in a paper entitled 'Considerations theoriques Bur la Polarisation de is Lumiere; which is printed in the 'Bulletin de la Societe Philo matiliqne; 1824, Freanel assumes that the eye is affected only by those vibratory motions of the particles of ether which take place trans versely, or in planes perpendicular to the direction of the motion of the wave. Unpolarised light he conceived to consist of a rapid suc cession of waves in which the vibrations are performed in every direction perpendicularly to that of the ray ; and common polarised light to consist in the transverse vibrations being parallel to one Plano passing through the direction of the ray. By combining the hypothesis of transverse vibrations with the theory of undulations, Fresuel obtained formulte for the intensity of reflected light at any angle of incidence.
lie conceived that the phenomena of double refraction in crystal( with one axis depend upon a modification of the actions of the ether by the action of the molecules of the crystal; the elasticity of the latter molecules in a direction perpendicular to the axis being sup posed to be different from the elasticity in a direction parallel to the axis. In the year 1821 Fresnel presented to the Academia des Sciences a Meuloire in which the properties of double refraction and polarisatiou in biaxal crystals were contemplated ; and bo investigated what he called the "surface of elasticity; " a superficies conceived te be such that the force of elasticity by which the vibrations of a mole. cute in the direction of the radius of such surface are regulated it proportional to the square of that radius. Ile also gave, for doubl] refracting crystals with two axes, an indication of the general equation to a wave surface, which has since been investigated by Ampere and Meccullagh.
Fre/nor' Inquiries were also directed to the subject of rotator] polarisation; and he found by experiments that the phenomena might be explained by conceiving the molecules of ether, which give rise to the rays In the direction of the axis of the quartz, or of tho fluids it which the like pheuomeca are exhibited, to revolve uniformly it circles, with different velocities, some from right to left, and others in • contrary direction. The colours produced by such media he conceived to be owing to the Interferences of two rays or pencils it which the molecules revolve in opposite directions.
Fresnel also proved that light was circularly polarised by two tots reflections from glass at an angle equal to about 54° 37'; an placing a crystallised plate between two rhomboids of glass, each o polarised the light circularly, and bad their planes of reflection at right angles to one another, he observed that the light transmitter through the system exhibited phenomena similar to those which an seeu along the axis of quartz. Ile succeeded in exhibiting before dm toademy, hi 1822, a division of the pencils, so that a line appeared loubla, by making the light pass through prism), of glass which were ubject to strong preasnre by means of screws ; and he was the first vho observed the change produced by heat on the tints of sulphate of line, a subject which has since been more completely investigated by d. Mitecherlich.
Apparently unacquainted with what had been previously proposed >y Brewster for a like purpose, this philosopher and eugiueer devised ,everal constructions of great lenses for lighthouse ; one of these tonstructions oonsisted of five concentric) spherical zones (01mo :4:lave%) of glass disposed about a central lens, the whole being cou ained in a square frame. Eight of these, in vertical positions, oustituted an octagonal case or lantern, which revolved about a (ordeal axis, and had in its centre a powerful lamp. Another con.
itruction consisted of two great lanterns, as they may be called, one within the other; the convex surface of each was formed uf thin tyliudrical refractor', and both revolved about a lamp in their common mitt : the cylinders were so disposed as to produce, by the revolution, ameartant flashes of light.
Fresnel made some experiments for the purpose of decomposing water by means of a magnet ; and the method which he pursued con sisted in producing a current in an electroonagnetio helix inclosing bar-magnet covered with silk ; on plunging the ends of the wire in water, he obtained some remarkable effects; but, being unable to account for certain anomalies which ho observed, he abandoned the project.
(1)uleau, Notice fur Francl ; Biographic Unirersclle.)