which several observers seemed at that time to be engaged,
was the establishment, on a satisfactory footing, of our acquaintance
with the dependent system of Uranus. Sir William Herschel, whose
researches formed, in so many distinct lines of astronomical inquiry,
the starting-points of future knowledge, detected, January 11,
1787,[236] two Uranian moons, since called Oberon and Titania, and
ascertained the curious circumstance of their motion in a plane almost
at right angles to the ecliptic, in a direction contrary to that of all
previously known denizens (other than cometary) of the solar kingdom. He
believed that he caught occasional glimpses of four more, but never
succeeded in assuring himself of their substantial existence. Even the
two first remained unseen save by himself until 1828, when his son
re-observed them with a 20-foot reflector, similar to that with which
they had been originally discovered. Thenceforward they were kept fairly
within view, but their four questionable companions, in spite of some
false alarms of detection, remained in the dubious condition in which
Herschel had left them. At last, on October 24, 1851,[237] after some
years of fruitless watching, Lassell espied "Ariel" and "Umbriel," two
Uranian attendants, interior to Oberon and Titania, and of about half
their brightness; so that their disclosure is still reckoned amongst the
very highest proofs of instrumental power and perfection. In all
probability they were then for the first time seen; for although
Professor Holden[238] made out a plausible case in favour of the fitful
visibility to Herschel of each of them in turn, Lassell's argument[239]
that the glare of the planet in Herschel's great specula must have
rendered almost impossible the perception of objects so minute and so
close to its disc, appears tolerably decisive to the contrary. Uranus is
thus attended by four moons, and, so far as present knowledge extends,
by no more. Among the most important of the "negative results"[240]
secured by Lassell's observations at Malta during the years 1852-53 and
1861-65, were the convincing evidence afforded by them that, without
great increase of optical power, no further Neptunian or Uranian
satellites can be perceived, and the consequent relegation of Herschel's
baffling quartette, notwithstanding the unquestioned place long assigned
to them in astronomical text-books, to the Nirvana of non-existence.

FOOTNOTES:

[Footnote 195: _Op.