Photograph of the Great Nebula in Orion, 1883 _Frontispiece_
Photographs of Jupiter, 1879, and of Saturn, 1885 _Vignette_
Plate I. Photographs of the Solar Chromosphere and Prominences _To face p. 198_
Plate II. Photograph of the Great Comet of May, 1901 (Taken at the Royal Observatory, Cape of Good Hope) " 343
Plate III. The Great Comet of September, 1882 (Photographed at the Cape of Good Hope) " 359
Plate IV. Photographs of Swift's Comet, 1892 " 368
Plate V. Photographic and Visual Spectrum of Nova Aurigæ " 396
Plate VI. Photograph of the Milky Way in Sagittarius " 424
HISTORY OF ASTRONOMY DURING THE NINETEENTH CENTURY
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.