Fig. 96.—The Globular Cluster in Hercules.
In the southern hemisphere there are two immense masses which are conspicuously visible to the naked eye, and resemble detached portions of the Milky Way. They cannot be seen by observers in our latitude, and are known as the Magellanic clouds or the two nubeculæ. Their structure, as revealed to an observer using a powerful telescope, is of great complexity. Sir John Herschel, who made a special study of these remarkable objects, gives the following description of them: "The general ground of both consists of large tracts and patches of nebulosity in every stage of resolution, from light irresolvable, in a reflector of eighteen inches aperture, up to perfectly separated stars like the Milky Way, and clustering groups sufficiently insulated and condensed to come under the designation of irregular and in some cases pretty rich clusters. But besides these there are also nebulæ in abundance and globular clusters in every state of condensation." It can hardly be doubted that the two nubeculæ, which are, roughly speaking, round, or, rather, oval, are not formed accidentally by a vast number of very different objects being ranged at various distances along the same line of sight, but that they really represent two great systems of objects, widely different in constitution, which here are congregated in each other's neighbourhood, whereas they generally do not co-exist close to each other in the Milky Way, with which the mere naked-eye view would otherwise lead us to associate the Magellanic clouds.
When we direct a good telescope to the heavens, we shall occasionally meet with one of the remarkable celestial objects which are known as nebulæ. They are faint cloudy spots, or stains of light on the black background of the sky. They are nearly all invisible to the naked eye. These celestial objects must not for a moment be confounded with clouds, in the ordinary meaning of the word. The latter exist only suspended in the atmosphere, while nebulæ are immersed in the depths of space. Clouds shine by the light of the sun, which they reflect to us; nebulæ shine with no borrowed light; they are self-luminous. Clouds change from hour to hour; nebulæ do not change even from year to year. Clouds are far smaller than the earth; while the smallest nebula known to us is incomparably greater than the sun. Clouds are within a few miles of the earth; the nebulæ are almost inconceivably remote.
Immediately after Herschel and his sister had settled at Slough he commenced his review of the northern heavens in a systematic manner. For observations of this kind it is essential that the sky be free from cloud, while even the light of the moon is sufficient to obliterate the fainter and more interesting objects. It was in the long and fine winter nights, when the stars were shining brilliantly and the pale path of the Milky Way extended across the heavens, that the labour was to be done. The telescope being directed to the heavens, the ordinary diurnal motion by which the sun and stars appear to rise and set carries the stars across the field of view in a majestic panorama. The stars enter slowly into the field of view, slowly move across it, and slowly leave it, to be again replaced by others. Thus the observer, by merely remaining passive at the eye-piece, sees one field after another pass before him, and is enabled to examine their contents. It follows, that even without moving the telescope a long narrow strip of the heavens is brought under review, and by moving the telescope slightly up and down the width of this strip can be suitably increased. On another night the telescope is brought into a different position, and another strip of the sky is examined; so that in the course of time the whole heavens can be carefully scrutinised.
Herschel stands at the eye-piece to watch the glorious procession of celestial objects. Close by, his sister Caroline sits at her desk, pen in hand, to take down the observations as they fall from her brother's lips. In front of her is a chronometer from which she can note the time, and a contrivance which indicates the altitude of the telescope, so that she can record the exact position of the object in connection with the description which her brother dictated. Such was the splendid scheme which this brother and sister had arranged to carry out as the object of their life-long devotion. The discoveries which Herschel was destined to make were to be reckoned not by tens or by hundreds, but by thousands. The records of these discoveries are to be found in the "Philosophical Transactions of the Royal Society," and they are among the richest treasures of those volumes. It was left to Sir John Herschel, the only son of Sir William, to complete his father's labour by repeating the survey of the northern heavens and extending it to the southern hemisphere. He undertook with this object a journey to the Cape of Good Hope, and sojourned there for the years necessary to complete the great work.
Fig. 97.—The Constellation of Orion, showing the Position of the Great Nebula.
As the result of the gigantic labours thus inaugurated and continued by other observers, there are now about eight thousand nebulæ known to us, and with every improvement of the telescope fresh additions are being made to the list. They differ from one another as eight thousand pebbles selected at random on a sea-beach might differ—namely, in form, size, colour, and material—but yet, like the pebbles, bear a certain generic resemblance to each other. To describe this class of bodies in any detail would altogether exceed the limits of this chapter; we shall merely select a few of the nebulæ, choosing naturally those of the most remarkable character, and also those which are representatives of the different groups into which nebulæ may be divided.