dition to the number of planets without overturning my
_Mysterium Cosmographicon_, published thirteen years ago, according
to which Euclid's five regular solids do not allow more than six
planets round the sun.

"But I am so far from disbelieving the existence of the four
circumjovial planets that I long for a telescope to anticipate you
if possible in discovering two round Mars (as the proportion seems
to me to require) six or eight round Saturn, and one each round
Mercury and Venus."

[Illustration: FIG. 45.--Eclipses of Jupiter's satellites. The diagram
shows the first (_i.e._ the nearest) moon in Jupiter's shadow, the
second as passing between earth and Jupiter, and appearing to transit
his disk, the third as on the verge of entering his shadow, and the
fourth quite plainly and separately visible.]

As an illustration of the opposite school, I will take the following
extract from Francesco Sizzi, a Florentine astronomer, who argues
against the discovery thus:--

"There are seven windows in the head, two nostrils, two eyes, two
ears, and a mouth; so in the heavens there are two favourable
stars, two unpropitious, two luminaries, and Mercury alone
undecided and indifferent. From which and many other similar
phenomena of nature, such as the seven metals, &c., which it were
tedious to enumerate, we gather that the number of planets is
necessarily seven.

"Moreover, the satellites are invisible to the naked eye, and
therefore can have no influence on the earth, and therefore would
be useless, and therefore do not exist.

"Besides, the Jews and other ancient nations as well as modern
Europeans have adopted the division of the week into seven days,
and have named them from the seven planets: now if we increase the
number of the planets this whole system falls to the ground."

To these arguments Galileo replied that whatever their force might be as
a reason for believing beforehand that no more than seven planets would
be discovered, they hardly seemed of sufficient weight to destroy the
new ones when actually seen.

Writing to Kepler at this time, Galileo ejaculates:

"Oh, my dear Kepler, how I wish that we could have one hearty laugh
together! Here, at Padua, is the principal professor of philosophy
whom I have repeatedly and urgently requested to look at the moon
and planets through