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 alaw. 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 
