218 HERSCHEL. 



to the progress of planetary astronomy. But on this subject two im- 

 portant discoveries especially added new glory to his name. 



Of the live known satellites of Saturn at the close of the seventeenth 

 century, Huygens had discovered the fourth ; Cassini the others. 



The tield seemed to be exhausted, when news from Slough announced 

 that this was a mistake. 



On the 28th of August, 1789, the great forty-foot telesco})e revealed 

 to Herschel a satellite still nearer to the ring than the other live already 

 observed. According to the i)rinci[)les of the nomenclature previously 

 adopted, the small body of the 2Sth August ought to have been called 

 the first satellite of Saturn ; the numbers indicating the places of the other 

 five would then have been each increased by a unity. But the fear of 

 introducing confusion into science by these continual changes of denom- 

 ination induced a preference for calling the new satellite the sixth. 



Thanks to the prodigious powers of the forty-foot telescope, a last 

 satellite, the seventh, showed itself on the 17th of September, 1789, 

 between the sixth and the ring. 



This seventh satellite is extremely faint. Herschel, however, snc- 

 ceeded in seeing it whenever circumstances were very favorable, even 

 by the aid of the twenty-foot telescope. 



The discovery of the planet Uranus, and the detection of its satellites, 

 will always occupy one of the highest i)laces among those by which 

 modei'u astrononiy is honored. 



On the loth of IMarch, 1781, between ten and eleven o'clock at night, 

 while Herschel vras examining the small stars near II Geminorum with a 

 seven-foot telescope, bearing a magnifying power of two hundred and 

 twent^'-seveu times, one of these stars seemed to have an unusual 

 diameter, and it was, therefore, thought to be a comet. It was under 

 this denomination that it was discussed at the Eoyal Society of London. 

 But the researches of Herschel and of Laplace showed later that the 

 orbit of the new body was nearly circular, and Uranns was conse- 

 quently elevated to the rank of a planet. 



The immense distance of Uranus, its small angular diameter, and the 

 feebleness of its light, scarcely allowed the hope that, if that body had 

 satellites, they could be perceived from the earth. Herschel was not 

 a man to be deterred by such discouraging conjectures. Therefore, since 

 powerful telescopes of the ordinary construction — that is to say, with 

 two mirrors conjugated — had not enabled him to discover anything, he 

 substituted, in the beginning of January, 1787, />•on^r/e^^ telescopes — 

 that is, telescopes throwing much more light on the objects, the small- 

 mirror being then suppressed, and with it one of the causes of loss of 

 light is got rid of. 



By this means, with patient labor and observations requiring a rare 

 perseverance, Herschel made (from the 11th of January, 1787, to the 

 28th of February, 1791) the discovery of the six satellites of his planet, 

 and thus completed the system of worlds that belongs entirely to him- 

 self. 



