had so sound a philosophical basis, evidenced the
iconoclastic nature of his genius. That he did not rest content until he
had demonstrated the validity of his revolutionary assumption shows how
truly this great theorizer made his hypotheses subservient to the most
rigid inductions.

GALILEO GALILEI

While Kepler was solving these riddles of planetary motion, there was
an even more famous man in Italy whose championship of the Copernican
doctrine was destined to give the greatest possible publicity to the
new ideas. This was Galileo Galilei, one of the most extraordinary
scientific observers of any age. Galileo was born at Pisa, on the 18th
of February (old style), 1564. The day of his birth is doubly memorable,
since on the same day the greatest Italian of the preceding epoch,
Michael Angelo, breathed his last. Persons fond of symbolism have found
in the coincidence a forecast of the transit from the artistic to
the scientific epoch of the later Renaissance. Galileo came of an
impoverished noble family. He was educated for the profession of
medicine, but did not progress far before his natural proclivities
directed him towards the physical sciences. Meeting with opposition in
Pisa, he early accepted a call to the chair of natural philosophy in the
University of Padua, and later in life he made his home at Florence. The
mechanical and physical discoveries of Galileo will claim our attention
in another chapter. Our present concern is with his contribution to the
Copernican theory.

Galileo himself records in a letter to Kepler that he became a convert
to this theory at an early day. He was not enabled, however, to make any
marked contribution to the subject, beyond the influence of his general
teachings, until about the year 1610. The brilliant contributions which
he made were due largely to a single discovery--namely, that of the
telescope. Hitherto the astronomical observations had been made with the
unaided eye. Glass lenses had been known since the thirteenth century,
but, until now, no one had thought of their possible use as aids to
distant vision. The question of priority of discovery has never been
settled. It is admitted, however, that the chief honors belong to the
opticians of the Netherlands.

As early as the year 1590 the Dutch optician Zacharias Jensen placed
a concave and a convex lens respectively at the ends of a tube about
eighteen inches long, and used this instrument for the purpose of
magnify