98 THE HISTORY OF THE TELESCOPE. 



of planetary bodies unknowu to the ancieuts. But his most important 

 astronomical discovery, made also in 105"), was the nature of tlie 

 rings of Saturn. This object had greatly puzzled Galileo, to whose 

 small telescope the planet appeared to consist of a larger s])here 

 flanked on either side by a smaller one; but when in the course of the 

 orbital motion of Saturn the rings entirely disappeared he was wholly 

 unable to suggest an explanation. This planet had thus j)resented 

 a remarkable problem to all astronomical observers for more than 

 forty years, and the records of the eftbrts to solve it during that inter- 

 val afford us a most excellent means of judging the progress in practi- 

 cal optics. Huyghens announced these discoveries early in 1650, but 

 that relating to the ring was given in the form of an anagram, the 

 solution of which was first published in 1659. This discovery was 

 contested in Italy by Divini, but was finally confirmed by members of 

 the Florentine Academy with one of Divini's own telescopes. 



A few years later the famous astronomer Cassini, having come to 

 Paris from Italy as royal astronomer, commenced a series of brilliant 

 discoveries with telescopes made by Campani, of Eome. AVith these, 

 varying in length from 35 feet to 13(5 feet, he discovered four satellites 

 to Saturn in addition to the one discovered by Huyghens. The whole 

 number was increased by Herschel's discovery of two smaller ones in 

 1789, a hundred and five years after Cassini's last discovery, and again 

 by Bond's discovery of an eighth in 1818. The Saturnian system, to 

 which the telescope has doubtless been directed luore frequently than 

 to anything else, thus serves as a record of the successive im])rove- 

 ments of the telescope. Highly significant is the fact that the discov- 

 eries of the eighteenth century were made with a reflecting telescope, 

 the otliers all being with refracting instruuients. 



Cassini's discovery in 1681 of the two satellites now known as Tethys 

 and Dione, was not accepted as conclusive until long afterwards, when 

 Pound, in 1718, wich a telescope 123 feet in length, which Huyghens 

 had made and presented to the Royal Society, saw all five. This par- 

 ticular instrument is of especial interest, because it is the only one of 

 those of the last half of the seventeenth century which has been care- 

 fully compared with modern instruments. Moreover, it is without 

 doubt quite equal in merit to any of that period. But we find that, 

 although it had a diameter of 6 inches, its performance was hardly 

 better than that of a perfect modern telescope of 4 inches in diameter, 

 and, ])erhaps, 4i feet in length, while in regard to convenience in use 

 the modern compact instrument is incomparably superior. 



Another notable discovery of this period was that of the duplicity 

 of the rings of Saturn by the Ball brothers in 1665, though its inde- 

 pendent discovery by Cassini ten years later first attracted the atten- 

 tion of astronomers. The earlier discovery was made by means of a 

 telescope 38 feet long which seems to have been of English manufac- 

 ture. We must regard Cassini's discovery of the third and loiuth sat- 



