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 11th 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 
xz of the diameter of the earth. 
Herschel saw no phase at an epoch when only 7% of 
