nd had it not been
that Professor Hall was engaged in observations upon the satellites, it
would not have been seen then. A similar spot on the planet was
recorded in 1902, and much more extensively noticed. On this occasion
the spot appeared in a higher latitude from the planet's equator than
did Professor Hall's. At this appearance the time of the planet's
revolution on its axis was found to be somewhat greater than in 1876,
in accordance with the general law exhibited in the rotations of the
sun and of Jupiter. Notwithstanding their transient character, these
two spots have afforded the only determination of the time of
revolution of Saturn which has been made since Herschel the elder.

[Illustration with caption: THE GREAT REFRACTOR OF THE NATIONAL
OBSERVATORY AT WASHINGTON]

Of the satellites of Saturn the brightest is Titan, which can be seen
with the smallest telescope, and revolves around the planet in fifteen
days. Iapetus, the outer satellite, is remarkable for varying greatly
in brilliancy during its revolution around the planet. Any one having
the means and ability to make accurate photometrical estimates of the
light of this satellite in all points of its orbit, can thereby render
a valuable service to astronomy.

The observations of Venus, by which the astronomers of the last century
supposed themselves to have discovered its time of rotation on its
axis, were made with telescopes much inferior to ours. Although their
observations have not been confirmed, some astronomers are still
inclined to think that their results have not been refuted by the
failure of recent observers to detect those changes which the older
ones describe on the surface of the planet. With a six-inch telescope
of the best quality, and with time to choose the most favorable moment,
one will be as well equipped to settle the question of the rotation of
Venus as the best observer. The few days near each inferior conjunction
are especially to be taken advantage of.

The questions to be settled are two: first, are there any dark spots or
other markings on the disk? second, are there any irregularities in the
form of the sharp cusps? The central portions of the disk are much
darker than the outline, and it is probably this fact which has given
rise to the impression of dark spots. Unless this apparent darkness
changes from time to time, or shows some irregularity in its outline,
it cannot indicate any rotation of the planet. The best ti