Apparently destined for a similar career as his father and brothers, Constantijn Huygens's second son, Christiaan, was sent to Leiden to study law. Actually, he spent most of his time studying mathematics with Frans Van Schooten (1615–60), the Latin translator of Descartes’ Geometry and one of the few who could understand it, in preparation of his first work, on the quadrature of the hyperbole and the ellipse (1651). With a powerful self-built telescope, he discovered in 1655 the first moon of Saturn (Titan) and made numerous other observations, which firmly established his reputation as an astronomer. Of great practical importance was his invention of the pendulum clock (patented in 1657). He was invited to become one of the founding members of the Académie des Sciences (1666) and until 1681 settled in Paris, returning several times to The Hague, though, for reasons of health. In Paris he became increasingly critical of Descartes’ physics, more particularly of his laws of collision (see law of nature). In 1689 he made a last journey to London, where he met Sir Isaac Newton. Although he admired Newton's mathematical genius, he remained skeptical of his “improbable principle of attraction” as well as his corpuscular theory of light. He died in 1695, his last years being devoted, among other things, to the problem of extraterrestrial life (published posthumously as Cosmotheoros, 1698). In his Traité de la lumière (1690), he provided a synthesis of his theory of light. Huygens was undoubtedly one of the greatest among the physicists, astronomers, and mathematicians of his age, bringing to Cartesian physics a mathematical as well as a practical and empirical mind, which allowed him to show the limits of the Cartesian method, unwillingly perhaps for in his heart he preferred a purely mechanical model.

See also Huygens, Constantijn; Laws of Nature; Light; Newton, Isaac; Physics