HISTORY OF PHYSICAL ASTRONOMY.

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bodies had been gradually diminishing, although they appeared to be im-moveable, both with respect to each other and to the central body*. Theycontinued to grow less and less during the course of the two followingyears, at the close of which they finally vanished altogether, leavingthe planet quite round, like Jupiter . Galileo announced this mysteriousdisappearance of the phenomenon in his third letter to Welser, datedDecember 4, 1012. The astonishment which he felt on the occasion wasmingled with a feeling of alarm lest the Aristotelians, taking advantageof his inability to adduce an adequate explanation of the cause of disap-pearance, should attempt to make his observations a subject of derision,by alleging them to have been destitute of any foundation in nature.“ What,” he remarks, “is to be said concerning so strange a metamor-phosis ? Are the two lesser stars consumed after the manner of the solarspots? Have they vanished and suddenly lied? Has Saturn perhapsdevoured his own children ? Or were the appearances indeed illusion orfraud, with which the glasses have so long deceived me, as well as manyothers, to whom I have shewn them ? Now, perhaps, is the time cometo revive the well nigh withered hopes of those who, guided by more pro-found contemplations, have discovered the fallacy of the new observations,and demonstrated the utter impossibility of their existence. I do notknow what to say in a case so surprising, so unlooked for, and so novel.The shortness of the time, the unexpected nature of the event, the weak-ness of my understanding, and the fear of being mistaken, have greatlyconfounded me.”j-

During the course of nearly half a century which elapsed after the in-vention of the telescope, the singular vicissitudes which the appendage ofSaturn underwent, continued to form an inexplicable enigma to astrono-mers. The only real progress made towards its explanation consistediu arriving at more accurate views of the appearances exhibited by itin the telescope and in ascertaining the time which it occupied inpassing through the cycle of its phases. Instruments superior to thoseof Galileo shewed that the objects composing the appendage were notround like the other celestial bodies, but rather resembled two luminouscrescents, attached by their cusps to the planet, and forming, as it were,two handles to it. The conformation of these handles, or ansa, wasfound to be subject to a slow alteration, which caused the appearance ofthe planet to be constantly changing. When the ansae first became visible,after the planet had passed through its round form, they appeared like twoarms extending on each side of the planet, but they continued gradually toopen out during the following seven or eight years, at the close of which theyagain began to contract, and after an equal lapse of time, vanished entirely,when the planet again assumed its round form. The appendage thereforecompleted the cycle of its various appearances in about fifteen years, or halfthe period of the planet's revolution round the sun. It was reserved forHuyghens to discover the true cause of these singular changes. He first com-municated to the world his explanation of them, under the form of an enigma,in a small tract, entitled, “ De SaturniLuna Observatio Nova,” publishedin 1656 ; but he subsequently gave an explicit announcement of the realnature of the appendage of the planet, with a detailed exposition of itstheory, in a book written expressly on the subject which appeared in theyear 1659 {. In this work, before proceeding to explain his own theory, he

* Opere di Galileo, Edit. Pad., tom. ii., p. 46. Ibid., p. 152.

1 Systema Saturnium, 4to. Hagas, 1659.