Christiaan Huygens (1629 - 1695), Dutch mathematician, physicist and astronomer.

Among many other things, Huygens worked on improving telescope lenses, and in 1655 he discovered Titan, a satellite of Saturn, and studied Saturn's impressive ring system. In 1659 he published his findings in Systema Saturnium.

Additionally, he developed a wave theory of light, as opposed to the corpuscular theory of Newton who saw light as particles, and formulated Huygen's Principle; "Concerning light waves, every point on a wave front is itself a source of new waves."

Huygens was also the inventor of the pendulum clock, and he was the first one to discover the polarization of light.

Christiaan Huygens was born on the 14th of April, 1629, at the Hague. He was the second son of Constantijn Huygens, a diplomat and a man of great learning, who was responsible for the initial instruction of his sons in science and other areas. Descartes was an occasional visitor at the Huygens’ house, but it does not appear that he ever met Christiaan. Huygens continued to be tutored at home until about the age of sixteen, at which point he was sent to the University of Leyden to study law. In 1646 or ’47 he transferred to Breda. In both places he had continued his studies in mathematics; in Leyden he was tutored by van Schooten, and in Breda by John Pell.

Huygens was in contact with Mersenne at about this time. Mersenne sent Huygens a number of problems, including that of the shape taken by a rope supported at both ends. Although Huygens did not solve this problem, he succeeded at a related one: how to attach weights to the rope so it would hang in the shape of a parabola. Huygens was introduced to the learned world by Mersenne, who lauded him as “the Dutch Archimedes”.

Huygens went to Denmark in 1649 as part of a diplomatic team, but time and especially the weather made it impossible for him to cross to Stockholm to visit Descartes, who was living there at the time. He did, however, exchange letters with Descartes, who called Huygens “a son of his own blood”.

Huygens published his first work on mathematics in 1651. It was an essay showing the fallacy in a system of quadratures proposed by Gregory of Saint-Vincent. Huygens followed this up with works on the quadrature of conic sections and an approximation to the area of the circle (the closest that had been made to that date).

In about 1654 Christiaan began to work with his brother on astronomy. They devised an improved method of grinding and polishing lenses. As a result of the improved telescopes they were able to build, Christiaan discovered a satellite to Saturn (Titan), and was able to correctly identify the observed elongation of that planet as a ring structure. Huygens' observations of the Moon and of Saturn's ring structure, as well as his discovery of the Orion Nebula, were published in his Systema Saturnium in 1659. It was the requirement for exact measurement of time when making astronomical observations which led Huygens to invent the pendulum-clock.

In 1655 Huygens made his home in Paris, receiving a pension from Louis XIV.

In 1666 the Royal Society set to a number of its members, including Huygens, the problem of the investigation of the laws of impact. Huygens' profound study of elastic impact (sent to the Royal Society in 1668) showed Descartes’ laws of impact to be erroneous. Huygens had doubted the validity of these for far longer, however; in 1654 he had written to van Schooten: “If all Descartes’ laws except the first are not false, I can only say that I cannot distinguish truth from falsehood”.

Perhaps the greatest of Huygens’ works was his Horologium oscillatorium, published in 1673. This work dealt with problems arising from the pendulum-clock. It contains the first attempt to apply dynamics to bodies of finite size, as opposed to particles. Among the many original discoveries in this work, Huygens showed that the centrifugal force on a body moving in a circle varies directly with the square of the velocity of the body and inversely with the radius of the circle.

In 1681 Huygens returned to Holland, possibly as a result of the increasing intolerance of the Catholics. At this point he took up again the study of astronomy, and especially optics, devoting himself to constructing lenses of enormous focal length. It was probably in this period that Huygens developed the almost perfectly achromatic eyepiece which still bears his name.

Huygens’ Treatise on Light was published in 1690, but most of it had been written back in 1678. This expounded the undulatory or wave theory of light, according to which space is filled with an ether, and the pulsations of a luminous body cause waves in this ether which are perceived as light. From this hypothesis Huygens deduced the laws of reflection and refraction, and explained certain other phenomena. He had also found the principal phenomena of polarisation by experiment.

Huygens died at the Hague on the eighth of June, 1695.

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