(484.) 
(485.) 
Fresnel. 
(486.) 
Diffraction 
of light. 
Fresnel’s 
first me- 
moir. 
Cuar. V., § 3.] 
bone. He does not appear to have noticed the phe- 
nomena of colour accompanying such depolarization, 
though he arrived at it so nearly that Arago, in all pro- 
bability, anticipated him by presenting a memoir on 
the subject to the Institute just one week previous to 
Malus’s announcement of what he had observed 
(19th August 1811). 
Malus was no more. 
The writings and discoveries of Malus present evi- 
In less than six months later 
OPTICS.—FRESNEL. 
903 
dence of great talent, but of far less fertility of com- Character 
bination than those of Fresnel, presently to be no- of Malus, 
ticed. He maintained the Newtonian theory of light. 
His reputation amongst his intimates was extremely 
high, and it was generally believed, that had he sur- 
vived, his discoveries would have extended much far- 
ther. To him was applied Newton's saying on the 
death of Cotes,—“ If Cotes had lived we should have 
learned something.” 
§ 3. Fresnen.—The Undulatory Theory of Light continued.—Difraction.—Transverse Vibra- 
tions ; Young.—Polarization and Double Refraction explained.—Lighthouse Illumination. 
Aveustin Fresyet was born at Broglie, in France, 
10th May 1788, of a feeble constitution, and he con- 
tinued throughout his too short life a prey to attacks 
of bad health. As a boy, his slow apprehension and 
uncertain memory gave no indication of the maturity 
of his judgment. He entered first the Polytechnic 
School, then that of Ponts et Chaussées. His fidelity 
to the Bourbon cause occasioned his being harshly 
treated by Napoleon, and he retired to Normandy in 
the beginning of 1815, to pursue the scientific studies 
which he had always loved. 
Diffracnon.—tThe theory of light in particular at- 
tracted his attention, and he had a steady belief that 
the Newtonian doctrine was erroneous, though in 
ignorance, as it appears, of the undulatory doctrines 
of Hooke, Huygens, and Young. The phenomena 
of diffraction, or the coloured fringes which are seen 
in the interior of the shadows of opake bodies when 
illuminated by a minute source of light, attracted 
his attention as most proper for deciding the deli- 
cate question of the molecular or undulatory cha- 
racter of light. The results of his experiments were 
detailed in a memoir confided in the first place to 
his friend Arago, and by him communicated to the 
Institute of France (October 1815). This remark- 
able paper contained much which Dr Young had al- 
ready discovered, and the explanations of the experi- 
ments which it described, both new and old, by the 
theory of undulations, were common to both. Dr 
Young having anticipated the publication by at least 
a dozen years, there could be no question of pri- 
ority ; but it is equally certain that Fresnel was un- 
aware of what Young had done until it was pointed 
out to him by Arago. His memoir, which was pub- 
lished in great part in the Annales de Chimie for 
1816, contains much which is interesting, The mode 
of observing the diffraction bands directly by means 
of a lens, without the intervention of a screen, was 
equally new and important. The observation that 
the interior fringes of the shadow of a narrow body, 
such as a wire, disappear when the light is intercepted 
on either side of the wire, leading to the conclusion 
that the union of the light from both sides is neces- 
sary for their occurrence, was (as we have seen) 
one of Young’s capital experiments, The explana- 
tion of Newton’s rings, by the interference of the light 
reflected from two adjacent surfaces, though partly 
anticipated by Hooke, was equally important. Nu- 
merous measures of the distances of the exterior 
diffraction bands from the geometrical shadow, as 
formed by homogeneous red light, are then given and 
compared with theory. Here Fresnel was on original 
ground. These accurate numerical comparisons, af- 
terwards pursued to a greater extent, constituted one 
of the most important bases of the new theory. In 
obtaining them he was materially aided by Arago, 
who, though considerably his senior, generously as- 
sisted him in every respect, and gave him the full ad- 
vantage of his station as a member of the Institute, 
and of his experience. 
Fresnel’s first memoir on diffraction justly excited 
so much notice that the subject was proposed by the 
Academy of Sciences in 1817 for one of their prizes. 
(487.) 
Second me- 
moir. 
proves 
The new essay which Fresnel then wrote was, as pro- upon 
bably had been anticipated, the successful one. 
this memoir he made an important step, by showing 
that the exterior fringes in diffraction shadows do not 
depend (as Young had supposed) upon the interfer- 
ence of the direct light with that reflected at a great 
obliquity from the edge of the diffracting body, but 
from the interference of the different elementary un- 
dulations which proceed from the disturbed surface 
forming the front of the grand wave. Decomposing 
the front of the wave into small portions after the 
manner of Huygens, he computed the disturbance 
produced by the integral effect of the whole at a given 
point of the screen where the picture of the shadow 
fell and was submitted to examination, and he found 
that such integral effects have a periodic character, 
presenting points of maximum and minimum distur- 
bance, or of greatest and least illumination as we re- 
cede from the geometrical shadow. These distances 
being measured in homogeneous red light were found 
to agree with the results of an arduous computation, 
requiring, as will easily be seen, an intimate acquain- 
tance with the integral caleulus and much skill in 
In Young’s 
th 
eory. 
Im- 
