FRESN EL, AUGUSTIN JEAN, a very distinguished French mathe matician and natural philosopher, was born in 1788, at Broglie near Berney : his father, who was an architect, endeavoured early to com municate to him the rudiments of education; but considerable dif ficulty was experienced in effectiog this desirable object, partly front the delicate state of the pupil's health, and partly, it is supposed, from a distaste in the latter for the acquisition of that kind of knowledge which depends chiefly on the exercise of the memory; hence the youth made small progress in the study of languages, and he was eight years of age before he could write in a legible manner. An inquiring faculty was however manifest in him even at that time by the experiments which he made to determine the best materials and the best construc tions for the small machines used in tho sports of children.
At the age of sixteen years and a half he was admitted a pupil in the );cols Polytechuique, where he soon made great progress in the study of the sciences, and where he attracted the notice of Legendre by his solution of a problem which had been proposed by that mathe• matician as a trial of the abilities of the students. On leaving that institution he was appointed engineer in the department of the Pouts et•Chaussees.
It is remarkable however that it was not till the year 1814 that Fresnel began to study the branch of science in which he afterwards became so much distinguished. In that year he requested a friend, by letter, to inquire of his uncle what was meant by ' polarisation of light ; ' and it is to be presumed that he obtained the informatiou he sought, for in eight months from that time he appears to have made himself fully acquainted with the subject. In 1823 he was made a member of the Academie des Sciences at Paris; in 1825 he was elected a Fellow of the Royal Society of London, and two years this learned body awarded him the Rumford modal for his optical dis coveries. At the time of his death, which happened in 1827, lie hell the post of secretary to the Commission for the Lighthouses of France, and he was succeeded In this post by his brother, M. Leonor Fresno'.
The phenomena of the colours exhibited by crystallised plates when exposed to polarised light, and the riogs which appear to surround their axes, were attentively examined by Fresnel, and, in conjunction with his friend Arago, ho succeeded in reducing the interferences of polarised light to a few simple laws, which were duly verified by experimental analysis. This subject, which has also beau investigated by Sir David Brewster, M. Blot, and subsequently by M. Mitscherlic.h,
is treated by Franc' in a Memoire which was read to the institute of France in 1816. Ile gave • formula for the intensity of a previouely polarised ray when reflected from a surface under any angle of inci dence in a plane inclined to the plane of primitive polarisation; and this, with a general account of the deviations which the plane of polarisation undergoes in consequence of the reflexion, is contained In two Mitmoiree which were preseuted to the Academic des Sciences in 1517 and 1818.
In 1810 he gained the prize which in the preceding year had been proposed by the Institute for the beet memoir ou the diffraction of light. In his Memoir's be showed that rays passing at a sensible distance from a reflecting body deviate from their primitive direc tion and interfere with the direct rays; and, on the principles of the undulatory theory, he ascribed the effect to a number of small waves which originate with each portion of the surface of the primitive wave when it arrives at the reflecting surface. In his'Mernoire sur la Diffraction de Is Luntiere,' 4to, Paris, Fresnel has given a complete explanation, on the undulatory hypothesis, of the coloured fringes produced by an opaque object when exposed to a luminous point : he has also given a table of the several maxima and minima of the intensity of light beyond the limits of the geometrical shadow, and he determined that, within those limits, the light gradually diminishes till total darkness takes place. In order to examine the effects pro duced by the diffraction of light when it is made to pass through a small aperture, he caused the imago of the sun at the focus of a glass lens to fall precisely at the spot where a small circular orifice was made in a plate of metal ; and, placing before his eye another glass lens, be suffered the cone of light from the orifice to fall on the lens, when the image of the orifice appeared as a bright spot, surrounded by rings of light of different colours. With a micrometer Framel measured the diameters of these rings, and he has given an explana tion of the variations produced in the intensity of the light of the central spot when the distance of the eye-glass from the orifice is varied, lie observed also the succession of bright and dark band. which are produced when light from a radiant point is reflected from two plane mirrors Inclined to one another at an angle nearly equal to 180 degrees.