HUYGENS (VAN ZUYLICHEM), CHRISTIAN (ante). The discovery of the ring of Saturn was not made with the first telescope with which Huygens discovered the fourth (then the first) satellite. That instrument, although the most powerful then made, had a focal length of only 10 feet. The ring was discovered in 1659 with a telescope of 22 ft. focal length, which besides being much more powerful than the first, had a greatly improved construction of the eye glasses—essentially the same as those used at the present day, with the exception of the achromatic combination of lenses, a comparatively recent discovery. The discovery of the ring of Saturn was one of the master exploits of science. The appearance of two luminous bodies on either side of • the planet at various times had been observed by Galileo, but his telescope did not enable him to make out anything more. The appearance of the two luminous points was found by lluygens to be caused by a position of the ring by which it reflected the sun's rays. From an examination of the subject he made calculations which foretold the disappearance of the ring from view in 1671. The_discovery was published in 1659 in a volume which con= tained, among several other discoveries, an account of the bands upon the disks of Jupiter and Mars, and of the great nebula in Orion. His Horologium Oscillatorium, a part of which was published in 1658 at the Hague, was not completed till 1673, and published at Paris. This book contains 13 theorems on centrifugal force, which led to most important results in the science of mechanics, and in the same work he gives a method for finding the center of oscillation. Mersenne had proposed the problem of the
center of oscillation in 1646, which was not solvable by any of the then known mathemati cal methods. It had attracted Huygens's attention at the time, but his youthful mind was unable to cope with the subject. However, when the last part of the Horologium appeared, in 1673, the problem was completely solved by a method which was the first solution of a dynamical problem in which connected material points are supposed to act on one another. Newton afterwards demonstrated these 13 theorems of Huygens in a manner of his own. Huygens's own demonstrations were found after his death among his papers. The first two theorems contain the assumption of the two first laws of motion, and in his treatise De Motu Corporum ex Percussion he assumed the third law of motion, which Newton afterwards expressed in the words " action and reaction are equal and opposite," an equivalent for saying that the impact of two bodies does not change the quantity of motion in them—one of the grand principles of the doctrine of conservation of forces. His works were edited by 'sGravesande in four volumes, entitled Opera, Maria, 2 vols. 4to; in one volume, Leyden, 1724; and Opera Beliqua, 2 vols. 4to, Amster dam, 1728.