e Englishman Gascoigne to produce the
first instrument on this "Keplerian" principle, universally known as the
Astronomical Telescope.

In 1606 came a second treatise on the new star, discussing various
theories to account for its appearance, and refusing to accept the
notion that it was a "fortuitous concourse of atoms". This was followed
in 1607 by a treatise on comets, suggested by the comet appearing that
year, known as Halley's comet after its next return. He regarded comets
as "planets" moving in straight lines, never having examined sufficient
observations of any comet to convince himself that their paths are
curved. If he had not assumed that they were external to the system and
so could not be expected to return, he might have anticipated Halley's
discovery. Another suggestive remark of his was to the effect that the
planets must be self-luminous, as otherwise Mercury and Venus, at any
rate, ought to show phases. This was put to the test not long afterwards
by means of Galileo's telescope.

In 1607 Kepler rushed into print with an alleged observation of Mercury
crossing the sun, but after Galileo's discovery of sun-spots, Kepler at
once cheerfully retracted his observation of "Mercury," and so far was
he from being annoyed or bigoted in his views, that he warmly adopted
Galileo's side, in contrast to most of those whose opinions were liable
to be overthrown by the new discoveries. Maestlin and others of Kepler's
friends took the opposite view.

CHAPTER V.

KEPLER'S LAWS.

When Gilbert of Colchester, in his "New Philosophy," founded on his
researches in magnetism, was dealing with tides, he did not suggest that
the moon attracted the water, but that "subterranean spirits and
humours, rising in sympathy with the moon, cause the sea also to rise
and flow to the shores and up rivers". It appears that an idea,
presented in some such way as this, was more readily received than a
plain statement. This so-called philosophical method was, in fact, very
generally applied, and Kepler, who shared Galileo's admiration for
Gilbert's work, adopted it in his own attempt to extend the idea of
magnetic attraction to the planets. The general idea of "gravity"
opposed the hypothesis of the rotation of the earth on the ground that
loose objects would fly off: moreover, the latest refinements of the old
system of planetary motions necessitated their orbits being described
about a mere empty point. Kepler very stron