Christiaan Huygens

Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of all time and a major figure in the Scientific Revolution. In physics, Huygens made groundbreaking contributions in optics and mechanics, while as an astronomer he is chiefly known for his studies of the rings of Saturn and the discovery of its moon Titan. As an engineer and inventor, he improved the design of telescopes and invented the pendulum clock, a breakthrough in timekeeping and the most accurate timekeeper for almost 300 years. An exceptionally talented mathematician and physicist, Huygens was the first to idealize a physical problem by a set of mathematical parameters, and the first to fully mathematize a mechanistic explanation of an unobservable physical phenomenon.Dijksterhuis, F.J. (2008) Stevin, Huygens and the Dutch republic. ''Nieuw archief voor wiskunde'', ''5'', pp. 100–10

/ref> For these reasons, he has been called the first Theoretical physics, theoretical physicist and one of the founders of modern mathematical physics.

Huygens first identified the correct laws of elastic collision in his work ''De Motu Corporum ex Percussione'', completed in 1656 but published posthumously in 1703.Gabbey, Alan (1980). Huygens and mechanics. In H.J.M. Bos, M.J.S. Rudwick, H.A.M. Snelders, & R.P.W. Visser (Eds.), ''Studies on Christiaan Huygens'' (pp. 166-199). Swets & Zeitlinger B.V. In 1659, Huygens derived geometrically the formula in classical mechanics for the centrifugal force in his work ''De vi Centrifuga'', a decade before Newton. In optics, he is best known for his wave theory of light, which he described in his ''Traité de la Lumière'' (1690). His theory of light was initially rejected in favour of Newton's corpuscular theory of light, until Augustin-Jean Fresnel adopted Huygens's principle to give a complete explanation of the rectilinear propagation and diffraction effects of light in 1821. Today this principle is known as the Huygens–Fresnel principle.

Huygens invented the pendulum clock in 1657, which he patented the same year. His horological research resulted in an extensive analysis of the pendulum in ''Horologium Oscillatorium'' (1673), regarded as one of the most important 17th century works on mechanics. While it contains descriptions of clock designs, most of the book is an analysis of pendular motion and a theory of curves. In 1655, Huygens began grinding lenses with his brother Constantijn to build refracting telescopes. He discovered the first of Saturn's moons, Titan, and was the first to explain Saturn's strange appearance as due to "a thin, flat ring, nowhere touching, and inclined to the ecliptic." In 1662 Huygens developed what is now called the Huygenian eyepiece, a telescope with two lenses to diminish the amount of dispersion.

As a mathematician, Huygens developed the theory of evolutes and wrote on games of chance and the problem of points in ''Van Rekeningh in Spelen van Gluck'', which Frans van Schooten translated and published as ''De Ratiociniis in Ludo Aleae'' (1657). The use of expectation values by Huygens and others would later inspire Jacob Bernoulli's work on probability theory.

Biography

Christiaan Huygens was born on 14 April 1629 in The Hague, into a rich and influential Dutch family, the second son of Constantijn Huygens. Christiaan was named after his paternal grandfather.Henk J. M. Bos (14 December 2012
Huygens, Christiaan (Also Huyghens, Christian)
, ''Complete Dictionary of Scientific Biography''. 2008. Encyclopedia.com. His mother, Suzanna van Baerle, died shortly after giving birth to Huygens's sister. The couple had five children: Constantijn (1628), Christiaan (1629), Lodewijk (1631), Philips (1632) and Suzanna (1637).

Constantijn Huygens was a diplomat and advisor to the House of Orange, in addition to being a poet and a musician. He corresponded widely with intellectuals across Europe; his friends included Galileo Galilei, Marin Mersenne, and René Descartes. Christiaan was educated at home until the age of sixteen, and from a young age liked to play with miniatures of mills and other machines. From his father he received a liberal education, studying languages, music, history, geography, mathematics, logic, and rhetoric, alongside dancing, fencing and horse riding.

In 1644, Huygens had as his mathematical tutor Jan Jansz Stampioen, who assigned the 15-year-old a demanding reading list on contemporary science. Descartes was later impressed by his skills in geometry, as was Mersenne, who christened him "the new Archimedes."

Student years

At sixteen years of age, Constantijn sent Huygens to study law and mathematics at Leiden University, where he studied from May 1645 to March 1647. Frans van Schooten was an academic at Leiden from 1646, and became a private tutor to Huygens and his elder brother, Constantijn Jr., replacing Stampioen on the advice of Descartes. Van Schooten brought Huygens's mathematical education up to date, introducing him to the work of Viète, Descartes, and Fermat.Bunge et al. (2003), ''Dictionary of Seventeenth and Eighteenth-Century Dutch Philosophers,'' p. 470.

After two years, starting in March 1647, Huygens continued his studies at the newly founded Orange College, in Breda, where his father was a curator. Constantijn Huygens was closely involved in the new College, which lasted only to 1669; the rector was André Rivet. Christiaan Huygens lived at the home of the jurist Johann Henryk Dauber while attending college, and had mathematics classes with the English lecturer John Pell. His time in Breda ended around the time when his brother Lodewijk, who was enrolled at the school, duelled with another student. Huygens left Breda after completing his studies in August 1649 and had a stint as a diplomat on a mission with Henry, Duke of Nassau. It took him to Bentheim, then Flensburg. He took off for Denmark, visited Copenhagen and Helsingør, and hoped to cross the Øresund to visit Descartes in Stockholm. It was not to be.

Although his father Constantijn had wished his son Christiaan to be a diplomat, circumstances kept him from becoming so. The First Stadtholderless Period that began in 1650 meant that the House of Orange was no longer in power, removing Constantijn's influence. Further, he realized that his son had no interest in such a career.Bunge et al. (2003), ''Dictionary of Seventeenth and Eighteenth-Century Dutch Philosophers,'' p. 469.

Early correspondence

Huygens generally wrote in French or Latin. In 1646, while still a college student at Leiden, he began a correspondence with his father's friend the intelligencer Mersenne, who died soon afterwards in 1648. Mersenne wrote to Constantijn on his son's talent for mathematics, and flatteringly compared him to Archimedes on 3 January 1647.

The letters show Huygens's early interest in mathematics. In October 1646 there is the suspension bridge and the demonstration that a hanging chain is not a parabola, as Galileo thought. Huygens would later label that curve the ''catenaria'' (catenary) in 1690 while corresponding with Gottfried Leibniz.

In the next two years (1647-48), Huygens's letters to Mersenne covered various topics, including a mathematical proof of the law of free fall, the claim by Grégoire de Saint-Vincent of circle quadrature, which Huygens showed to be wrong, the rectification of the ellipse, projectiles, and the vibrating string. Some of Mersenne's concerns at the time, such as the cycloid (he sent Huygens Torricelli's treatise on the curve), the centre of oscillation, and the gravitational constant, were matters Huygens only took seriously towards the end of the 17th century. Mersenne had also written on musical theory. Huygens preferred meantone temperament; he innovated in 31 equal temperament (which was not itself a new idea but known to Francisco de Salinas), using logarithms to investigate it further and show its close relation to the meantone system.

In 1654, Huygens returned to his father's house in The Hague, and was able to devote himself entirely to research. The family had another house, not far away at Hofwijck, and he spent time there during the summer. Despite being very active, his scholarly life did not allow him to escape bouts of depression.

Subsequently, Huygens developed a broad range of correspondents, though picking up the threads after 1648 was hampered by the five-year ''Fronde'' in France. Visiting Paris in 1655, Huygens called on Ismael Boulliau to introduce himself, who took him to see Claude Mylon. The Parisian group of savants that had gathered around Mersenne held together into the 1650s, and Mylon, who had assumed the secretarial role, took some trouble to keep Huygens in touch. Through Pierre de Carcavi Huygens corresponded in 1656 with Pierre de Fermat, whom he admired greatly, though this side of idolatry. The experience was bittersweet and somewhat puzzling, since it became clear that Fermat had dropped out of the research mainstream, and his priority claims could probably not be made good in some cases. Besides, Huygens was looking by then to apply mathematics to physics, while Fermat's concerns ran to purer topics.

Scientific debut

Like some of his contemporaries, Huygens was often slow to commit his results and discoveries to print, preferring to disseminate his work through letters instead. In his early days, his mentor Frans van Schooten provided technical feedback and was cautious for the sake of his reputation.

Between 1651 and 1657, Huygens published a number of works that showed his talent for mathematics and his mastery of both classical and analytical geometry, increasing his reach and reputation among mathematicians. Around the same time, Huygens began to question Descartes's laws of collision, which were largely wrong, deriving the correct laws algebraically and later by way of geometry. He showed that, for any system of bodies, the centre of gravity of the system remains the same in velocity and direction, which Huygens called the conservation of "quantity of movement". While others were studying impact around the same time, Huygens's theory of collisions was more general. These results were known through correspondence and in a short article in '' Journal des Sçavans'' but would remain largely unpublished until after his death, with the publication of ''De Motu Corporum ex Percussione'' (''Concerning the motion of colliding bodies'').

In addition to his work on mechanics, he made important scientific discoveries, such as the identification of Saturn's moon Titan in 1655, and the invention of the pendulum clock in 1657, both of which brought him fame across Europe. On 3 May 1661, Huygens observed the planet Mercury transit over the Sun, using the telescope of instrument maker Richard Reeve in London, together with astronomer Thomas Streete and Reeve. Streete then debated the published record of the transit of Hevelius, a controversy mediated by Henry Oldenburg. Huygens passed to Hevelius a manuscript of Jeremiah Horrocks on the transit of Venus, 1639, which was printed for the first time in 1662.

Sir Robert Moray sent Huygens John Graunt's life table in 1662, and in time Huygens and his brother Lodewijk dabbled on life expectancy. Huygens eventually created the first graph of a continuous distribution function under the assumption of a uniform death rate, and used it to solve problems in joint annuities. In the same year, Huygens, who played the harpsichord, took an interest in Simon Stevin's theories on music; however, he showed very little concern to publish his theories on consonance, some of which were lost for centuries. For his contributions to science, the Royal Society of London elected Huygens a Fellow in 1665, making him its first foreign member when he was just 36 years old.

France

The Montmor Academy was the form the old Mersenne circle took after the mid-1650s. Huygens took part in its debates, and supported its "dissident" faction who favoured experimental demonstration to curtail fruitless discussion, and opposed amateurish attitudes. During 1663 he made what was his third visit to Paris; the Montmor Academy closed down, and Huygens took the chance to advocate a more Baconian program in science. Three years later, in 1666, he moved to Paris on an invitation to fill a position at King Louis XIV's new French Académie des sciences.

While in Paris, Huygens had an important patron and correspondent in Jean-Baptiste Colbert, First Minister to Louis XIV. However, his relationship with the French Academy was not always easy, and in 1670 Huygens, seriously ill, chose Francis Vernon to carry out a donation of his papers to the Royal Society in London, should he die. The aftermath of the Franco-Dutch War (1672–78), and particularly England's role in it, may have damaged his relationship with the Royal Society. Robert Hooke, as a Royal Society representative, lacked the finesse to handle the situation in 1673.

The physicist and inventor Denis Papin was assistant to Huygens from 1671. One of their projects, which did not bear fruit directly, was the gunpowder engine. Papin moved to England in 1678 to continue work in this area. Also in Paris, Huygens made further astronomical observations using the observatory recently completed in 1672. He introduced Nicolaas Hartsoeker to French scientists such as Nicolas Malebranche and Giovanni Cassini in 1678.

Huygens met Leibniz as a young diplomat, visiting Paris in 1672 on a vain mission to meet the French Foreign Minister Arnauld de Pomponne. At this time Leibniz was working on a calculating machine, and he moved on to London in early 1673 with diplomats from Mainz. From March 1673, Leibniz was tutored in mathematics by Huygens, who taught him analytical geometry. An extensive correspondence ensued, in which Huygens showed at first reluctance to accept the advantages of Leibniz's infinitesimal calculus.

Final years

Huygens moved back to The Hague in 1681 after suffering another bout of serious depressive illness. In 1684, he published ''Astroscopia Compendiaria'' on his new tubeless aerial telescope. He attempted to return to France in 1685 but the revocation of the Edict of Nantes precluded this move. His father died in 1687, and he inherited Hofwijck, which he made his home the following year.

On his third visit to England, Huygens met Isaac Newton in person on 12 June 1689. They spoke about Iceland spar, and subsequently corresponded about resisted motion.

Huygens returned to mathematical topics in his last years and observed the acoustical phenomenon now known as flanging in 1693. Two years later, on 8 July 1695, Huygens died in The Hague and was buried in an unmarked grave in the Grote Kerk there, as was his father before him.

Huygens never married.

Mathematics

Huygens first became internationally known for his work in mathematics, publishing a number of important results that drew the attention of many European geometers.Bos, H. J. M. (2004). Huygens and mathematics. ''Titan: From discovery to encounter'', pp. 67–8

Huygens's preferred method in his published works was that of Archimedes, though he used Descartes's analytic geometry and Fermat's infinitesimal techniques more extensively in his private notebooks.

Published works

''Theoremata de Quadratura''

Huygens's first publication was ''Theoremata de Quadratura Hyperboles, Ellipsis et Circuli'' (''Theorems on the quadrature of the hyperbola, ellipse, and circle''), published by the Elzeviers in Leiden in 1651. The first part of the work contained theorems for computing the areas of hyperbolas, ellipses, and circles that paralleled Archimedes's work on conic sections, particularly his ''Quadrature of the Parabola''. The second part included a refutation to Grégoire de Saint-Vincent's claims on circle quadrature, which he had discussed with Mersenne earlier.

Huygens demonstrated that the centre of gravity of a segment of any hyperbola, ellipse, or circle was directly related to the area of that segment. He was then able to show the relationships between triangles inscribed in conic sections and the centre of gravity for those sections. By generalizing these theorems to all conic sections, Huygens extended classical methods to generate new results.

Quadrature was a live issue in the 1650s and, through Mylon, Huygens intervened in the discussion of the mathematics of Thomas Hobbes. Persisting in trying to explain the errors Hobbes had fallen into, he made an international reputation.

''De Circuli Magnitudine Inventa''

Huygens's next publication was ''De Circuli Magnitudine Inventa'' (''New findings in the measurement of the circle''), published in 1654. In this work, Huygens was able to narrow the gap between the circumscribed and inscribed polygons found in Archimedes's ''Measurement of the Circle'', showing that the ratio of the circumference to its diameter or '' π'' must lie in the first third of that interval.

Using a technique equivalent to Richardson extrapolation, Huygens was able to shorten the inequalities used in Archimedes's method; in this case, by using the centre of the gravity of a segment of a parabola, he was able to approximate the centre of gravity of a segment of a circle, resulting in a faster and accurate approximation of the circle quadrature. From these theorems, Huygens obtained two set of values for ''π'': the first between 3.1415926 and 3.1415927, and the second between 3.1415926538 and 3.1415926533.

Huygens also showed that, in the case of the hyperbola, the same approximation with parabolic segments produces a quick and simple method to calculate logarithms. He appended a collection of solutions to classical problems at the end of the work under the title ''Illustrium Quorundam Problematum Constructiones'' (''Construction of some illustrious problems'').

''De Ratiociniis in Ludo Aleae''

Huygens became interested in games of chance after he visited Paris in 1655 and encountered the work of Fermat, Blaise Pascal and Girard Desargues years earlier. He eventually published what was, at the time, the most coherent presentation of a mathematical approach to games of chance in ''De Ratiociniis in Ludo Aleae'' (''On reasoning in games of chance''). Frans van Schooten translated the original Dutch manuscript into Latin and published it in his ''Exercitationum Mathematicarum'' (1657).

The work contains early game-theoretic ideas and deals in particular with the problem of points. Huygens took from Pascal the concepts of a "fair game" and equitable contract (i.e., equal division when the chances are equal), and extended the argument to set up a non-standard theory of expected values. His success in applying algebra to the realm of chance, which hitherto seemed inaccessible to mathematicians, demonstrated the power of combining Euclidean synthetic proofs with the symbolic reasoning found in the works of Viète and Descartes.

Huygens included five challenging problems at the end of the book that became the standard test for anyone wishing to display their mathematical skill in games of chance for the next sixty years. People who worked on these problems included Abraham de Moivre, Jacob Bernoulli, Johannes Hudde, Baruch Spinoza, and Leibniz.

Unpublished work

Huygens had earlier completed a manuscript in the manner of Archimedes's ''On Floating Bodies'' entitled ''De Iis quae Liquido Supernatant'' (''About parts floating above liquids''). It was written around 1650 and was made up of three books. Although he sent the completed work to Frans van Schooten for feedback, in the end Huygens chose not to publish it, and at one point suggested it be burned. Some of the results found here were not rediscovered until the eighteenth and nineteenth centuries.

Huygens first re-derives Archimedes's results for the stability of the sphere and the paraboloid by a clever application of Torricelli's principle (i.e., that bodies in a system move only if their centre of gravity descends). He then proves the general theorem that, for a floating body in equilibrium, the distance between its centre of gravity and its submerged portion its at a minimum. Huygens uses this theorem to arrive at original solutions for the stability of floating cones, parallelepipeds, and cylinders, in some cases through a full cycle of rotation. His approach was thus equivalent to the principle of virtual work. Huygens was also the first to recognize that, for homogeneous solids, their specific weight and their aspect ratio are the essentials parameters of hydrostatic stability.

Natural Philosophy

Huygens was the leading European natural philosopher between Descartes and Newton. However, unlike many of his contemporaries, Huygens had no taste for grand theoretical or philosophical systems, and generally avoided dealing with metaphysical issues (if pressed, he adhered to the Cartesian and mechanical philosophy of his time). Instead, Huygens excelled in extending the work of his predecessors, such as Galileo, to derive solutions to unsolved physical problems that were amenable to mathematical analysis. In particular, he sought explanations that relied on contact between bodies and avoided action at a distance.

In common with Robert Boyle and Jacques Rohault, Huygens advocated an experimentally oriented, corpuscular-mechanical natural philosophy during his Paris years. This approach was sometimes labelled "Baconian," without being inductivist or identifying with the views of Francis Bacon in a simple-minded way.

After his first visit to England in 1661 and attending a meeting at Gresham College where he learned directly about Boyle's air pump experiments, Huygens spent time in late 1661 and early 1662 replicating the work. It proved a long process that brought to the surface both an experimental issue ("anomalous suspension") and a theoretical issue ("'' horror vacui''"), and which ended in July 1663 as Huygens became a Fellow of the Royal Society. Huygens came to accept Boyle's view of the void against the Cartesian denial of it, while the replication of results of Boyle's experiments with the air pump trailed off messily.

Newton's influence on John Locke was mediated by Huygens, who assured Locke that Newton's mathematics was sound, leading to Locke's acceptance of a corpuscular-mechanical physics.

Laws of motion, impact, and gravitation

The general approach of the mechanical philosophers was to postulate theories of the kind now called "contact action." Huygens adopted this method, but not without seeing its difficulties and failures. Leibniz, his student in Paris, later abandoned the theory. Seeing the universe this way made the theory of collisions central to physics. Matter in motion made up the universe, and only explanations in those terms could be truly intelligible. While he was influenced by the Cartesian approach, he was less doctrinaire. He studied elastic collisions in the 1650s but delayed publication for over a decade.

Huygens concluded quite early that Descartes's laws for the elastic collision of two bodies must be wrong, and he formulated the correct laws, including the conservation of the product of mass times the square of the speed for hard bodies, and the conservation of quantity of motion in one direction for all bodies. An important step was his recognition of the Galilean invariance of the problems. Huygens had actually worked out the laws of collision from 1652 to 1656 in a manuscript entitled ''De Motu Corporum ex Percussione'', though his results took many years to be circulated. In 1661, he passed them on in person to William Brouncker and Christopher Wren in London. What Spinoza wrote to Henry Oldenburg about them in 1666, during the Second Anglo-Dutch War, was guarded. The war ended in 1667, and Huygens announced his results to the Royal Society in 1668. He later published them in the ''Journal des Sçavans'' in 1669.

In 1659 Huygens found the constant of gravitational acceleration and stated what is now known as the second of Newton's laws of motion in quadratic form. He derived geometrically the now standard formula for the centrifugal force, exerted on an object when viewed in a rotating frame of reference, for instance when driving around a curve. In modern notation:

:$F_=$

with ''m'' the mass of the object, ''w'' the angular velocity, and ''r'' the radius. Huygens collected his results in a treatise under the title ''De vi Centrifuga'', unpublished until 1703, where the kinematics of free fall were used to produce the first generalized conception of force prior to Newton. The general formula for the centrifugal force, however, was published in 1673 and was a significant step in studying orbits in astronomy. It enabled the transition from Kepler's third law of planetary motion to the inverse square law of gravitation. Yet, the interpretation of Newton's work on gravitation by Huygens differed from that of Newtonians such as Roger Cotes: he did not insist on the ''a priori'' attitude of Descartes, but neither would he accept aspects of gravitational attractions that were not attributable in principle to contact between particles.

The approach used by Huygens also missed some central notions of mathematical physics, which were not lost on others. In his work on pendulums Huygens came very close to the theory of simple harmonic motion; the topic, however, was covered fully for the first time by Newton in Book II of the ''Principia Mathematica'' (1687). In 1678 Leibniz picked out of Huygens's work on collisions the idea of conservation law that Huygens had left implicit.

Horology

Pendulum clock

In 1657, inspired by earlier research into pendulums as regulating mechanisms, Huygens invented the pendulum clock, which was a breakthrough in timekeeping and became the most accurate timekeeper for almost 300 years until the 1930s. The pendulum clock was much more accurate than the existing verge and foliot clocks and was immediately popular, quickly spreading over Europe. He contracted the construction of his clock designs to Salomon Coster in The Hague, who built the clock. However, Huygens did not make much money from his invention. Pierre Séguier refused him any French rights, while Simon Douw in Rotterdam and Ahasuerus Fromanteel in London copied his design in 1658. The oldest known Huygens-style pendulum clock is dated 1657 and can be seen at the Museum Boerhaave in Leiden.van den Ende, H., Hordijk, B., Kersing, V., & Memel, R. (2018)
''The invention of the pendulum clock: A collaboration on the real story.''
/ref>

Part of the incentive for inventing the pendulum clock was to create an accurate marine chronometer that could be used to find longitude by celestial navigation during sea voyages. However, the clock proved unsuccessful as a marine timekeeper because the rocking motion of the ship disturbed the motion of the pendulum. In 1660, Lodewijk Huygens made a trial on a voyage to Spain, and reported that heavy weather made the clock useless. Alexander Bruce elbowed into the field in 1662, and Huygens called in Sir Robert Moray and the Royal Society to mediate and preserve some of his rights. Trials continued into the 1660s, the best news coming from a Royal Navy captain Robert Holmes operating against the Dutch possessions in 1664. Lisa Jardine doubts that Holmes reported the results of the trial accurately, as Samuel Pepys expressed his doubts at the time.

A trial for the French Academy on an expedition to Cayenne ended badly. Jean Richer suggested correction for the figure of the Earth. By the time of the Dutch East India Company expedition of 1686 to the Cape of Good Hope, Huygens was able to supply the correction retrospectively.

''Horologium Oscillatorium''

Sixteen years after the invention of the pendulum clock, in 1673, Huygens published his major work on horology entitled ''Horologium Oscillatorium, Horologium Oscillatorium: Sive de Motu Pendulorum ad Horologia Aptato Demonstrationes Geometricae (The Pendulum Clock: or Geometrical demonstrations concerning the motion of pendula as applied to clocks''). It is the first modern work on mechanics where a physical problem is idealized by a set of parameters then analysed mathematically.

Huygens's motivation came from the observation, made by Mersenne and others, that pendulums are not quite isochronous: their period depends on their width of swing, with wide swings taking slightly longer than narrow swings. He tackled this problem by finding 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 curve, tautochrone problem. By geometrical methods which anticipated the calculus, Huygens showed it to be a cycloid, rather than the circular arc of a pendulum's bob, and therefore that pendulums needed to move on a cycloid path in order to be isochronous. The mathematics necessary to solve this problem led Huygens to develop his theory of evolutes, which he presented in Part III of his ''Horologium Oscillatorium''.

He also solved a problem posed by Mersenne earlier: how to calculate the period of a pendulum made of an arbitrarily-shaped swinging rigid body. This involved discovering the center of oscillation, centre 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 Frequency, period of an ideal mathematical pendulum (with mass-less rod or cord and length much longer than its swing), in modern notation:

:$T = 2 \pi \sqrt$

with ''T'' the period, ''l'' the length of the pendulum and ''g'' the gravitational acceleration. By his study of the oscillation period of compound pendulums Huygens made pivotal contributions to the development of the concept of moment of inertia.Ernst Mach, ''The Science of Mechanics'' (1919), e.g. pp. 143, 172, 187 .

Huygens also observed coupled oscillations: two of his pendulum clocks mounted next to each other on the same support often became synchronized, swinging in opposite directions. He reported the results by letter to the Royal Society, and it is referred to as "odd sympathy, an odd kind of sympathy" in the Society's minutes. This concept is now known as Entrainment (physics), entrainment.

Balance spring watch

In 1675, while investigating the oscillating properties of the cycloid, Huygens was able to transform a cycloidal pendulum into a vibrating spring through a combination of geometry and higher mathematics. In the same year, Huygens designed a spiral balance spring and patented a pocket watch. These watches are notable for lacking a fusee (horology), fusee for equalizing the mainspring torque. The implication is that Huygens thought his spiral spring would isochronize the balance in the same way that cycloid-shaped suspension curbs on his clocks would isochronize the pendulum.

He later used spiral springs in more conventional watches, made for him by Thuret family, Thuret in Paris. Such springs are essential in modern watches with a detached lever escapement because they can be adjusted for isochronism. Watches in Huygens's time, however, employed the very ineffective verge escapement, which interfered with the isochronal properties of any form of balance spring, spiral or otherwise.

Huygens's design came around the same time as, though independently of, Robert Hooke's. Controversy over the priority of the balance spring persisted for centuries. In February 2006, a long-lost copy of Hooke's handwritten notes from several decades of Royal Society meetings was discovered in a cupboard in Hampshire, England, presumably tipping the evidence in Hooke's favour.

Optics

Dioptrics

Huygens had a long-term interest in the study of Refraction, light refraction and lenses or dioptrics.Bunge et al. (2003), ''Dictionary of Seventeenth and Eighteenth-Century Dutch Philosophers,'' p. 472. From 1652 date the first drafts of a Latin treatise on the theory of dioptrics, known as the ''Tractatus'', which contained a comprehensive and rigorous theory of the telescope. Huygens was one of the few to raise theoretical questions regarding the properties and working of the telescope, and almost the only one to direct his mathematical proficiency towards the actual instruments used in astronomy.

Huygens repeatedly announced its publication to his colleagues but ultimately postponed it in favor of a much more comprehensive treatment, now under the name of the ''Dioptrica''. It consisted of three parts. The first part focused on the general principles of refraction, the second dealt with Spherical aberration, spherical and chromatic aberration, while the third covered all aspects of the construction of telescopes and microscopes. In contrast to Descartes' dioptrics which treated only ideal (elliptical and hyperbolical) lenses, Huygens dealt exclusively with spherical lenses, which were the only kind that could really be made and incorporated in devices such as microscopes and telescopes.

Huygens also worked out practical ways to minimize the effects of spherical and chromatic aberration, such as long focal distances for the objective of a telescope, internal stops to reduce the aperture, and a new kind of ocular in the form of a set of two planoconvex lenses, now known as the Huygens's eyepiece. The ''Dioptrica'' was never published in Huygens’s lifetime and only appeared in press in 1703, when most of its contents were already familiar to the scientific world.

Lenses

Together with his brother Constantijn, Huygens began grinding his own lenses in 1655 in an effort to improve telescopes. He designed in 1662 what is now called the Huygenian eyepiece, with two lenses, as a telescope ocular.Bunge et al. (2003), ''Dictionary of Seventeenth and Eighteenth-Century Dutch Philosophers,'' p. 473. Lenses were also a common interest through which Huygens could meet socially in the 1660s with Spinoza, who ground them professionally. They had rather different outlooks on science, Spinoza being the more committed Cartesian, and some of their discussion survives in correspondence. He encountered the work of Antoni van Leeuwenhoek, another lens grinder, in the field of microscopy which interested his father.

Huygens also investigated the use of lenses in projectors. He is credited as the inventor of the magic lantern, described in correspondence of 1659. There are others to whom such a lantern device has been attributed, such as Giambattista della Porta and Cornelis Drebbel, though Huygens's design used lens for better projection (Athanasius Kircher has also been credited for that).

''Traité de la Lumière''

Huygens is especially remembered in optics for his wave theory of light, which he first communicated in 1678 to the Académie des sciences in Paris. Originally a preliminary chapter of his ''Dioptrica'', Huygens's theory was published in 1690 under the title ''Traité de la Lumière'' (''Treatise on light''), and contains the first fully mathematized, mechanistic explanation of an unobservable physical phenomenon (i.e., light propagation).C. Huygens (1690), translated by Silvanus P. Thompson (1912), ''iarchive:treatiseonlight031310mbp, Treatise on Light'', London: Macmillan, 1912
Project Gutenberg edition
, 2005

, 2016. Huygens refers to Ignace-Gaston Pardies, whose manuscript on optics helped him on his wave theory.

The challenge at the time was to explain geometrical optics, as most physical optics phenomena (such as diffraction) had not been observed or appreciated as issues. Huygens had experimented in 1672 with double refraction (birefringence) in the Iceland spar (a calcite), a phenomenon discovered in 1669 by Rasmus Bartholin. At first, he could not elucidate what he found but was later able to explain it using his wavefront theory and concept of evolutes. He also developed ideas on caustic (optics), caustics. Huygens assumes that the speed of light is finite, based on a report by Ole Rømer, Ole Christensen Rømer in 1677 but which Huygens is presumed to have already believed. Huygens's theory posits light as radiating wavefronts, with the common notion of light rays depicting propagation normal to those wavefronts. Propagation of the wavefronts is then explained as the result of spherical waves being emitted at every point along the wave front (known today as the Huygens–Fresnel principle). It assumed an omnipresent luminiferous aether, ether, with transmission through perfectly elastic particles, a revision of the view of Descartes. The nature of light was therefore a longitudinal wave.

His theory of light was not widely accepted, while Newton's rival corpuscular theory of light, as found in his ''Opticks'' (1704), gained more support. One strong objection to Huygens's theory was that longitudinal waves have only a single polarization (waves), polarization which cannot explain the observed birefringence. However, Thomas Young (scientist), Thomas Young's Young's interference experiment, interference experiments in 1801, and François Arago's detection of the Poisson spot in 1819, could not be explained through Newton's or any other particle theory, reviving Huygens's ideas and wave models. Fresnel became aware of Huygens's work and in 1821 was able to explain birefringence as a result of light being not a longitudinal (as had been assumed) but actually a transverse wave. The thus-named Huygens–Fresnel principle was the basis for the advancement of physical optics, explaining all aspects of light propagation until James Clerk Maxwell, Maxwell's History of electromagnetic theory, electromagnetic theory culminated in the development of quantum mechanics and the discovery of the photon.

Astronomy

''Systema Saturnium''

In 1655, Huygens discovered the first of Saturn's moons, Titan, and observed and sketched the Orion Nebula using a refracting telescope with a 43x magnification of his own design. Huygens succeeded in subdividing the nebula into different stars (the brighter interior now bears the name of the ''Huygenian region'' in his honour), and discovered several nebula, interstellar nebulae and some double stars. He was also the first to propose that the rings of Saturn, appearance of Saturn, which have baffled astronomers, was due to "a thin, flat ring, nowhere touching, and inclined to the ecliptic”.

More than three years later, in 1659, Huygens published his theory and findings in ''Systema Saturnium''. It is considered the most important work on telescopic astronomy since Galileo's ''Sidereus Nuncius'' fifty years earlier. Much more than a report on Saturn, Huygens provided measurements for the relative distances of the planets from the Sun, introduced the concept of the Filar micrometer, micrometer, and showed a method to measure angular diameters of planets, which finally allowed the telescope to be used as an instrument to measure (rather than just sighting) astronomical objects. He was also the first to question the authority of Galileo in telescopic matters, a sentiment that was to be common in the years following its publication.

In the same year, Huygens was able to observe Syrtis Major Planum, Syrtis Major, a volcanic plain on Mars. He used repeated observations of the movement of this feature over the course of a number of days to estimate the length of day on Mars, which he did quite accurately to 24 1/2 hours. This figure is only a few minutes off of the actual length of the Martian day of 24 hours, 37 minutes.

Planetarium

At the instigation of Jean-Baptiste Colbert, Huygens undertook the task of constructing a mechanical planetarium that could display all the planets and their moons then known circling around the Sun. Huygens completed his design in 1680 and had his clockmaker Johannes van Ceulen built it the following year. However, Colbert passed away in the interim and Huygens never got to deliver his planetarium to the French Academy of Sciences as the new minister, François-Michel le Tellier, Marquis de Louvois, François-Michel le Tellier, decided not to renew Huygens's contract.van den Bosch, D. (2018). The application of continued fractions in Christiaan Huygens planetariu

In his design, Huygens made an ingenious use of continued fractions to find the best rational approximations by which he could choose the gears with the correct number of teeth. The ratio between two gears determined the orbital periods of two planets. To move the planets around the Sun, Huygens used a clock-mechanism that could go forwards and backwards in time. Huygens claimed his planetarium was more accurate that a similar device constructed by Ole Rømer around the same time, but his planetarium design was not published until after his death in the ''Opuscula Posthuma'' (1703).

''Cosmotheoros''

Shortly before his death in 1695, Huygens completed his most speculative work entitled ''Cosmotheoros''. At his direction, it was to be published only posthumously by his brother, which Constantijn Jr. did in 1698. In this work, Huygens speculated on the existence of extraterrestrial life, which he imagined similar to that on Earth. Such speculations were not uncommon at the time, justified by Copernicanism or the plenitude principle, but Huygens went into greater detail. However, it did so without the benefit of understanding Newton's laws of gravitation, or the fact that the atmospheres on other planets are composed of different gases. ''Cosmotheoros,'' translated into English as ''The celestial worlds discover’d'', has been seen as part of speculative fiction in the tradition of Francis Godwin, John Wilkins, and Cyrano de Bergerac. Huygens's work was fundamentally Utopia, utopian and owes some inspiration from the cosmography and Planet, planetary speculation of Peter Heylin.

Huygens wrote that availability of water in liquid form was essential for life and that the properties of water must vary from planet to planet to suit the temperature range. He took his observations of dark and bright spots on the surfaces of Mars and Jupiter to be evidence of water and ice on those planets. He argued that extraterrestrial life is neither confirmed nor denied by the Bible, and questioned why God would create the other planets if they were not to serve a greater purpose than that of being admired from Earth. Huygens postulated that the great distance between the planets signified that God had not intended for beings on one to know about the beings on the others, and had not foreseen how much humans would advance in scientific knowledge.

It was also in this book that Huygens published his estimates for the relative sizes of the Solar System, solar system and his method for calculating stellar distances. He made a series of smaller holes in a screen facing the Sun, until he estimated the light was of the same intensity as that of the star Sirius. He then calculated that the angle of this hole was 1/27,664th the diameter of the Sun, and thus it was about 30,000 times as far away, on the (incorrect) assumption that Sirius is as luminous as the Sun. The subject of photometry (astronomy), photometry remained in its infancy until the time of Pierre Bouguer and Johann Heinrich Lambert.

Legacy

During his lifetime, Huygens's influence was considerable but began to fade shortly after his death. His skills as a geometer and his mechanical insights elicited the admiration of many of his contemporaries, including Newton, Leibniz, Guillaume de l'Hôpital, l'Hôpital, and the Bernoulli (disambiguation), Bernoullis. For his work in physics, Huygens has been deemed one of the greatest scientists in history and a prominent figure in the Scientific Revolution, rivaled only by Newton in both depth of insight and the number of results obtained. Huygens was also instrumental in the development of institutional frameworks for scientific research on the Continental Europe, European continent, making him a leading actor in the establishment of modern science.

Mathematics and physics

In mathematics, Huygens mastered the methods of ancient Greek mathematics, Greek geometry, particularly the work of Archimedes, and was an adept user of the analytic geometry and infinitesimal techniques of Descartes, Fermat, and others. His mathematical style can be characterized as geometrical infinitesimal analysis of curves and of motion. Drawing inspiration and imagery from mechanics, it remained pure mathematics in form. Huygens brought this type of geometrical analysis to its greatest height but also to its conclusion, as more mathematicians turned away from classical geometry to the calculus for handling infinitesimals, limit processes, and motion.

Huygens was moreover one of the first to fully employ mathematics to answer questions of physics. Often this entailed introducing a simple Mathematical model, model for describing a complicated situation, then analyzing it starting from simple arguments to their logical consequences, developing the necessary mathematics along the way. As he wrote at the end of a draft of ''De vi Centrifuga'':

Huygens favoured Axiomatic system, axiomatic presentations of his results, which require rigorous methods of geometric demonstration: although in the selection of primary axioms and hypotheses he allowed levels of uncertainty, the proofs of theorems derived from these could never be in doubt. Huygens's published works were seen as precise, unambiguous, and elegant, and exerted a big influence in Newton's presentation of his own Philosophiæ Naturalis Principia Mathematica, major Opticks, works.

Besides the application of mathematics to physics and physics to mathematics, Huygens relied on mathematics as methodology, particularly its power to generate new knowledge about the world. Unlike Galileo, who used mathematics primarily as rhetoric or synthesis, Huygens consistently employed mathematics as a method of discovery and analysis, and insisted that the reduction of the physical to the geometrical satisfy exacting standards of fit between the real and the ideal. In demanding such mathematical tractibility and precision, Huygens set an example for eighteenth-century scientists such as Johann Bernoulli, Jean le Rond d'Alembert, and Charles-Augustin de Coulomb.

Although never intended for publication, Huygens made use of algebraic expressions to represent physical entities in a handful of his manuscripts on collisions. This would make him one of the first to employ mathematical formulae to describe relationships in physics, as it is done today. Huygens also came close to the modern idea of Limit (mathematics), limit while working on his ''Dioptrica,'' though he never used the notion outside geometrical optics.

Later influence

Huygens's standing as the greatest scientist in Europe was eclipsed by Newton's at the end of the seventeenth century, despite the fact that, as Hugh Aldersey-Williams notes, "Huygens's achievement exceeds that of Newton in some important respects". His very idiosyncratic style and reluctance to publish his work did much to diminish his influence in the aftermath of the Scientific Revolution, as adherents of Leibniz’ calculus and Newton's physics took centre stage.

His analysis of curves that satisfy certain physical properties, such as the cycloid, led to later studies of many other such curves like the caustic, the Brachistochrone curve, brachistochrone, the sail curve, and the catenary. His application of mathematics to physics, such as in his analysis of birefringence, would inspire new developments in mathematical physics and Classical mechanics, rational mechanics in the following centuries (albeit in the language of the calculus). Additionally, Huygens developed the oscillating timekeeping mechanisms, the pendulum and the balance spring, that have been used ever since in mechanical Watch, watches and clocks. These were the first reliable timekeepers fit for Scientific instrument, scientific use (e.g., it was possible for the first time to make accurate measurements of the Synodic day, inequality of the solar day, which astronomers in the past could not do). His work on this area anticipated the union of applied mathematics with mechanical engineering in the centuries that followed.

Portraits

During his lifetime, Huygens and his father had a number of portraits commissioned. These included:

* 1639 – Constantijn Huygens in the midst of his five children by Adriaen Hanneman, painting with medallions, Mauritshuis, The Hague
* 1671 – Portrait by Caspar Netscher, Museum Boerhaave, Leiden, loan from Haags Historisch Museum
* c.1675 – Depiction of Huygens in ''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. Château de Versailles, Musée National du Château et des Trianons de Versailles, Versailles (commune), Versailles
* 1679 – Locket, Medaillon portrait in relief by the French sculptor Jean-Jacques Clérion
* 1686 – Portrait in pastel by Bernard Vaillant, Hofwijck, Museum Hofwijck, Voorburg
* 1684 to 1687 – Engravings by G. Edelinck after the painting by Caspar Netscher
* 1688 – Portrait by Pierre Bourguignon (painter), Royal Netherlands Academy of Arts and Sciences, Amsterdam

Commemorations

The European Space Agency spacecraft that landed on Titan, Saturn's largest moon, in 2005 was named after him.

A number of monuments to Christiaan Huygens can be found across important cities in the Netherlands, including Rotterdam, Delft, and Leiden.

File:Christiaan Huygens Statue Rotterdam.jpg, Rotterdam
File:Christiaan Huygens Statue Delft 1.jpg, Delft
File:Christiaan Huygens by Frank Letterie.jpg, Leiden
File:Tuin, standbeeld van Christiaan Huygens - Haarlem - 20097899 - RCE.jpg, Haarlem
File:Voorburg monument huygensmonument.jpg, Voorburg

Works

Source(s):
* 1650 – ''De Iis Quae Liquido Supernatant'' (''About parts floating above liquids'', unpublished).
* 1651 – ''Theoremata de Quadratura Hyperboles, Ellipsis et Circuli'', republished in ''Oeuvres Complètes'', Tome XI.
* 1651 – ''Epistola, qua diluuntur ea quibus 'Εξέτασις [Exetasis] Cyclometriae Gregori à Sto. Vincentio impugnata fuit'' (supplement).
* 1654 – ''De Circuli Magnitudine Inventa.''
* 1654 – ''Illustrium Quorundam Problematum Constructiones'' (supplement).
* 1655 – ''Horologium'' (''The clock'' – short pamphlet on the pendulum clock).
* 1656 – ''De Saturni Luna Observatio Nova'' (''About the new observation of the Titan (moon), moon of Saturn,'' describing the discovery of Titan).
* 1656 – ''De Motu Corporum ex Percussione'', published posthumously in 1703.
* 1657 – ''De Ratiociniis in Ludo Aleae'' (''Van probability, reeckening in spelen van geluck'', translated into Latin by Frans van Schooten).
* 1659 – ''Systema Saturnium'' (''System of Saturn'').
* 1659 – ''De vi Centrifuga'' (''Concerning the centrifugal force''), published posthumously in 1703.
* 1673 – ''Horologium Oscillatorium Sive de Motu Pendulorum ad Horologia Aptato Demonstrationes Geometricae'' (includes his theory of evolutes and designs of pendulum clocks, dedicated to Louis XIV of France).
* 1684 – ''Astroscopia Compendiaria Tubi Optici Molimine Liberata'' (''Compound telescopes without a tube'').
* 1685 – ''Memoriën aengaende het slijpen van glasen tot verrekijckers'' (dealing with the grinding of lenses).
* 1686 – ''Old nl, Kort onderwijs aengaende het gebruijck der horologiën tot het vinden der lenghten van Oost en West'' (instructions on how to use clocks to establish the longitude at sea).
* 1690 – ''Traité de la Lumière'' (dealing with the nature of light propagation).
* 1690 – ''Discours de la Cause de la Pesanteur'' (''Discourse about gravity'', supplement).
* 1691 – ''Lettre Touchant le Cycle Harmonique'' (short tract concerning the 31 equal temperament, 31-tone system).
* 1698 – ''Cosmotheoros'' (dealing with the solar system, cosmology, and extraterrestrial life).
* 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'' (a treatise on music, published in Leiden after Huygens's death).
* 1728 – ''Christiani Hugenii Zuilichemii, dum viveret Zelhemii Toparchae, Opuscula Posthuma ...'' (pub. 1728) Alternate title: ''Opera Reliqua'', including works in optics and physics
* 1888–1950 – ''Huygens, Christiaan. Oeuvres complètes.'' Complete works, 22 volumes. Editors D. Bierens de Haan (1–5), J. Bosscha (6–10), Diederik Johannes Korteweg, D.J. Korteweg (11–15), Albertus Antonie Nijland, A.A. Nijland (15), J.A. Vollgraf (16–22). The Hague:
** ''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).

See also

* History of the internal combustion engine
* List of largest optical telescopes historically
* Adriaan Fokker#Musical instruments, Fokker Organ
*Seconds pendulum

References

Further reading

* Cornelis Dirk Andriesse, Andriesse, C.D. (2005). ''Huygens: The Man Behind the Principle''. Foreword by Sally Miedema. Cambridge University Press.
* Bell, A. E. (1947). ''Christian Huygens and the Development of Science in the Seventeenth Century''
* Carl Benjamin Boyer, Boyer, C.B. (1968). ''A History of Mathematics'', New York.
* Eduard Jan Dijksterhuis, Dijksterhuis, E. J. (1961). ''The Mechanization of the World Picture: Pythagoras to Newton''
* Hooijmaijers, H. (2005). ''Telling time – Devices for time measurement in Museum Boerhaave – A Descriptive Catalogue'', Leiden, Museum Boerhaave.
* Dirk Jan Struik, Struik, D.J. (1948). ''A Concise History of Mathematics''
* Van den Ende, H. et al. (2004). ''Huygens's Legacy, The golden age of the pendulum clock'', Fromanteel Ltd, Castle Town, Isle of Man.
* Yoder, J G. (2005). "Book on the pendulum clock" in Ivor Grattan-Guinness, ed., ''Landmark Writings in Western Mathematics''. Elsevier: 33–45.

External links

Primary sources, translations

* :
** C. Huygens (translated by Silvanus P. Thompson, 1912),
Treatise on Light
'


*
*
*

De Ratiociniis in Ludo Aleae or The Value of all Chances in Games of Fortune, 1657
Christiaan Huygens's book on probability theory. An English translation published in 1714. Text pdf file.
*
Horologium oscillatorium
' (German translation, pub. 1913) or

' (English translation by Ian Bruce) on the pendulum clock
*

' (''Cosmotheoros''). (English translation of Latin, pub. 1698; subtitled ''The celestial worlds discover'd: or, Conjectures concerning the inhabitants, plants and productions of the worlds in the planets.'')
* C. Huygens (translated by Silvanus P. Thompson), ''Traité de la lumière'' or ''Treatise on light'', London: Macmillan, 1912
archive.org/details/treatiseonlight031310mbp
New York: Dover, 1962; Project Gutenberg, 2005
gutenberg.org/ebooks/14725


a digital edition of Smithsonian Libraries
*

' (1703)

Huygens's work at WorldCat

The Correspondence of Christiaan Huygens
i
EMLO

Christiaan Huygens biography and achievements

Portraits of Christiaan Huygens
* Huygens's books, in digital facsimile from the Linda Hall Library:
** (1659
''Systema Saturnium''
(Latin)
** (1684
''Astroscopia compendiaria''
(Latin)
** (1690
''Traité de la lumiére''
(French)
** (1698
''ΚΟΣΜΟΘΕΩΡΟΣ, sive De terris cœlestibus''
(Latin)

Museums

Huygensmuseum Hofwijck
in Voorburg, Netherlands, where Huygens lived and worked.

Huygens Clocks
exhibition from the Science Museum, London

Online exhibition
on Huygens in Leiden University Library

Other

*

Huygens and music theory
Huygens–Fokker Foundation —on Huygens's 31 equal temperament and how it has been used

Christiaan Huygens on the 25 Dutch Guilder banknote of the 1950s.
*

{{DEFAULTSORT:Huygens, Christiaan
Christiaan Huygens,
1629 births
1695 deaths
Discoverers of moons
Dutch music theorists
17th-century Dutch scientists
Optical physicists
Theoretical physicists
Geometers
Dutch clockmakers
Dutch scientific instrument makers
Original Fellows of the Royal Society
Members of the French Academy of Sciences
Leiden University alumni
Huygens family, Christiaan
Dutch members of the Dutch Reformed Church
Scientists from The Hague
17th-century Latin-language writers
Age of Enlightenment
Astronomy in the Dutch Republic
17th-century Dutch inventors
17th-century Dutch mathematicians
17th-century Dutch philosophers
17th-century Dutch writers