EPOCH OF YOUNG AND FRESNEL. 445 



of these philosophers threw himself upon the sub- 

 ject with a zeal and intelligence which peculiarly 

 belonged to him. He verified the laws announced 

 by Fresnel : " laws," he says, " which appear to me 

 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 occu- 

 pied in it. Grimaldi, Newton, Maraldi, he states, 

 had observed the facts, and tried in vain to reduce 

 them to rule or cause. "Such 2 was the state of 

 our knowledge on this difficult question, when Dr. 

 Thomas Young made the very remarkable experi- 

 ment which is described in the Philosophical Trans- 

 actions 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 

 observation, that the same obliteration takes place, 

 if we stop the rays 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 3 that he was 

 not, at the time, aware of Young's previous labours. 

 After stating nearly the same reasonings concerning 

 fringes which Young had put forward in 1801, he 

 adds, " it is therefore the meeting, the actual cross- 

 ing of the rays, which produces the fringes. This 



3 Ann. Chim. 1815, Febr. 3 Ib. torn. xvii. p. 402. 



