Nova Aurigae " 396

Plate VI. Photograph of the Milky Way in Sagittarius " 424

HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY

_INTRODUCTION_

We can distinguish three kinds of astronomy, each with a different
origin and history, but all mutually dependent, and composing, in their
fundamental unity, one science. First in order of time came the art of
observing the returns, and measuring the places, of the heavenly bodies.
This was the sole astronomy of the Chinese and Chaldeans; but to it the
vigorous Greek mind added a highly complex geometrical plan of their
movements, for which Copernicus substituted a more harmonious system,
without as yet any idea of a compelling cause. The planets revolved in
circles because it was their nature to do so, just as laudanum sets to
sleep because it possesses a _virtus dormitiva_. This first and oldest
branch is known as "observational," or "practical astronomy." Its
business is to note facts as accurately as possible; and it is
essentially unconcerned with schemes for connecting those facts in a
manner satisfactory to the reason.

The second kind of astronomy was founded by Newton. Its nature is best
indicated by the term "gravitational"; but it is also called
"theoretical astronomy."[1] It is based on the idea of cause; and the
whole of its elaborate structure is reared according to the dictates of
a single law, simple in itself, but the tangled web of whose
consequences can be unravelled only by the subtle agency of an elaborate
calculus.

The third and last division of celestial science may properly be termed
"physical and descriptive astronomy." It seeks to know what the heavenly
bodies are in themselves, leaving the How? and the Wherefore? of their
movements to be otherwise answered. Now, such inquiries became possible
only through the invention of the telescope, so that Galileo was, in
point of fact, their originator. But Herschel first gave them a
prominence which the whole progress of science during the nineteenth
century served to confirm and render more exclusive. Inquisitions begun
with the telescope have been extended and made effective in unhoped-for
directions by the aid of the spectroscope and photographic camera; and a
large part of our attention in the present volume will be occupied with
the brilliant results thus achieved.

The unexpected development of this