Tycho made those invaluable observations
that enabled Kepler to deduce the true laws of planetary motion. But
after all these centuries the sidereal world embraced no objects,
barring an occasional comet or temporary star, that lay beyond
the vision of the earliest astronomers. The conceptions of the
stellar universe, except those that ignored the solid ground of
observation, were limited by the small aperture of the human eye.
But the dawn of another age was at hand.

[Illustration: Fig. 2. The Great Nebula in Orion (Pease).

Photographed with the 100-inch telescope. This short-exposure photograph
shows only the bright central part of the nebula. A longer exposure
reveals a vast outlying region.]

The dominance of the sun as the central body of the solar system,
recognized by Aristarchus of Samos nearly three centuries before
the Christian era, but subsequently denied under the authority of
Ptolemy and the teachings of the Church, was reaffirmed by the
Polish monk Copernicus in 1543. Kepler's laws of the motions of the
planets, showing them to revolve in ellipses instead of circles,
removed the last defect of the Copernican system, and left no room for
its rejection. But both the world and the Church clung to tradition,
and some visible demonstration was urgently needed. This was supplied
by Galileo through his invention of the telescope.

[Illustration: Fig. 3. Model by Ellerman of summit of Mount Wilson,
showing the observatory buildings among the trees and bushes.

The 60-foot tower on the extreme left, which is at the edge of
a precipitous canon 1,500 feet deep, is the vertical telescope
of the Smithsonian Astrophysical Observatory. Above it are the
"Monastery" and other buildings used as quarters by the astronomers
of the Mount Wilson Observatory while at work on the mountain. (The
offices, computing-rooms, laboratories, and shops are in Pasadena.)
Following the ridge, we come successively to the dome of the 10-inch
photographic telescope, the power-house, laboratory, Snow horizontal
telescope, 60-foot-tower telescope, and 150-foot-tower telescope,
these last three used for the study of the sun. The dome of the
60-inch reflecting telescope is just below the 150-foot tower,
while that of the 100-inch telescope is farther to the right. The
altitude of Mount Wilson is about 5,900 feet.]

The crystalline lens of the human eye, limited by the iris to a
maximum opening about one-quarter of an inch in diameter, w