NO name in the history of science is associated with more material advance, or with advances in more various directions, than that of Huygens. To him we owe important improvements in the telescope, which in his time was a very crude instrument; the discovery of the first satellite of Saturn and of the nature of his ring; the accepted theory of the character of the surface of the moon; the undulatory theory of light, which had to wait till our day to be verified or even accepted; the theory of the pendulum and of the properties of the cycloidal curve; continuous fractions; with Newton, the determination of the shape of the earth; the knowledge of the properties of double refraction and polarization; many other discoveries of practical use or theoretical value; and a few ingenious speculations which have been used to lend attraction to some works of popular science.

Christian Huygens van Zuylichem was born at the Hague, April 14, 1629, and died June 8, 1695. He was the second son of Constantine Huygens, secretary and counselor of three successive Princes of Orange, who was also a distinguished Dutch poet and writer of Latin verses. His grandfather, too, was a secretary to the great William the Silent; and his elder brother Constantine, serving in the corresponding capacity, accompanied Prince William Henry to England, where he went, in 1688, to become King William III.

His earlier instruction was attended to by his father, who, remarking, the signs of promise in him, taught him music, arithmetic, and geometry, and, when thirteen years old, mechanics. At fifteen, he was given an instructor in mathematics; at sixteen, he was sent to Leyden to study law under Vinnius; and he attended the University at Breda from 1646 to 1648. In these cities he enjoyed the instructions of the skilled geometricians, François Schooten and Jean Pell, and his first essays in that branch of mathematics were so fortunate as to attract the attention of Descartes, who wrote concerning it: "A little while ago Professor Schooten sent me a tract by the second son of M. de Zuylichem, touching a mathematical invention which he had sought out; yet he did not find in it what he was looking for (and this was not strange, for he was seeking what no one has ever yet found); but he went at it so straightway that I am sure he will become excellent in that science, in which I hardly ever see any one who knows anything." Huygens also had unbounded admiration for the great philosopher, but never enjoyed the privilege of meeting him.

The prediction of Descartes was very speedily fulfilled, for, within a few years after his graduation, having taken a short journey with Henry, Count of Nassau, Huygens began the series of labors and publications that have made his name immortal, with his theorems, in 1651, on the quadrature of the hyperbola, ellipse, and circle, following it with a criticism of Père Gregory de Saint Vincent's treatise on the same subject, and, three years afterward, with his discoveries on the magnitude of the circle (de circuli magnitudine inventa nova).

In 1655 he went to France, and received a degree in law from the Protestant Academy at Angers. Returning to Holland, he engaged with his brother in the manufacture of large lenses. With one of these, an objective of twelve feet focal distance, he discovered the first satellite of Saturn (the sixth in the order of distance), and announced the fact, after the manner of his time, in an anagram. It is said that, in the excitement attending his achievement, he engraved his anagram upon the glass itself by the aid of which the discovery was made. He afterward made glasses with one hundred, one hundred and seventy, and two hundred and ten feet of focal distance, which could not be inclosed in a telescopic tube on account of the swagging, to which so long an instrument would be subject, but for which he contrived a kind of framework support, while the observer stood at the focal point, eye-glass in hand. The necessity of using such cumbrous contrivances has happily been dispensed with by the introduction of reflecting telescopes.

In 1656, Huygens published, in Dutch, a memoir on the calculation of probabilities, for which Pascal and Feitnat had prepared the way, and which was translated into Latin by his preceptor, Schooten, to be inserted as an appendix to his "Mathematical Exercises," in illustration of the usefulness of algebra. In the same year he invented the escapement of watches and clocks. Galileo had already recognized the synchronism of the motion of pendulums, and experimenters had begun to avail themselves of it in timing their observations; but they knew of no better way of using the pendulums than to employ a man to keep them in motion and count their vibrations. Huygens connected them with clock-work, very much as we now have them, and made the whole operation automatic.

In 1659, having constructed an objective of twenty-two feet focal distance, Huygens turned his attention to Saturn's ring, which Galileo had perceived but dimly, discovered its true character, calculated its elements, and predicted its temporary disappearancee in 1671; a prediction which his fellow-astronomers saw fulfilled twelve years after it was made, with great admiration for his genius. In his work, giving an account of these observations, "Systema Saturninum," he also described the nebula in Orion, and the bands of Jupiter and Mars, announced that the fixed stars had no perceptible diameter, and made known his device for measuring the apparent diameters of the planets, an incipient micrometer. He discovered but one of the satellites of Saturn, and did not seem to care to look for any other; for his enterprise in this direction was bound by the opinion he entertained that there was a relation between the number of planets and of satellites; and there were already six planets—Mercury, Venus, Earth, Mars, Jupiter, and Saturn; and six satellites—one for the Earth, four for Jupiter, and one for Saturn. This fancy did not, however, prevent his afterward accepting Cassini's discovery of four other satellites of Saturn, and speculating from it upon the possibility of there being still others, either between some of those already discovered, or beyond the orbits of all.

Huygens, having now attained a very high and extensive reputation, visited France and England in 1660 and 1661. He explained his method of grinding lenses to the scientific men of England, and, finding them occupied with the recently introduced air-pump, took back with him the idea of that instrument when he returned to Holland, after two years, to develop it and improve upon it. Remarking in his experiments the close adherence of two plates of polished metal in vacuo, he conceived that it was due to the same cause as that which, operating at still closer quarters, produces cohesion. At about the same period he developed a rule for estimating the height of a place by the local pressure, and reciprocally, for calculating the pressure at a given place from its elevation above the sea. He was made a member of the Royal Society of London, and communicated to it the solution of the law of impact of bodies, at which Descartes had made an unsuccessful attempt. His own solution involved the laws of motion, and of action and reaction, in the main as they are now understood, and contained the germ of the law of the conservation of forces.

In 1665 he accepted an invitation from Colbert to go to Paris and reside in the Bibliothèque Royale. There he wrote his treatises on dioptrics and the law of percussion, in a literary style which won from Newton the remark that it more nearly approached the style of the ancients than that of any other modern author. Subsequently he composed the greatest of his works, the "Horologium Oscillatorium," which was published in 1673, and has been pronounced, with the exception of Newton's "Principia," the finest work on the exact sciences of the seventeenth century. In the dedication of this work to King Louis XIV, he revealed the dominant characteristic of his mind, making it the great object of all his researches to find out useful things, to promote the knowledge of nature, and add to the comforts of living. "I shall not waste any time, great king," he said, "in demonstrating to you the usefulness of these things, for my automatons (clocks) placed in your apartments will impress you every day with the regularity of their indications and the consequences they promise you in the progress of astronomy and navigation." The first chapter of this work was devoted to the description of pendulum-clocks; the second chapter embodied a study of the motion of a grave body moving along a given curve, in which was established the tautochronism of motion in a cycloid. In the third chapter, concerning the evolution and dimension of linear curves, was introduced the idea from which the author deduced the theory of evolutes. In the fourth chapter he determined the center of oscillation of a pendulum, and consequently the length of the simple isochronous pendulum; and in the fifth chapter was estimated the measure of the centrifugal force in circular motion.

We next find Huygens devising the application of the spiral spring to clock-movements, and making pocket watches and sea chronometers possible, and then disputing for the priority of the invention with the Abbé Hautefeuille, "one of those schemers who begin everything and finish nothing."

Huygens turned his attention to the study of the properties of light and weight and of the magnet, and communicated his results to the French Academy and the Royal Society. His theory of light was the one which is now generally accepted after having slept for a hundred and fifty years. Double refraction attracted his attention, and he explained that it was occasioned by an ellipsoidal form given to the light-waves, while in ordinary refraction the waves were spherical. To account for gravity he accepted the Cartesian vortices, and supposed that those bodies which were too unwieldy to keep up with the motion of the outside circles were forced to fall back into the inner circles, where the motion was slower, thus approaching the center. Considering the phenomena of terrestrial gravity exhibited in the variations of the oscillations of the pendulum, he concluded that the earth was a spheroid and not a sphere. He accounted for magnetism in a paper which has never been published, by a theory that has not endured. He left France in 1681, some say on account of the Edict of Nantes, others because his health was bad and he needed a change. At home in Holland he constructed an automatic planetarium to represent the motions of the solar system, and in doing it discovered the theory of continuous fractions.

In the mean time a revolution was taking place in the world of mathematics, through the discovery of the differential calculus by Leibnitz, a philosopher who has said of his intercourse with Huygens, some ten years previous to this time (1672 and 1673), that it opened a new world to him and made him feel like another man. The use of the new method would have greatly facilitated the calculations Huygens was making, but he had become skilled in the old ways, imperfect as they were, and not always of universal application, and, being too old to change his method readily, continued to employ them. But, after a discussion of the merits of the new system in correspondence with Leibnitz, he came to a full appreciation of its value, which he expressed freely by saying that he observed "with surprise and admiration the extent and fruitfulness of that art; on whatever side he turned, he discovered new uses for it; and conceived it destined to infinite progress and speculation."

The "Cosmotheoros," or "Observer of the World," which was not published till after the death of Huygens, was chiefly a treatise on the habitability of other worlds than ours, and was marked by curious and ingenious speculations, of a character from which his other works were almost entirely free. In this work, after expressing his belief in the existence upon the planets of living bodies in no way inferior to those on the earth, he added: "What obliges me to believe also that there is a rational animal in the planets is that, if there is not, the earth would have too great advantages (while it is one of the smallest of the planets) and would be too much elevated in dignity (while it is neither the nearest to the sun nor the most distant from it) over the other planets, if it had an animal so much superior to all that they have. . . . Finally, is it reasonable to suppose that the heavenly bodies among which our earth occupies so modest a rank have been created only in order that we other little men may enjoy their light and contemplate their situation and motion?" He also gave some vivid pictures of the scenery of the heavens as observed from the different planets, paraphrases of which had wide circulation in an English work of popular astronomy of the last generation. In observing the moon he made a study of its mountains and plains, and, remarking that the latter were too rough to be lakes or oceans, concluded, what is now generally believed, that the moon has no bodies of water; also that it has no atmosphere—none at least that rises above the valleys.

At the beginning of the year 1695, Huygens lost his faculties—an affliction he had suffered once before while residing in Paris, but from which he had recovered after removal to his native land. This time the affliction was permanent, except for a few lucid intervals which he employed in making testamentary dispositions of his property, and in consigning the care of his manuscripts to his friends Bürcher de Volder and Bernard Fullen.

Like his illustrious contemporaries Descartes, Leibnitz, and Newton, Huygens was never married. He is described as having had a good figure, and been possessed of a noble and elevated character. He was affable and frank in his disposition, and gave a warm welcome to inquiring young men, whom he was always ready to direct in the way of discovery. It was thus that Leibnitz came to him and received the inspiration of which we have quoted the acknowledgment. Though qualified by birth and fortune to shine in society, and constrained to figure there for a part of his life, he preferred retreat, and passed all of his time that he could in the country, immersed in his studies and experiments.