HUYGENS, CHRISTIAAN (1629-1695), Dutch mathe matician, astronomer and physicist, was born at The Hague on April 14, 1629. From his father Constantijn Huygens, he re ceived the rudiments of his education, which was continued at Leyden. In 1651 he published an essay in which he attacked the unsound methods of Gregory of St. Vincent in his method of quadratures. This was followed by other papers on mathematical subjects.
In 1655, Huygens, working with his brother Constantijn on the improvement of the telescope, hit upon a new method of grinding and polishing lenses. The immediate results of the clearer defi nition obtained were the detection of a satellite to Saturn (the sixth in order of distance from its primary), and the resolution into their true form of the abnormal appendages to that planet. Huygens was also in 1656 the first effective observer of the Orion nebula; he delineated the bright region still known by his name, and detected the multiple character of its nuclear star. His appli cation of the pendulum to regulate the movement of clocks sprang from his experience of the need for an exact measure of time in observing the heavens. The invention dates from 1656; on June 16, 1657, Huygens presented his first "pendulum-clock" to the States-General ; and the Horologium, containing a description of the requisite mechanism, was published in 1658.
His reputation now became cosmopolitan. In 1663 he was elected a fellow of the Royal Society, and delivered before that body in Jan. 1669 a clear and concise statement of the laws governing the collision of elastic bodies. Although these con clusions were arrived at independently, they were in great measure anticipated by the communications on the same subject of John Wallis and Christopher Wren, made respectively in Nov. and Dec. 1668.
Huygens had before this time fixed his abode in France. In 1665 Colbert made to him on behalf of Louis XIV. an offer too tempting to be refused, and from 1666 to 1681 his residence at the Bibliotheque du Roi was only interrupted by two short visits to his native country. His magnum opus dates from this period. The Horologium oscillatorium (1673) contained many original discoveries. This work contains the first successful attempt to deal with the dynamics of a system. The determination of the true relation between the length of a pendulum and the time of its oscillation; the invention of the theory of evolutes; the discovery, hence ensuing, that the cycloid is its own evolute, and is tauto chronous; the ingenious although practically inoperative idea of correcting the "circular error" of the pendulum by applying cy cloidal cheeks to clocks—were all contained in this remarkable treatise. The theorems on centrifugal force in circular motion with which it concluded helped Newton to formulate his law of gravitation.
In 1681 he returned to Holland; perhaps because of the treat ment of his co-religionists in France. He now spent six years on the production of lenses of enormous focal distance, which, mounted on high poles, and connected with the eye-piece by means of a cord, formed what were called "aerial telescopes." Three of his object-glasses, of respectively 123, 18o and 210 ft. focal length, are in the possession of the Royal Society. He also succeeded in constructing an almost perfectly achromatic eye-piece, still known by his name. But his researches in physical optics constitute his chief title-deed to immortality. He developed the wave theory of light which had already been adopted by Hooke in 1665; he assumed that all the points of a wave-front originate secondary waves, the aggregate effect of which is to reconstitute the primary disturbance at the subsequent stages of its advance, thus accom plishing its propagation; so that each primary wave-front is the envelope of an indefinite number of secondary undulations. This resolution of the original wave is the well-known "Principle of Huygens," and by its means he was enabled to prove the funda mental laws of optics, and to assign the correct construction for the direction of the extraordinary ray in uniaxial crystals. These investigations, together with his experiments on polarization, are recorded in his Traite de la lumiere, published at Leyden in 169o, but composed in 1678. In the appended treatise Sur la Cause de la pesanteur, he rejected gravitation as a universal quality of mat ter, although admitting the Newtonian theory of the planetary revolutions. From his views on centrifugal force he deduced the oblate figure of the earth, estimating its compression, however, at little more than one-half its actual amount.
Huygens never married. He died at The Hague on June 8, 1695, bequeathing his manuscripts to the University of Leyden. In addition to the works already mentioned, his Cosmotheoros a speculation concerning the inhabitants of the planets—was printed posthumously at The Hague in 1698, and appeared almost simultane ously in an English translation. A volume entitled Opera posthuma (Leyden, 1703) contained his "Dioptrica," in which he calculated the magnifying power of a telescope, together with the shorter essays De vitris figurandis, De corona et parheliis, etc. An early tract De ratiociniis in ludo aleae, printed in 1657 with Schooten's Exercitationes mathematicae, is notable as one of the first formal treatises on the theory of probabilities ; nor should his investigations of the properties of the cissoid, logarithmic and catenary curves be left unnoticed. His invention of the spiral watch-spring was explained in the Journal des savants (Feb. 25, 1675) . An edition of his works was published by G. J. 's Gravesande, in four quarto volumes entitled Opera varia (Ley den, 1724) and Opera reliqua (Amsterdam, 1728) . His scientific corre spondence was edited by P. J. Uylenbroek from manuscripts preserved at Leyden. A complete edition of his works and correspondence was published by the Societe Hollandaise des Sciences (1888-1905, 10 vols.) . See Boscha, Christian Huygens (1895).