as the
only collector of starlight available to the Greek and Arabian
astronomers. Galileo's telescope, which in 1610 suddenly pushed
out the boundaries of the known stellar universe and brought many
thousands of stars into range, had a lens about 2-1/4 inches in
diameter. The area of this lens, proportional to the square of
its diameter, was about eighty-one times that of the pupil of the
eye. This great increase in the amount of light collected should
bring to view stars down to magnitude 10.5, of which nearly half
a million are known to exist.

It is not too much to say that Galileo's telescope revolutionized
human thought. Turned to the moon, it revealed mountains, plains,
and valleys, while the sun, previously supposed immaculate in its
perfection, was seen to be blemished with dark spots changing from
day to day. Jupiter, shown to be accompanied by four encircling
satellites, afforded a picture in miniature of the solar system,
and strongly supported the Copernican view of its organization,
which was conclusively demonstrated by Galileo's discovery of the
changing phases of Venus and the variation of its apparent diameter
during its revolution about the sun. Galileo's proof of the Copernican
theory marked the downfall of mediaevalism and established astronomy on
a firm foundation. But while his telescope multiplied a hundredfold
the number of visible stars, more than a century elapsed before
the true possibilities of sidereal astronomy were perceived.

[Illustration: Fig. 4. The 100-inch Hooker telescope.]

STRUCTURE OF THE UNIVERSE

Sir William Herschel was the first astronomer to make a serious
attack upon the problem of the structure of the stellar universe.
In his first memoir on the "Construction of the Heavens," read
before the Royal Society in 1784, he wrote as follows:

"Hitherto the sidereal heavens have, not inadequately for the purpose
designed, been represented by the concave surface of a sphere in
the centre of which the eye of an observer might be supposed to be
placed.... In future we shall look upon those regions into which we
may now penetrate by means of such large telescopes, as a naturalist
regards a rich extent of ground or chain of mountains containing
strata variously inclined and directed as well as consisting of
very different materials."

On turning his 18-inch reflecting telescope to a part of the Milky
Way in Orion, he found its whitish appearance to be completely
resolved into