296 HERSCHEL. 



The immense distance of Uranus, its small angular 

 diameter, the feebleness of its light, did not allow the 

 hope, that if that body had satellites, the magnitudes of 

 which were, relatively to its own size, what the satellites 

 of Jupiter, of Saturn are, compared to those two large 

 planets, any observer could perceive them, from the 

 earth. Herschel was not a man to be deterred by such, 

 discouraging conjectures. Therefore, since powerful tel- 

 escopes of the ordinary construction, that is to say, with 

 two mirrors conjugated, had not enabled him to discover 

 any thing, he substituted, in the beginning of January, 

 1787, front view 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 patient labour, by observations requiring a rare 

 perseverance, Herschel attained (from the llth of Janu- 

 ary, 1787, to the 28th of February, 1794,) to the dis- 

 covery of the six satellites of his planet, and thus to 

 complete the world of a system that belongs entirely to 

 himself. 



There are several of Herschel's memoirs on comets. 

 In analyzing them, we shall see that this great observer 

 could not touch any thing without making further dis- 

 coveries in the subject. 



Herschel applied some of his fine instruments to the 

 study of the physical constitution of a comet discovered 

 by Mr. Pigott, on the 28th September, 1807. 



The nucleus was round and well determined. Some 

 measures taken on the day when the nucleus subtended 

 only an angle of a single second, gave as its real angle 

 of the diameter of the earth. 



Herschel saw no phase at an epoch when only T 7 5 of 



