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# Encyclopedia

Christiaan Huygens

Christiaan Huygens
Born 14 April 1629
The Hague, Netherlands
Died 8 July 1695 (aged 66)
Netherlands
Residence Netherlands, France
Nationality Dutch
Fields Physics
Mathematics
Astronomy
Horology
Science fiction
Institutions Royal Society of London
Alma mater University of Leiden
College of Orange
John Pell
Known for Titan
Explanation Saturn's rings
Centrifugal force
Collision formulae
Pendulum clock
Huygens–Fresnel principle
Wave theory
Birefringence
First theoretical physicist
Influences René Descartes
Frans van Schooten
Blaise Pascal
Marin Mersenne
Influenced Gottfried Wilhelm Leibniz
Isaac Newton

Christiaan Huygens, FRS (English pronunciation: /ˈhaɪɡənz/, Dutch: [ˈhœyɣəns]; 14 April 1629 – 8 July 1695) was a prominent Dutch mathematician, astronomer, physicist, horologist, and writer of early science fiction. His work included early telescopic studies elucidating the nature of the rings of Saturn and the discovery of its moon Titan, the invention of the pendulum clock and other investigations in timekeeping, and studies of both optics and the centrifugal force.

Huygens achieved note for his argument that light consists of waves,[1] now known as the Huygens–Fresnel principle, which became instrumental in the understanding of wave-particle duality. He generally receives credit for his discovery of the centrifugal force, the laws for collision of bodies, for his role in the development of modern calculus and his original observations on sound perception (see repetition pitch). Huygens is seen as the first theoretical physicist as he was the first to use formulae in physics.[citation needed]

## Life

Christiaan Huygens by Bernard Vaillant, Museum Hofwijck, Voorburg
Christiaan Huygens. Cut from the engraving following the painting of Caspar Netscher by G. Edelinck, between 1684 and 1687.
Huygens' giant telescope without tube. Picture from his 1684 Astroscopia Compendiaria tubi optici molimine liberata (compound telescopes without a tube)
Huygens' explanation for the aspects of Saturn, Systema Saturnium, 1659.

Christiaan Huygens was born in April 1629 at The Hague, the second son of Constantijn Huygens, (1596–1687), a friend of mathematician and philosopher René Descartes, and of Suzanna van Baerle (deceased 1637), whom Constantijn had married on 6 April 1627. Christiaan studied law and mathematics at the University of Leiden and the College of Orange in Breda. After a stint as a diplomat, Huygens turned to science.

### French Academy of Sciences and Royal Society

The Royal Society elected Huygens a member in 1663. In the year 1666 Huygens moved to Paris where he held a position at the French Academy of Sciences under the patronage of Louis XIV. Using the Paris Observatory (completed in 1672) he made further astronomical observations. In 1684 he published "Astroscopia Compendiaria" which presented his new aerial (tubeless) telescope.

### Death

Huygens moved back to The Hague in 1681 after suffering serious illness. He attempted to return to France in 1685 but the revocation of the Edict of Nantes precluded this move. Huygens died in The Hague on 8 July 1695, and was buried in the Grote Kerk.[2]

## Scientific work

### Mathematics

#### Probability theory

After Blaise Pascal encouraged him to do so, Huygens wrote the first book on probability theory,[3] De ratiociniis in ludo aleae ("On Reasoning in Games of Chance"),[4] which he had published in 1657.

### Physics

#### Mechanics

Huygens formulated what is now known as the second law of motion of Isaac Newton in a quadratic form. Newton reformulated and generalized that law. In 1659 Huygens derived the now well-known formula for the centrifugal force, exerted by an object describing a circular motion, for instance on the string to which it is attached, in modern notation:

$F_{cf}=\frac{m\ v^2}{r}$

with m the mass of the object, v the velocity and r the radius. Furthermore, Huygens concluded that Descartes' laws for the elastic collision of two bodies must be wrong and formulated the correct laws.

#### Wave theory

Huygens is remembered especially for his wave theory of light, expounded in his Traité de la lumière (see also Huygens-Fresnel principle). The later theory of light by Isaac Newton in his Opticks proposed a different explanation for reflection, refraction and interference of light assuming the existence of light particles. The interference experiments of Thomas Young vindicated Huygens' wave theory in 1801, as the results could no longer be explained with light particles (see however wave-particle duality).

#### Optics

Huygens experimented with double refraction (birefringence) in Icelandic crystal (calcite) and explained it with his wavetheory and polarised light.

#### Clocks

He also worked on the design of accurate clocks, suitable for naval navigation. His invention of the pendulum clock, patented in 1657, was a breakthrough in timekeeping. Huygens was a scholar, scientist, and inventor, not a clockmaker, and is not known to have ever made any clock himself; he contracted the construction of his clock designs to Salomon Coster in The Hague, who actually built the first pendulum clocks.

In 1673 he published his mathematical analysis of pendulums, Horologium Oscillatorium sive de motu pendulorum, his greatest work on horology. It had been discovered that pendulums are not isochronous for large swings; that is, their period depends on the width of swing. Huygens analysed this problem by finding the shape of the curve down which a mass will slide under the influence of gravity in the same amount of time, regardless of its starting point; the so-called tautochrone problem. By geometrical methods which were an early use of calculus, he showed that this curve is a cycloid, not the circular arc of a pendulum's bob, so pendulums are not isochronous. He also solved the problem of how to calculate the period of a pendulum made of an arbitrarily-shaped swinging rigid body, discovering the center of oscillation and its reciprocal relationship with the pivot point. In the same work, he analysed the conical pendulum, consisting of a weight on a cord moving in a circle, using the concept of centrifugal force.

Huygens was the first to derive the formula for the period of an ideal mathematical pendulum (with massless rod or cord), in modern notation:

$T = 2 \pi \sqrt{\frac{l}{g}}$

with T the period, l the length of the pendulum and g the gravitational acceleration.

Huygens also observed that two pendulums mounted next to each other on the same support will become synchronized, swinging in opposite directions, which he referred to as "odd sympathy". This was the first observation of what is now called coupled oscillations.

The oldest known Huygens style pendulum clock is dated 1657 and can be seen at the Museum Boerhaave in Leiden[5][6][7][8] which also shows an important astronomical clock owned and used by Huygens.

Huygens also developed a balance spring clock more or less contemporaneously with, though separately from, Robert Hooke, and controversy over whose invention was the earlier persisted for centuries. In February 2006, a long-lost copy of Hooke's handwritten notes from several decades' Royal Society meetings was discovered in a cupboard in Hampshire, and the balance-spring controversy appears by evidence contained in those notes to be settled in favor of Hooke's claim.[9][10]

#### Internal combustion and other inventions

In 1673, Huygens carried out experiments with internal combustion. Although he designed a basic form of internal combustion engine, fueled by gunpowder, he never successfully built one.

In 1675, Christiaan Huygens patented a pocket watch. He also invented numerous other devices, including a 31 tone to the octave keyboard instrument which made use of his discovery of 31 equal temperament.

### Astronomy

#### Saturn's rings and Titan

In 1655, Huygens proposed that Saturn was surrounded by a solid ring, "a thin, flat ring, nowhere touching, and inclined to the ecliptic." Using a 50 power refracting telescope that he designed himself, Huygens also discovered the first of Saturn's moons, Titan.[11] In the same year he observed and sketched the Orion Nebula. His drawing, the first such known of the Orion nebula, was published in Systema Saturnium in 1659. Using his modern telescope he succeeded in subdividing the nebula into different stars. (The brighter interior of the Orion Nebula bears the name of the Huygens Region in his honour.) He also discovered several interstellar nebulae and some double stars.

#### Transit of Mercury

Possible depiction of Huygens left of center, detail from l'Établissement de l'Académie des Sciences et fondation de l'observatoire', 1666 by Henri Testelin. Colbert presents the members of the newly founded Académie des Sciences to king Louis XIV of France, around 1675.
Huygens on the Dutch 25-guilder banknote from the 1950's, showing his country house Hofwijck, Saturn, its moon Titan and an eclipse.

On May 3, 1661, he observed planet Mercury transit over the Sun, using the telescope of telescope maker Richard Reeves in London together with astronomer Thomas Streete and Richard Reeves.[12]

## Extraterrestrial Life

Christiaan Huygens believed in existence of extraterrestrial life. Prior to his death in 1695, he completed a book entitled Cosmotheoros in which he discussed his notions on extraterrestrial life. Huygens was of the opinion that life on other planets is pretty much similar to that on Earth. He thought that availability of water in liquid form was essential for existence of life and therefore the properties of water should vary from planet to planet, since the kind of water that is found on Earth would instantly freeze on Jupiter and vaporize on Venus. He even reported observing dark and bright spots on the surface of planet Mars and Jupiter. This he explained could only be justified by existence of water and ice on those planets.[13]

## Works

• 1649 - De iis quae liquido supernatant (About the parts above the warer, unpublished)
• 1651 - Cyclometriae
• 1651 - Theoremata de quadratura hyperboles, ellipsis et circuli (theorems concerning the quadrature of the hyperbola, ellipse and circle, Huygens' first publication)
• 1654 - De circuli magnitudine inventa
• 1656 - De Saturni Luna observatio nova (About the new observation of the moon of Saturn - discovery of Titan)
• 1656 - De motu corporum ex percussione, published only in 1703
• 1657 - De ratiociniis in ludo aleae = Van reeckening in spelen van geluck (translated by Frans van Schooten)
• 1659 - Systema saturnium
• 1673 - Horologium oscillatorium sive de motu pendularium (theory and design of the pendulum clock, dedicated to Louis XIV of France)
• 1673 - De vi centrifuga (Concerning the centrifugal force)
• 1684 - Astroscopia Compendiaria tubi optici molimine liberata (compound telescopes without a tube)
• 1685 - Memoriën aengaende het slijpen van glasen tot verrekijckers (How to grind telescope lenses)
• 1686 - Kort onderwijs aengaende het gebruijck der horologiën tot het vinden der lenghten van Oost en West (How to use clocks to establish the longitude
• 1690 - Traité de la lumière
• 1690 - Discours de la cause de la pesanteur (Discourse about gravity, from 1669?)
• 1691 - Lettre touchant le cycle harmonique (Rotterdam, concerning the 31-tone system)
• 1698 - Cosmotheoros , sciencefiction
• 1703 - Opuscula posthuma including
• De motu corporum ex percussione (Concerning the motions of colliding bodies - contains the first correct laws for collision, dating from 1656).
• Descriptio automati planetarii (description and design of a planetarium)
• 1724 - Novus cyclus harmonicus (Leiden, after Huygens' death)
• 1728 - Christiani Hugenii Zuilichemii, dum viveret Zelhemii toparchae, opuscula posthuma ... (pub. 1728) Alternate title: Opera reliqua, concerning optics and physics
• 1888-1950 - Huygens, Christiaan. Oeuvres complètes. The Hague Complete work, editors D. Bierens de Haan (tome=deel 1-5), J. Bosscha (6-10), D.J. Korteweg (11-15), A.A. Nijland (15), J.A. Vollgraf (16-22).
Tome I: Correspondance 1638-1656 (1888). Tome II: Correspondance 1657-1659 (1889). Tome III: Correspondance 1660-1661 (1890). Tome IV: Correspondance 1662-1663 (1891). Tome V: Correspondance 1664-1665 (1893). Tome VI: Correspondance 1666-1669 (1895). Tome VII: Correspondance 1670-1675 (1897). Tome VIII: Correspondance 1676-1684 (1899). Tome IX: Correspondance 1685-1690 (1901). Tome X: Correspondance 1691-1695 (1905).
Tome XI: Travaux mathématiques 1645-1651 (1908). Tome XII: Travaux mathématiques pures 1652-1656 (1910).
Tome XIII, Fasc. I: Dioptrique 1653, 1666 (1916). Tome XIII, Fasc. II: Dioptrique 1685-1692 (1916).
Tome XIV: Calcul des probabilités. Travaux de mathématiques pures 1655-1666 (1920).
Tome XV: Observations astronomiques. Système de Saturne. Travaux astronomiques 1658-1666 (1925).
Tome XVI: Mécanique jusqu’à 1666. Percussion. Question de l’existence et de la perceptibilité du mouvement absolu. Force centrifuge (1929). Tome XVII: L’horloge à pendule de 1651 à 1666. Travaux divers de physique, de mécanique et de technique de 1650 à 1666. Traité des couronnes et des parhélies (1662 ou 1663) (1932). Tome XVIII: L'horloge à pendule ou à balancier de 1666 à 1695. Anecdota (1934). Tome XIX: Mécanique théorique et physique de 1666 à 1695. Huygens à l’Académie royale des sciences (1937).
Tome XX: Musique et mathématique. Musique. Mathématiques de 1666 à 1695 (1940).
Tome XXI: Cosmologie (1944).
Tome XXII: Supplément à la correspondance. Varia. Biographie de Chr. Huygens. Catalogue de la vente des livres de Chr. Huygens (1950).

## References

1. ^ Christiaan Huygens, Traité de la lumiere (Leiden, Netherlands: Pieter van der Aa, 1690), Chapter 1. (Note: In the preface to his Traité, Huygens states that in 1678 he first communicated his book to the French Royal Academy of Sciences.)
2. ^ GroteKerkDenHaag.nl
3. ^ "I believe that we do not know anything for certain, but everything probably." —Christiaan Huygens, Letter to Pierre Perrault, 'Sur la préface de M. Perrault de son traité del'Origine des fontaines' [1763], Oeuvres Complétes de Christiaan Huygens (1897), Vol. 7, 298. Quoted in Jacques Roger, The Life Sciences in Eighteenth-Century French Thought, ed. Keith R. Benson and trans. Robert Ellrich (1997), 163. Quotation selected by W.F. Bynum and Roy Porter (eds., 2005), Oxford Dictionary of Scientific Quotations ISBN 0-19-858409-1 p. 317 quotation 4.
4. ^ p963-965, Jan Gullberg, Mathematics from the birth of numbers, W. W. Norton & Company; ISBN 039304002X ISBN 978-0393040029
5. ^ Hans van den Ende: "Huygens's Legacy, The Golden Age of the Pendulum Clock", Fromanteel Ldt., 2004,
6. ^ van Kersen, Frits & van den Ende, Hans: Oppwindende Klokken - De Gouden Eeuw van het Slingeruurwerk 12 September - 29 November 2004 [Exhibition Catalog Paleis Het Loo]; Apeldoorn: Paleis Het Loo,2004,
7. ^ Hooijmaijers, Hans; Telling time - Devices for time measurement in museum Boerhaave - A Descriptive Catalogue; Leiden: Museum Boerhaave, 2005
8. ^ No Author given; Chistiaan Huygens 1629-1695, Chapter 1: Slingeruurwerken; Leiden: Museum Boerhaave, 1988
9. ^ nature - International Weekly Journal of Science, number 439, pages 638-639, 9 February 2006
10. ^ Notes and Records of the Royal Society (2006) 60, pages 235-239, 'Report - The Return of the Hooke Folio' by Robyn Adams and Lisa Jardine
11. ^ Ron Baalke, Historical Background of Saturn's Rings
12. ^ Peter Louwman, Christiaan Huygens and his telescopes, Proceedings of the International Conference from discovery to Encounter, 13 – 17 April 2004, ESTEC, Noordwijk, Netherlands, ESA, sp 1278, Paris 2004
13. ^ Johar Huzefa (2009) Nothing But The Facts - Christiaan Huygens

• Andriesse, C.D., 2005, Huygens The Man Behind the Principle. Foreword by Sally Miedema. Cambridge University Press.
• Boyer, C.B.: A history of mathematics, New York, 1968
• Dijksterhuis, E. J.: The Mechanization of the World Picture: Pythagoras to Newton
• Hooijmaijers, H.: Telling time - Devices for time measurement in Museum Boerhaave - A Descriptive Catalogue, Leiden, Museum Boerhaave, 2005
• Struik, D.J.: A history of mathematics
• Van den Ende, H. et al.: Huygens's Legacy, The golden age of the pendulum clock, Fromanteel Ltd, Castle Town, Isle of Man, 2004
• Yoder, J G., 2005, "Book on the pendulum clock" in Ivor Grattan-Guinness, ed., Landmark Writings in Western Mathematics. Elsevier: 33-45.
• Christiaan Huygens (1629-1695) : Library of Congress Citations. Retrieved 2005-03-30.

# Quotes

Up to date as of January 14, 2010

### From Wikiquote

Christiaan Huygens () was a Dutch astronomer.

## Sourced

• Since 'tis certain that Earth and Jupiter have their Water and Clouds, there is no reason why the other Planets should be without them. I can't say that they are exactly of the same nature with our Water; but that they should be liquid their use requires, as their beauty does that they be clear. This Water of ours, in Jupiter or Saturn, would be frozen up instantly by reason of the vast distance of the Sun. Every Planet therefore must have its own Waters of such a temper not liable to Frost.
• What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock’d with so many Herbs, Trees and Animals, and adorn’d with so many Seas and Mountains! And how must our wonder and admiration be encreased when we consider the prodigious distance and multitude of the Stars?

## Disputed

• The world is my country, to promote science is my religion.
• The earliest citation of this remark yet found is in The Making of Modern Europe, 1648-1780 (1985) by Geoffrey Treasure, p. 474, where it is declared that this was Huygens' "motto" — but this seems very similar to the much more famous and long attested declaration of Thomas Paine in Rights of Man (1791): "My country is the world, and my religion is to do good." which has long been paraphrased "The world is my country, and to do good is my religion."
• The world is my country, science my religion. from Cosmos: A Personal Voyage - Episode 6 - Huygens

# 1911 encyclopedia

Up to date as of January 14, 2010

### From LoveToKnow 1911

CHRISTIAAN HUYGENS (1629-1695), Dutch mathematician, mechanician, astronomer and physicist, was born at the Hague on the 14th of April 1629. He was the second son of Sir Constantijn Huygens. From his father he received the rudiments of his education, which was continued at Leiden under A. Vinnius and F. van Schooten, and completed in the juridical school of Breda. His mathematical bent, however, soon diverted him from legal studies, and the perusal of some of his earliest theorems enabled Descartes to predict his future greatness. In 1649 he accompanied the mission of Henry, count of Nassau, to Denmark, and in 1651 entered the lists of science as an assailant of the unsound system of quadratures adopted by Gregory of St Vincent. This first essay (Exetasis quadraturae circuli, Leiden, 1651) was quickly succeeded by his Theoremata de quadratura hyperboles, ellipsis, et circuli; while, in a treatise entitled De circuli magnitudine inventa, he made, three years later, the closest approximation so far obtained to the ratio of the circumference to the diameter of a circle.

Another class of subjects was now to engage his attention. The improvement of the telescope was justly regarded as a sine qua non for the advancement of astronomical knowledge. But the difficulties interposed by spherical and chromatic aberration had arrested progress in that direction until, in 1655, Huygens, working with his brother Constantijn, hit upon a new method of grinding and polishing lenses. The immediate results of the clearer definition 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. Each discovery in turn was, according to the prevailing custom, announced to the learned world under the veil of an anagram - removed, in the case of the first, by the publication, early in 1656, of the little tract De Saturni luna observatio nova; but retained, as regards the second, until 1659, when in the Systema Saturnium the varying appearances of the so-called "triple planet" were clearly explained as the phases of a ring inclined at an angle of 28° to the ecliptic. 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 application 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 the 16th of June 1657 Huygens presented his first "pendulumclock" to the states-general; and the Horologium, containing a description of the requisite mechanism, was published in 1658.

His reputation now became cosmopolitan. As early as 1655 the university of Angers had distinguished him with an honorary degree of doctor of laws. In 1663, on the occasion of his second visit to England, he was elected a fellow of the Royal Society, and imparted to that body in January 1669 a clear and concise statement of the laws governing the collision of elastic bodies. Although these conclusions were arrived at independently, and, as it would seem, several years previous to their publication, they were in great measure anticipated by the communications on the same subject of John Wallis and Christopher Wren, made respectively in November and December 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 between the following year and 1681 his residence in the philosophic seclusion of 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, published with a dedication to his royal patron in 1673, contained original discoveries sufficient to have furnished materials for half a dozen striking disquisitions. His solution of the celebrated problem of the "centre of oscillation" formed in itself an important event in the history of mechanics. Assuming as an axiom that the centre of gravity of any number of interdependent bodies cannot rise higher than the point from which it fell, he arrived, by anticipating in the particular case the general principle of the conservation of vis viva, at correct although not strictly demonstrated conclusions. His treatment of the subject was 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 strictly isochronous; the ingenious although practically inoperative idea of correcting the "circular error" of the pendulum by applying cycloidal cheeks to clocks - were all contained in this remarkable treatise. The theorems on the composition of forces in circular motion with which it concluded formed the true prelude to Newton's Principia, and would alone suffice to establish the claim of Huygens to the highest rank among mechanical inventors.

In 1681 he finally severed his French connexions, and returned to Holland. The harsher measures which about that time began to be adopted towards his co-religionists in France are usually assigned as the motive of this step. He now devoted himself during six years to 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, 180 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. Although Robert Hooke in 1668 and Ignace Pardies in 1672 had adopted a vibratory hypothesis of light, the conception was a mere floating possibility until Huygens provided it with a sure foundation. His powerful scientific imagination enabled him to realize that all the points of a wavefront originate partial waves, the aggregate effect of which is to reconstitute the primary disturbance at the subsequent stages of its advance, thus accomplishing its propagation; so that each primary undulation 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 fundamental laws of optics, and to assign the correct construction for the direction of the extraordinary ray in uniaxial crystals. These investigations, together with his discovery of the "wonderful phenomenon" of polarization, are recorded in his Traite de la lumiere, published at Leiden in 1690, but composed in 1678. In the appended treatise Sur la Cause de la pesanteur, he rejected gravitation as a universal quality of matter, 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 the 8th of June 1695, bequeathing his manuscripts to the university of Leiden, and his considerable property to the sons of his younger brother. In character he was as estimable as he was brilliant in intellect. Although, like most men of strong originative power, he assimilated with difficulty the ideas of others, his tardiness sprang rather from inability to depart from the track of his own methods than from reluctance to acknowledge the merits of his competitors.

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 simultaneously in an English translation. A volume entitled Opera posthuma (Leiden, 1703) contained his "Dioptrica," in which the ratio between the respective focal lengths of object-glass and eye-glass is given as the measure of magnifying power, together with the shorter essays De vitris figurandis, De corona et parheliis, &c. An early tract De ratiociniis tin ludo aleae, printed in 16J7 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 (Leiden, 1724) and Opera reliqua (Amsterdam, 1728). His scientific correspondence was edited by P. J. Uylenbroek from manuscripts preserved at Leiden, with the title Christiani Hugenii aliorumque seculi XVII. virorum celebrium exercitationes mathematicae et philosophicae (the Hague, 1833).

The publication of a monumental edition of the letters and works of Huygens was undertaken at the Hague by the Societe Hollandaise des Sciences, with the heading ¦uvres de Christian Huygens (1888), &c. Ten quarto volumes, comprising the whole of his correspondence, had already been issued in 1905. A biography of Huygens was prefixed to his Opera varia (1724); his Eloge in the character of a French academician was printed by J. A. N. Condorcet in 1773. Consult further: P. J. Uylenbroek, Oratio de fratribus Christiano atque Constantino Hugenio (Groningen, 1838); P. Harting, Christiaan Huygens in zijn Leven en Werken geschetzt (Groningen, 1868); J. B. J. Delambre, Hist. de l'astronomie moderne (ii. 549); J. E. Montucla, Hist. des mathemaliques (ii. 84, 4 12, 549); M. Chasles, Apercu historique sur l'origine des methodes en geometrie, pp. 101-109; E. Daring, Kritische Geschichte der allgemeinen Principien der Mechanik, Abschnitt (ii. 120, 163, iii. 227); A. Berry, A Short History of Astronomy, p. 200; R. Wolf, Geschichte der Astronomic, passim; Houzeau, Bibliographie astronomique (ii. 169); F. Kaiser, Astr. Nach. (xxv. 245, 1847); Tijdschrift voor de Wetenschappen (i. 7, 1848); Allgemeine deutsche Biographie (M. B. Cantor); J. C. Poggendorff, Biog. lit. Handworterbuch. (A. M. C.)

 << Huy

# Simple English

File:Christiaan
Painted portrait of Christiaan Huygens.

Christiaan Huygens (April 14, 1629July 8, 1695) was a Dutch physicist, mathematician and astronomer, born in The Hague. He was noted for his arguments that light was in the form of waves. He discovered Saturn's largest moon Titan in 1655. He also did detailed studies on Saturn's rings, and in 1656, he discovered that they are made up of rocks. He worked on the making of accurate clocks, and invented the pendulum clock on Christmas 1656.