d his kingdom's destiny.
He seems to have been politically a weak and superstitious prince, who
was letting his kingdom get into hopeless confusion, and entangling
himself in all manner of political complications. While Bohemia
suffered, however, the world has benefited at his hands; and the tables
upon which Tycho was now engaged are well called the Rudolphine tables.

These tables of planetary motion Tycho had always regarded as the main
work of his life; but he died before they were finished, and on his
death-bed he intrusted the completion of them to Kepler, who loyally
undertook their charge.

The Imperial funds were by this time, however, so taxed by wars and
other difficulties that the tables could only be proceeded with very
slowly, a staff of calculators being out of the question. In fact,
Kepler could not get even his own salary paid: he got orders, and
promises, and drafts on estates for it; but when the time came for them
to be honoured they were worthless, and he had no power to enforce his
claims.

So everything but brooding had to be abandoned as too expensive, and he
proceeded to study optics. He gave a very accurate explanation of the
action of the human eye, and made many hypotheses, some of them shrewd
and close to the mark, concerning the law of refraction of light in
dense media: but though several minor points of interest turned up,
nothing of the first magnitude came out of this long research.

The true law of refraction was discovered some years after by a Dutch
professor, Willebrod Snell.

We must now devote a little time to the main work of Kepler's life. All
the time he had been at Prague he had been making a severe study of the
motion of the planet Mars, analyzing minutely Tycho's books of
observations, in order to find out, if possible, the true theory of his
motion. Aristotle had taught that circular motion was the only perfect
and natural motion, and that the heavenly bodies therefore necessarily
moved in circles.

So firmly had this idea become rooted in men's minds, that no one ever
seems to have contemplated the possibility of its being false or
meaningless.

When Hipparchus and others found that, as a matter of fact, the planets
did _not_ revolve in simple circles, they did not try other curves, as
we should at once do now, but they tried combinations of circles, as we
saw in Lecture I. The small circle carried by a bigger one was called an
Epicycle. The carrying circle was ca