KINETIC OR MECHANICAL VIEW OF NATURE. 25 



Sciences in 1 8 1 7 issued for the competition on the grand 

 mathematical prize for 1819 the subject of Diffraction, 

 " persuaded that a deeper investigation of these phenomena, 

 which seemed opposed to their cherished doctrine, would 

 give occasion for new triumphs." * In this they were 

 doomed to disappointment. At the request of Arago and 

 Ampere, Fresnel entered for this competition, and his 

 ' Memoire 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, 2 one of the judges and a believer in the emission 

 theory, drew certain apparently very paradoxical conse- 



18. 



Fresnel's 



1 Verdet in ' (Euvres de Fresnel,' 

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



* The commissinn consisted of 

 Biot, Arago, Laplace, Gay-Lussac. 

 and Poisson. Arago drew up the 

 report, which is published in the 

 first volume of the ' (Euvres de 

 Fresnel,' No. 13. It closes with 

 the following note: "M. Poisson, 

 depuis le rapport de la commission, 

 ay ant fait remarquer a M. Fresnel 

 que l'inte"grale qui repre'sente 

 I'lntensite* de la lumiere diffractie 

 peut aise"ment s'obtenir pour le 

 centre de 1'ombre d'un ecran ou 

 d'une ouverture circulaires, celui-ci 

 fit le calcul pour ce dernier cas, et 

 trouva que 1'expression ge'ne'rale 

 d' in ten sit 6 devenait alors semblable 

 h. celle de la lumiere refle'chie dans 

 le phe"noinene des anneaux colored ; 

 que ses minima e"taient tout & fait 

 nul et devaient presenter ainsi un 

 Doir a peu pres parfait dans une 



lumiere sensiblement homogene ; du 

 moins pour les trois premiers ordres, 

 oil le de'faut d'homoge'neite' de la 

 lumiere rouge employee ne se fais- 

 ait pas encore trop sentir : c'est 

 aussi ce que 1'experience a confirme ; 

 en placant le foyer de la loupe du 

 micrometre aux distances calculees 

 on appercevait comme une tache 

 d'encre au centre de 1'ouverture 

 circulaire. . . . On peut regarder 

 cette experience comme une ve"rifi- 

 cation des formules de M. Fresnel," 

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

 See also the note which Fresnel 

 attached to his memoir (ibid., p. 

 365). The memoir was crowned in 

 1819, but not published till 1826. 

 An abstract of the first and a re- 

 print of the second part had been 

 published in the llth vol. of the 

 ' Annales de Chimie et de Physique. ' 

 Fresnel sent two copies to Young, 

 19th September 1819. 



