The Dutch mathematician, astronomer, and physicist Christiaan Huygens (1629-1695) was the first to recognize the rings of Saturn, made pioneering studies of the dynamics of moving bodies, and was the leading advocate of the wave, or pulse, theory of light.
Born in The Hague on April 14, 1629, Christiaan Huygens was the second son of Constantin Huygens, a brilliant diplomat and Renaissance scholar. Privately tutored at home until he was 16, Christiaan early showed signs of intellectual brilliance, devoting much time to drawing and making mechanical models and devices as well as demonstrating exceptional skill in geometry. He studied law and mathematics at the University of Leiden and after 2 years moved on to Breda, where he completed his studies.
Huygens's first published work, on the quadrature of various mathematical curves, appeared in 1651. In addition, as a result of his study of collision phenomena between hard, elastic bodies, by 1656 he had demonstrated the incorrectness of René Descartes's laws of motion and impact, although he did not announce his conclusions until some 12 years later, and his complete study of such phenomena was published posthumously. The best-known of his early researches, however, were his efforts to improve telescope lenses and his observations of the planet Saturn.
In 1655 Huygens spent several months in Paris. He attended the informal gatherings of the so-called Montmort Academy, an important precursor of the French Academy of Sciences and, to at least some of its members, reported on his discovery of Titan, the first of Saturn's moons to be observed. He had initially been attracted to Saturn by its apparently anomalous shape, described by Galileo as "three spheres which almost touch each other, which never change their relative positions, and are arranged in a row along the zodiac so that the middle sphere is three times as large as the others." Intrigued by this peculiar shape, Huygens realized that its resolution would depend on constructing improved telescopes, less subject to various aberrations and more capable of producing detailed images.
Upon returning from Paris, Huygens devoted full time to his efforts to construct such improved eyepieces and lens systems, and although he was unsuccessful in his attempts to produce lenses with hyperbolic or elliptical surfaces, he and his elder brother did succeed in figuring and polishing lenses with an accuracy never before attained. With telescopes utilizing these improved components, great progress was made toward solving the problem of Saturn's appearance. What had originally appeared as a "trispherical" form now appeared as simply some sort of band passing across the middle of the planet, and, early in 1656, utilizing a still better telescope, Huygens was able to clearly distinguish a thin ring surrounding the planet at a slight angle to the ecliptic. In 1659 he published his complete study of Saturn in a work entitled Systema Saturnium.
Huygens's interest in improving telescopes continued throughout his life. For measuring the angular diameter of planets, he invented a type of micrometer consisting of a series of small brass plates of varying widths which could be slipped across the focal plane of the telescope. Recognizing that the eyepiece could be made to partially correct for certain defects in the objective lens of a telescope, he designed a special eyepiece which still bears his name, and his improved methods of grinding lenses allowed him to construct longer telescopes with greater powers of magnification. These "aerial telescopes" exceeded 30 feet in length and dispensed entirely with the usual tubular enclosure, utilizing instead two shorter tubes, one for the eyepiece and one for the objective lens. Huygens never was, however, a regular astronomical observer; with the exception of his observations of Saturn, his contributions to astronomy exhibited a strong practical bias.
This practical aspect of Huygens's work is also manifested in the great time and energy he devoted to the perfection of the pendulum clock. Although he had been working on it for some years, Huygens first described his successful application of the pendulum to the escapement mechanism of the standard mechanical clock in 1658.
Sometime after he had produced the first pendulum clock, Huygens became interested in its obvious application to the problem of determining longitude at sea. One of the simplest solutions to this important navigation problem involved the construction of an extremely accurate time keeping device with which local time could be compared with a standard time at, say, Paris or London. Although the pendulum clock was the most accurate such device then available, its motion was easily disturbed by the movement of the ship at sea. In an effort to overcome this difficulty, Huygens invented a pendulum whose period of oscillation was independent of the amplitude of its swing (for regular pendulums this isochronous property exists only for very small amplitudes). Although the discovery of a pendulum whose path was a cycloidal arc provided such an isochronous device, it did not solve the problem of constructing a marine clock or chronometer whose accuracy would not be affected by the pitching of a ship.
In 1660 Huygens returned to Paris, where he again attended meetings of the Montmort Academy. By 1661 he had discovered a basic principle of mechanics which allowed him to solve with ease certain types of problems which English mathematicians at the time found especially difficult. Now fundamental, this principle stated that the center of gravity of a body or system of bodies, acting solely under the influence of gravity, cannot rise above the level from which it initially falls. In recognition of the significance of this and other aspects of his work, he was in 1663 elected a fellow of the Royal Society.
The French Academy of Sciences was officially formed in 1666, and at the invitation of Louis XIV's chief minister, Huygens returned to Paris and a position as a leader and foundation member of the new academy. He was one of the chief influences in guiding the early affairs of the academy and, profiting from his contacts with English men of science, he emphasized the need for careful observation and experiment. With the exception of two trips to The Hague because of illness, he remained in Paris until 1681.
In the years after his return to Paris, Huygens's interests turned increasingly from astronomy to terrestrial mechanics, and as the result of his work in this field, he has rightfully been regarded as one of the founders of the science of dynamics. His earliest studies in this area dealt with impact phenomena, and although he had completed this work as early as 1656, his results were reported only in 1669, when he presented to the Royal Society a clear and concise statement of the laws governing the collision of elastic bodies. Although of great significance because of their statement of the conservation of mechanical energy in the collision of perfectly elastic bodies and because of their refutation of the incorrect laws of impact earlier presented by Descartes, Huygens's results were presented without proof, and their complete demonstration was published posthumously in 1703.
One of the great scientific treatises of the 17th century, Huygens's masterpiece, Horologium oscillatorium, appeared in 1673. More than just a summary of his researches on the pendulum clock, it in fact was a general work on dynamics containing numerous original discoveries. In it he demonstrated the isochronous nature of a body moving freely under the influence of gravity along a cycloidal path. He showed how to calculate the period of oscillation of a simple pendulum. He provided a definitive solution to the problem of compound and physical pendulums, demonstrating how to calculate the "center of oscillation" and the length of an equivalent simple pendulum. And, in an appendix, he presented the basic laws of centrifugal force governing bodies moving with uniform circular motion. The significance of this monumental work was immediately recognized.
Perhaps the best-known of Huygens's varied pursuits is his work on physical optics and his development of the wave, or, more accurately, pulse, theory of light. First presented before the Academy of Sciences in 1678, his Traité de la lumière (Treatise on Light) was, characteristically, not published until 1690. The theory of light put forth in it, however, was the direct result of his study of impact phenomena and represented the union of the physical and mathematical aspects of the study of optics.
Light, Huygens suggested, consisted of the longitudinal vibrations of an all-pervasive ether composed of small, hard, elastic particles, each of which transmitted the impulses it received to all contiguous particles without itself suffering any permanent displacement. The propagation of light was thus reduced to the transmission of motion. Each particle of a luminous body, such as a candle flame, sent out its own set of concentric, spherical wavelets. Formulating what is today known as Huygens's principle, he conceived of each ether particle itself as also being the source of a new wavelet, which was likewise propagated to the adjacent particles. Within the boundaries where these individual wavelets reinforced each other and formed a coherent wave envelope, light was propagated. Outside the boundaries, there being no reinforcement of the wavelets, was shadow and no propagation of light.
In 1681 Huygens returned to The Hague. Although ill health was the immediate cause, additional personal and religious pressures combined to make permanent his return to his native country. He had never married, and his later years were characterized by considerable solitariness; in his correspondence he often lamented the absence of anyone with whom to discuss scientific topics. He did, however, maintain his extensive correspondence, and although his mathematical and abstract studies suffered a marked diminution after 1680, the general pattern of his life remained little changed until his death on July 8, 1695.
Unlike many men of science in the 17th century, Huygens never occupied himself to any significant extent with either philosophy or theology. He devoted his efforts entirely to the pursuit of science, and his contributions to astronomy, dynamics, and optics were of fundamental importance.
Huygens's correspondence and collected works were published in Dutch in a 22-volume edition under the auspices of the Dutch Academy of Sciences (1888-1950). His Treatise on Light was translated into English by Silvanus P. Thompson (1912). Huygens's life and the historical significance of his work are covered in Arthur Ernest Bell, Christian Huygens and the Development of Science in the Seventeenth Century (1947).