Born Apr. 14, 1629, in The Hague; died there July 8, 1695. Dutch engineer, physicist, and mathematician; originator of the wave theory of light. First foreign member of the London Royal Society (1663). He studied at the Universities of Leiden and Breda, devoting himself to jurisprudence and mathematics.

At the age of 22, Huygens published a paper on the determination of the length of the arcs of circles, ellipses, and hyperbola. In 1654 he published the work On the Determination of the Value of the Circumference, which was an extremely important contribution to the theory of determination of the ratio of the circumference to the diameter of a circle (the calculation of π). This was followed by other important mathematical treatises on the study of the cycloid and logarithmic and catenary curves. His treatise On Calculations Made While Playing Dice(1657) is one of the first investigations in probability theory. Together with R. Hooke, Huygens established the fixed points of the thermometer— the melting point and boiling point of water.

During this period Huygens also worked on improving the lenses of astronomical telescopes, trying to increase their aperture ratios and remove chromatic aberration. In 1665, using these telescopes, Huygens discovered a satellite of the planet Saturn (Titan), ascertained its period of rotation, and established that Saturn is surrounded by a thin ring that does not touch it at any point and is inclined toward the ecliptic. Huygens gave all his observations in the classical work The System of Saturn(1659), in which he also described for the first time the nebulas in the constellation Orion and reported bands on the surfaces of Jupiter and Mars.

Astronomical observations required accurate and convenient measurement of time. In 1657, Huygens invented the first pendulum clocks, equipped with an escapement mechanism; he described his invention in the work Pendulum Clocks(1658). The second enlarged edition of this work was published in Paris in 1673. In its first four parts, Huygens studied a number of problems connected with the motion of a pendulum. He gave a solution to the problem of finding the center of oscillation of a physical pendulum, the first problem in the history of mechanics about the motion of a system of connected mass points in a given force field. In the same work Huygens established the tautochronous nature of motion along a cycloid and, having worked out the theory of evolutes of plane curves, showed the evolute of a cycloid to be another cycloid, but situated differently with respect to the axis.

In 1665, when the French Academy of Sciences was founded, Huygens was invited to Paris to be its president, and he lived there almost continously for 16 years (1665–81). In 1680, Huygens worked to create a “planetary machine,” the prototype of the modern planetarium, for the construction of which he developed a rather complete theory of chain, or continued, fractions. This was the last work he completed in Paris.

In 1681, having returned to his native land, Huygens again occupied himself with work on optics. In 1681–87 he ground lenses with large focal lengths (37, 54, and 63 m). At the same time he constructed the eyepiece bearing his name, which is still being used. Huygens completed the whole cycle of optical labors with the famous Treatise on Light(1690), in which the wave theory of light was for the first time expounded clearly and applied to explain optical phenomena. In Chapter 5 of the Treatise on Light,Huygens explained the phenomenon of double refraction, which was discovered in crystals of Iceland spar; the classical theory of refraction in optically uniaxial crystals is still propounded on the basis of that chapter.

Huygens added an appendix to the Treatise on Light in the form of the discussion “On the Causes of Gravity,” in which he came close to discovering the law of universal gravitation. In his last treatise, Cosmotheoros(1698), published posthumously, Huygens took as a basis the theory of multiplicity of worlds and their habitation. In 1717 the treatise was translated into Russian by an order of Peter I.