18. 



1 Yerdet in ' Qiuvres de Fresnel,' 

 vol. i.. Preface, p. xxxv., &c. 



- The commissinn cousisted of 

 Biot, Arago, Laplace, Gay-Lussac, 

 and Poissuii. Arago drew up the 

 report, which is published in the 

 first volume of the ' Oiuvres de 

 Fresnel,' No. 13. It closes with 

 the following note: "M. Poisson, 

 depuis le rajjport de la commission, 

 ayant fait remarquer ii M. Fresnel 

 que I'intdgrale qui represente 

 rintensit(5 de la luniicre difYractie 

 peut aisomeut s'obtenir pour le 

 centre de I'ombre d'un ecran ou 

 d'une ouverture circulaires, celui-ci 

 fit le calcul pour ce dernier cas, et 

 trouva (|ue I'expression gendi-ale 

 d'intensite devenait alors seinlilable 

 h celle de la lumierc reflechie dans 

 le phononiene des anneaux colorcs ; 

 que 868 minima dtaient tout h fait 

 nul et devaient presenter ainsi un 

 jioir h. peu prds parfait dans une 



lumicre sensiblenient honn)g6ne ; du 

 moins pour les trois premiers ordres, 

 oil le d^faut d'homogoneitA? de la 

 lumi6re rouge employee ne se fais- 

 ait pas encore trop sentir : c'est 

 aussi ce que Texperience a confirmo ; 

 en plarant le foyer de la loupe du 

 micrometre aux distances calculoes 

 ou appercevait comme une tache 

 d'encre au centre de I'ouverture 

 circulaire. . . . Ou peut rcgarder 

 cette experience comme une verifi- 

 cation des foi-mules de M. Fresnel," 

 &c. ('Qiuvres,' vol. i. p. 245). 

 See also the note which Fresnel 

 attached to his memoir (il>id., p. 

 365). The memoir was crowned in 

 1819, Imt not publislied till 1826. 

 An abstract of the first and a re- 

 print of the second ]>art had been 

 publislied in tiie 11th vol uf the 

 ' Annales de Chimie et ile Pliysique.' 

 Fresnel sent two co]iie8 to Vounp, 

 19th September 1819. 



