204 FRESNEL. 



The complete experimental demonstration of the fact 

 of interferences will always be the principal title of 

 Dr. Thomas Young to the recognition of posterity. The 

 researches of this illustrious physicist (whose recent loss 

 the sciences have to deplore) had already led to the 

 general principles which I do not think I ought here to 

 abstain from announcing, although the genius of Fresnel 

 seized upon them, extended them, and showed their 

 great fertility.* 



plained from the extremely vague nature of that explanation ; it, in 

 fact, amounted to no more than a general notion that some such peri- 

 odical action might be occasioned by a concurrence of waves or pul- 

 ses. It did not amount to a theory; it had no reference to measures 

 of the phenomenon, and indicated nothing like a law. At the time 

 Hooke does not appear to have been aware of the composition of white 

 light, and thus all accurate analysis of the phenomenon was out of 

 the question. Newton pursued the subject on professedly experi- 

 mental grounds alone ; it was not his plan to enter on any theoreti- 

 cal considerations ; he, therefore, could not be expected to refer to 

 Hooke' s, which must necessarily have seemed to him wholly gratui- 

 tous, and even visionary. Translator. 



* Young's investigations of diffraction was rather general, and 

 qualitative, though the demonstration as to the nature of the effect 

 was perfectly conclusive ; but the later researches of Fresnel carried 

 out the subject to a quantitative determination. This being made to 

 include the combined effect of an infinite number of interferences 

 acting at every point, involved the use of the higher calculus; and the 

 result was established by means of integrations giving the intensity 

 of light at all parts of the screen or image. This remark applies not 

 merely to the particular case of diffraction, but to that of thin plates, 

 and other analogous cases, in which the principle of interference is 

 applied. This analytical extension constituted one of the most char- 

 acteristic excellences of Fresnel's researches. In an experimental 

 point of view, Fresnel's researches are characterized by scarcely less 

 improvements. The most material modifications he introduced were 

 those of (1.) viewing the image of the stripes directly by an eye lens, 

 instead of throwing them on a'screen; (2.) discarding any interposi- 

 tion of an opaque body, and causing two rays simply to act on each 

 other, by causing the sun's light diverging from a minute aperture or 



