Such was the construction of the telescopes with which Hevelius, Huygens, Cassini, and other eminent astronomers of the seventeenth century made their principal discoveries. With such telescopes, Huygens discovered the fourth satellite of Saturn, and determined that this planet was surrounded with a ring; and with the same kind of instrument Cassini detected the first, second, third, and fifth, satellites of Saturn, and made his other discoveries. When the night was very dark, they were obliged to make the object-glass visible, by means of a lantern so constructed as to throw the rays of light up to it in a parallel direction. In making such observations, they must have taken incredible pains, endured much cold and fatigue, and subjected themselves to very great labour and expense--which almost makes us wonder at the discoveries they were instrumental in bringing to light--and should make modern philosophers sensible of the obligations they are under to such men as Newton and Dollond, through whose inventions such unwieldy instruments are no longer necessary. Telescopes of the description now stated were made of all sizes, from 30 to above 120 feet in length. Divini at Rome, and Campani at Bologna, were famed as makers of the object-glasses of the long focal distance to which we have alluded, who sold them for a great price, and took every method to keep the art of making them a secret. It was with telescopes made by Campani, that Cassini made his discoveries. They were made by the express order of Louis XIV, and were of 86, 100, and 136 Paris feet in focal length. M. Auzout made one object-glass of 600 feet focus; but he was never able to manage it, so as to make any practical observations with it. Hartsocker is said to have made some of a still greater focal length. The famous aerial telescope of Huygens was 123 feet in focal length, with six inches of aperture. At his death, he bequeathed it to the Royal Society of London, in whose possession it still remains. It required a pole of more than a hundred feet high, on which to place the object-glass for general observations. It was with this glass, that Dr. Derham made the observations to which he alludes in his preface to his ‘Astro-Theology.’ When this glass was in the possession of Mr. Cavendish, it was compared with one of Mr. Dollond’s forty-six inch treble object-glass Achromatics, and the gentlemen who were present at the trial, said that ‘the Dwarf was fairly a match for the Giant.’ It magnified 218 times, and the trouble of managing it, was said to be extremely tiresome and laborious. SECT. 4.--THE COMMON REFRACTING TELESCOPE FOR TERRESTRIAL OBJECTS. [Illustration: _figure 47._]