446 HISTORY OF OPTICS. 



consequence, which is only, so to speak, the trans- 

 lation of the phenomenon, seems to me entirely 

 opposed to the hypothesis of emission, and confirms 

 the system which makes light consist in the vibra- 

 tions of a peculiar fluid." And thus the Principle 

 of Interferences, and the theory of undulations, so 

 far as that principle depends upon the theory, was a 

 second time established by Fresnel in France, four- 

 teen years after it had been discovered, fully proved, 

 and repeatedly published by Young in England. 



In this Memoir of Fresnel's, he takes very nearly 

 the same course as Young had done; considering 

 the interference of the direct light with that re- 

 flected at the edge, as the cause of the external 

 fringes ; and he observes, that in this reflection it is 

 necessary to suppose half an undulation lost : but a 

 few years later, he considered the propagation of 

 undulations in a more true and general manner, 

 and obtained the solution of this difficulty of the 

 half-undulation. His more complete Memoir on 

 Diffraction was delivered to the Institute of France, 

 July 29, 1818 ; and had the prize awarded it in 

 1819 4 : but by the delays which at that period 

 occurred in the publication of the Parisian Acade- 

 mical Transactions, it was not published 5 till 1826, 

 when the theory was no longer generally doubtful 

 or unknown in the scientific world. In this Memoir, 

 Fresnel observes, that we must consider the effect 

 of every portion of a wave of light upon a distant 



4 Ann. Chim. May, 1819. * Mem. Inst for 1821-2. 



