Page:The American Cyclopædia (1879) Volume VII.djvu/498

 486 FRESNEL FRESNEL, Angnstin Jean, a French physicist, born at Broglie, in Normandy, May 10, 1788, died at Ville d'Avray, near Paris, July 14, 182V. At a very early age he exhibited a taste for mechanical and physical science. In his 17th year he entered the polytechnic school, where he gained the applause of Legen- dre by a peculiar solution of a question in geometry. He passed thence to the school of bridges and roads. After graduating, he su- perintended the engineering operations of the government in the department of Vendee for eight years. His first memoir (1814) was a demonstration of the phenomenon of the stellar aberration. He went to Paris in 1815, in which year his first experimental researches were made, and from this time until his death his discoveries and scientific memoirs followed each other rapidly. At the commencement of 1815 he did not know what was meant by the term polarization of light, and in less than a year he stood at the head of investigators of the subject. In 1819 he gained a prize offered by the French academy of sciences for an article on diffraction. In 1823 he was elected member of the academy by a unanimous vote. In 1825 he was made an associate of the royal society of London, and in 1827 that society awarded him the Rumford medal, which was presented to him upon his deathbed by his friend and collaborator Arago. In May ,,1824, he was appointed secretary of the commission of light- houses. He was at the same time engineer of the pavements of Paris and one of the exami- ners of the polytechnic school. From the end of 1824 until his death his health was so bad from the effects of unremitting labor that he was obliged to give up all work. The true laws of the complicated phenomena of double refrac- tion were demonstrated by Fresnel. It is now known that nearly all crystals possess the prop- erty of double refraction. Before Fresnel's in- vestigations it was supposed that it belonged only to Iceland spar and quartz. Fresnel in con- junction with Arago explained the interferences of polarized light, giving the phenomena and determining their laws. He proved that all the colors engendered in doubly refracting crys- tals are particular cases of the interference of polarized light, and also discovered the phe- nomena which are called circular polarization, and explained their laws. He was an able and enthusiastic advocate of the wave theory of light, against that of emission or material em- anations. In 1811 a lighthouse board or com- mission of lighthouses was formed in France. One of the duties of this commission was to de- termine whether the lighting apparatus might not be improved. In 1819 Arago volunteered to take charge of the experiments on the sub- ject, provided Fresnel and Mathieu were joined with him. The proposition was accepted, and Fresnel devoted the whole strength of his mind to the subject. The result was the system of lens-lighting apparatus which has changed the mode of lighthouse illumination throughout the world, and is universally known as the Fres- nel system. The most perfect system known before Fresnel's was that of parabolic reflectors. In this, for a fixed light, the reflectors are ar- ranged around one or more horizontal circles with their axes parallel to the horizon, and passing (produced) through the centres of the circles. In a revolving light the reflectors are arranged with their axes parallel to each other and to the horizon. By making the system revolve, a bright flash is produced by the combined action of all the reflectors, when the eye is in or near the axis of one of them. As the rays proceeding from a lamp at the focus of a parabolic reflector are parallel to the axis after deviation by the re- flector, it is evident that systems arranged as above indicated will show a bright light in the horizon to an observer situated in or near the axis of any one 'of the reflectors, since the re- flected beam does not lose its intensity except by atmospheric absorption. Therefore the greater the number of reflectors, the better will be the light ; and to produce as nearly as possible a uniform light at the horizon, the number of reflectors in important fixed lights is sometimes very great, as many as 24 having been used. In all cases the reflectors are made of copper care- fully shaped to the form of a paraboloid of rev- olution, and covered with a uniform coating of pure silver. The objections to the reflector sys- tem are : 1, the want of uniformity of the light ; 2, the great expense, each lamp requiring 50 gallons of sperm oil per annum ; 3, the rapid deterioration of the reflectors from the necessity of daily cleaning the silvered surface, the silver- ing requiring entire renewal at least once in ten years ; 4, the great loss of light caused by the reflection and by the necessary imperfections in form in a parabolic reflecting surface. As soon as he began to study the subject, Fresnel con- ceived the idea of substituting lenses for the reflectors. A convex lens possesses the prop- erty of making all rays proceeding from its principal focus parallel after deviation. It pro- duces the effect by refraction that parabolic reflectors produce by reflection. If therefore a plano-convex lens could be formed which would not much exceed in thickness ordinary plate glass, the loss of light by absorption in passing through such a lens would be much less than in the case of reflection. For the two refracting surfaces the loss does not much exceed ^, while by reflection it is about. But if the exterior surface of the lens is spherical, it is evident that, supposing the lens to embrace all rays which are contained in a belt 22-J- above and 22^ below the horizon, and in a horizontal angle of 45, the thickness would become so great for a large principal focal distance that much of the light would be absorbed, and the lens would become useless. The weight, too, would be so great, that it would be nearly im- possible to make the apparatus revolve by ma- chinery available at the top of a lighthouse. For these reasons a lens light which existed in