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ment. In some places they are crowded together, in others thinly dispersed. A few groups more closely condensed form very beautiful objects even to the naked eye, of which the Pleiades and the constellation Coma Berenices are the most striking examples; but the greater number of these clusters of stars appear to unassisted vision like thin white clouds or vapors: such is the milky way, which, as Sir William Herschel has proved, derives its brightness from the diffused light of the myriads of stars that form it. Most of them are extremely small on account of their enormous distances, and they are so numerous that, according to his estimation, no fewer than 50000 passed through the field of his telescope in the course of one hour in a zone 2° broad. This singular portion of the heavens, constituting part of our firmament, consists of an extensive stratum of stars, whose thickness is small compared with its length and breadth; the earth is placed about midway between its two surfaces, near the point where it diverges into two branches. Many clusters of stars appear like white clouds or round comets without tails, either to unassisted vision or with ordinary telescopes; but with powerful instruments Sir John Herschel describes them as conveying the idea of a globular space filled full of stars insulated in the heavens, and constituting a family or society apart from the rest, subject only to its own internal laws. To attempt to count the stars in one of these globular clusters, he says, would be a vain task,—that they are not to be reckoned by hundreds, and, on a rough computation, it appears that many clusters of this description must contain ten or twenty thousand stars compacted and wedged together in a round space whose area is not more than a tenth part of that
covered by the moon; so that its centre, where the stars are seen projected on each other, is one blaze of light. If each of these stars be a sun, and if they be separated by intervals equal to that which separates our sun from the nearest fixed star, the distance which renders the whole cluster barely visible to the naked eye must be so great, that the existence of this splendid assemblage can only be known to us by light which must have left it at least a thousand years ago. Occasionally these clusters are so irregular and so undefined in their outline as merely to suggest the idea of a richer part of the heav They contain fewer stars than the globular clusters, and sometimes a red star forms a conspicuous object among them. These Sir William Herschel regarded as the rudiments of globular clusters in a less advanced state of condensation, but tending to that form by their mutual attraction.
Multitudes of nebulous spots are to be seen on the clear vault of heaven which have every appearance of being clusters like those described, but are too distant to be resolved into stars by the most excellent telescopes. This nebulous matter exists in vast abundance in space. No fewer than 2000 nebulæ and clusters of stars were observed by Sir William Herschel, whose places have been computed from his observations, reduced to a common epoch, and arranged into a catalogue in order of right ascension by his sister, Miss Caroline Herschel, a lady so justly eminent for astronomical knowledge and discovery. Six or seven hundred nebulæ have already been ascertained in the southern hemisphere; of these the magellanic clouds are the most remarkable. The nature and use of this matter, scattered over the heavens in such a variety
of forms, is involved in the greatest obscurity. That it is a self-luminous, phosphorescent, material substance, in a highly dilated or gaseous state, but gradually subsiding by the mutual gravitation of its particles into stars and sidereal systems, is the hypothesis which seems to be most generally received; but the only way that any real knowledge on this mysterious subject can be obtained is by the determination of the form, place, and present state of each individual nebula; and a comparison of these with future observations will show generations to come the changes that may now be going on in these supposed rudiments of future systems. With this view, Sir John Herschel began in the year 1825 the arduous and pious task of revising his illustrious father's observations, which he finished a short time before he sailed for the Cape of Good Hope, in order to disclose the mysteries of the southern hemisphere, because he considers our firmament to be exhausted till farther improvements in the telescope shall enable asstronomers to penetrate deeper into space. In a truly splendid paper read before the Royal Society on the 21st of November, 1833, he gives the places of 2500 nebulæ and clusters of stars. Of these, 500 are new,—the rest he mentions with peculiar pleasure as having been most accurately determined by his father. This work is the more. extraordinary, as, from bad weather, fogs, twilight, and moonlight, these shadowy appearances are not visible, at an average, above thirty nights in the year.
The nebulæ have a great variety of forms. Vast multitudes are so faint as to be with difficulty discerned at all till they have been for some time in the field of the teleescope, or are just about to quit it. Many present an illdefined surface, in which it is difficult to say where the
centre of the greatest brightness is. Some cling to stars like wisps of cloud; others exhibit the wonderful appearance of an enormous flat ring seen very obliquely, with a lenticular vacancy in the centre. A very remarkable instance of an annular nebula is to be seen exactly half-way between ẞ and y Lyræ. It is elliptical in the ratio of 4 to 5, is sharply defined, the internal opening occupying about half the diameter. This opening is not entirely dark, but filled up with a faint hazy light, aptly compared by Sir John Herschel to fine gauze stretched over a hoop. Two are described as most amazing objects:—one like a dumb bell or hour-glass of bright matter, surrounded by a thin hazy atmosphere, so as to give the whole an oval form, or the appearance of an oblate spheroid. This phenomenon bears no resemblance to any known object. The other consists of a bright round nucleus, surrounded at a distance by a nebulous ring split through half its circumference, and having the split portions separated at an angle of 45° each to the plane of the other. This nebula bears a strong similitude to the milky-way, and suggested to Sir John Herschel the idea of a brother system bearing a real physical resemblance and strong analogy of structure to our own.' It appears that double nebulæ are not unfrequent, exhibiting all the varieties of distance, position, and relative brightness with their counterparts the double stars. The rarity of single nebulæ as large, faint, and as little condensed in the centre as these, makes it extremely improbable that two such bodies should be accidentally so near as to touch, and often in part to overlap each other as these do. It is much more likely that they constitute systems; and if so, it will form an interesting subject of future inquiry to discover whether they possess orbitual motion round one another.
Stellar nebulæ form another class. These have a round or oval shape, increasing in density towards the centre. Sometimes the matter is so rapidly condensed as to give the whole the appearance of a star with a blur, or like a candle shining through horn. In some instances the central matter is so highly and suddenly condensed, so vivid and sharply defined, that the nebula might be taken for a bright star surrounded by a thin atmosphere. Such are nebulous stars. The zodiacal light, or lenticular shaped atmosphere of the sun, which may be seen extending beyond the orbits of Mercury and Venus soon after sunset in the months of April and May, is supposed to be a condensation of the etherial medium by his attractive force, and seems to place our sun among the class of stellar nebulæ. The stellar nebulæ and nebulous stars assume all degrees of ellipticity. Not unfrequently they are long and narrow, like a spindle shaped ray, with a bright nucleus in the centre. The last class are the planetary nebulæ. These bodies have exactly the appearance of planets, with sensibly round or oval discs, sometimes sharply terminated, at other times hazy and ill-defined. Their surface, which is blue or bluish white, is equable or slightmottled, and their light occasionally rivals that of the planets in vividness. They are generally attended by minute stars which give the idea of accompanying satellites. These nebulæ are of enormous dimensions. One of them near Aquarii, has a sensible diameter of about 20", and another presents a diameter of 12". Sir John Herschel has computed that, if these objects be as far from us as the stars, their real magnitude, must, even on the lowest estimation, be such as would fill the orbit of Uranus. He concludes that, if they be solid bodies of a solar nature,