96 HISTORY OF OPTICS. 



the supposed judges of science and letters. Its author went ou labor- 

 ing at the completion and application of the theory in other parts of 

 the subject ; but his extraordinary success in unravelling the complex 

 phenomena of which we have been speaking, appears to have excited 

 none of the notice and admiration which properly belonged to it, till 

 Fresnel's Memoir On Diffraction was delivered to the Institute, in 

 October, 1815. 



MM. Arago and Poinsot were commissioned to make a report upon 

 this Memoir; and the former of these philosophers threw himself upon 

 the subject with a zeal and intelligence which peculiarly belonged to 

 him. He verified the laws announced by Fresnel : "laws," he says, 

 " which appear to be destined to make an epoch in science." He then 

 cast a rapid glance at the history of the subject, and recognized, at 

 once, the place which Young occupied in it. Grimaldi, Newton, 

 Maraldi, he states, had observed the facts, and tried in vain to reduce 

 them to rule or cause. " Such 3 was the state of our knowledge on this 

 difficult question, when Dr. Thomas Young made the very remarkable 

 experiment which is described in the Philosophical Transactions for 

 1803 ;" namely, that to obliterate all the bands within the shadow, we 

 need only stop the ray which is going to graze, or has grazed, one 

 border of the object. To this, Arago added the important observa- 

 tion, that the same obliteration takes place, if we stop the ray, with a 

 transparent plate ; except the plate be very thin, in which case the 

 bands are displaced, -and not extinguished. "Fresnel," says he, 

 " guessed the effect which a thin plate would produce, when I had told 

 him of the effect of a thick glass." Fresnel himself declares 4 that he 

 was not, at the time, aware of Young's previous labors. After stating 

 nearly the same reasonings concerning fringes which Young had put 

 forward in 1801, he adds, "it is therefore the meeting, the actual 

 crossing of the rays, which produces the fringes. This consequence, 

 which is only, so to speak, the translation of the phenomena, seems to 

 me entirely opposed to the hypothesis of emission, and confirms the 

 system which makes light consist in the vibrations of a peculiar fluid." 

 And thus the Principle of Interferences, and the theory of undula- 

 tions, so far as that principle depends upon the theory, was a second 

 time established by Fresnel in France, fourteen years after it had been 

 discovered, fully proved, and. repeatedly published by Young in 

 England. 



An. Chlm. 1815, Febr. 4 Ib. torn. xvii. p. 402. 



