conds.[1119] This, he was careful to add, represented the
period, not necessarily of the _planet_, but only of the individual
spot. The only previous determination of Saturn's axial movement
(setting aside some insecure estimates by Schroeter) was Herschel's in
1794, giving a period of ten hours sixteen minutes. The substantial
accuracy of Hall's result was verified by Mr. Denning in 1891.[1120] In
May and June of that year, ten vague bright markings near the equator
were watched by Mr. Stanley Williams, who derived from them a rotation
period only two seconds shorter than that determined at Washington.
Nevertheless, similarly placed spots gave in 1892 and 1893 notably
quicker rates;[1121] so that the task of timing the general drift of the
Saturnian surface by the displacements of such objects is hampered, to
an indefinite extent, by their individual proper motions.

Saturn's outermost satellite, Japetus, is markedly variable--so variable
that it sends us, when brightest, just 4-1/2 times as much light as when
faintest. Moreover, its fluctuations depend upon its orbital position in
such a way as to make it a conspicuous telescopic object when west, a
scarcely discernible one when east of the planet. Herschel's
inference[1122] of a partially obscured globe turning always the same
face towards its primary seems the only admissible one, and is confirmed
by Pickering's measurements of the varying intensity of its light. He
remarked further that the dusky and brilliant hemispheres must be so
posited as to divide the disc, viewed from Saturn, into nearly equal
parts; so that this Saturnian moon, even when "full," appears very
imperfectly illuminated over one-half of its surface.[1123]

Zoellner estimated the albedo of Saturn at 0.51, Mueller at 0.88, a value
impossibly high, considering that the spectrum includes no vestige of
original emissions. Closely similar to that of Jupiter, it shows the
distinctive dark line in the red (wave-length 618), which we may call
the "red-star line"; and Janssen, from the summit of Etna in 1867[1124]
found traces in it of aqueous absorption. The light from the ring
appears to be pure reflected sunshine unmodified by original atmospheric
action.[1125]

Uranus, when favourably situated, can easily be seen with the naked eye
as a star between the fifth and sixth magnitudes. There is indeed, some
reason to suppose that he had been detected as a wandering orb by savage
"watchers of the skies" i