of the nebulous matter in the state in which my observations represented it; they seemed to be the natural result of the observations under consideration, and were not given with a view to establish a systematic opinion, such as will admit of complete demonstration. The observations themselves are arranged so conveniently that any astronomer, chemist, or philosopher, after having considered my critical remarks, may form what judgment appears inost probable to him. At all events, the subject is of such a nature as cannot fail to attract the notice of every inquisitive mind to a contemplation of the stupendous construction of the heavens; and what I have said may at least serve to throw some new light upon the organization of the celestial bodies. Synopsis of the Contents of this Paper. Diffused nebulosity exists in great abundance Its abundance exceeds all imagination page 63 66 68 68 Nebulous matter consists of substances that give out light, The range of its visibility is confined to very moderate 69, 70 69 Much of it may be at distances where it cannot be seen 70 may exist, deserves investigation 70 Either greater depth or greater compression of the nebulous matter may occasion greater brightness Condensation will best account for greater brightness tion 73 When a nebula has more than one preponderating seat of attracting matter, it will probably in time be divided 78 This opinion is supported, by the remarkable situation of nebulæ Which may be seen in Mr. Bode's Atlas Cœlestis 77 78 The real form of the nebulous matter of which nebulæ are composed may be inferred from their figure 79 The form of the nebulous matter of round nebulæ is globular 84 This form is caused by gravitation The central brightness of nebulæ points out the seat of attrac- Three different stages of condensation pointed out Progressive condensation takes place 84 Concentric brightness as well as globular form indicates the 91 Nebulous matter is probably capable of being consolidated; the act of shining proves it to have chemical properties It will stop light, and is partly opaque Queries relating to the subsidence of the nebulous matter, the The uniform light of nuclei, and of much condensed nebulæ, proves that the nebulous matter is considerably opaque When the nebulous matter assumes hardness, the progress of condensation will be impeded 95 95 Three indications of the compression of the nebulous matter 96 96 97 97 In common good telescopes planetary nebulæ cannot be distinguished from stars 98 Perhaps they may in the end be so condensed as actually to become stars 93 The nebulous matter in a cubical space of 10' will admit of a condensation of two trillion and 208 thousand billion times before it can go into the compass of a globe of the diameter of our sun Planetary nebulæ have a rotatory motion on their axes The nebulous star in Orion is fictitious 56 99 99 99 99 Two out of three nebulous stars in Orion have lost their former nebulous appearance 100 Their fictitious appearance was owing to a dispersion of their light in passing through nebulous matter 100 The faintest appearance of the nebulosity in Orion is perhaps Nebulæ are not permanent celestial bodies Additional proof of the opacity of the nebulous matter 101 102 102 103 103 Conversion of planetary into bright stellar nebulæ, into stars with burs, or stars with haziness 104 Conversion of more distant ones into faint stellar nebulæ; into stars with burs, or with faint chevelure 106 When it is doubtful whether an object is a star or a nebula, it 106 107 When we cannot ascertain whether the doubtful object is a star or a nebula, of which several instances are given, the similitude is as great as any we can expect; for were it greater, there could be no doubt 107 POSTSCRIPT. It will be seen that in this paper I have only considered the nebulous part of the construction of the heavens, and have taken a star for the limit of my researches. The rich collection of clusters of stars contained in the 6th, 7th, and 8th classes of my Catalogues, and many of the Connoissance des Temps, have as yet been left un⚫ noticed. Several other objects, in which stars and nebulosity are mixed, such as nebulous stars, nebulæ containing stars, or suspected clusters of stars which yet may be nebulæ, have not been introduced, as they appeared to belong to the sidereal part of the con. struction of the heavens, into a critical examination of which it was not my intention to enter in this paper. Slough, near Windsor, May 26, 1811. WILLIAM HERSCHEL. [Herschel, Phil. Trans. 1811.1 CHAP. VIII. RECAPITULATION OF DR. HERSCHEL'S VERY INGENIOUS AND IMPORTANT OBSERVATIONS IN THE PRECEDING CHAPTER. DR. HERSCHEL supposes that the luminous fluid which so often appears in different parts of the heavens, and throws streaks athwart them, is not light immediately issuing from stars too remote to be traced by the telescope, but existing independently of stars or planets, though perhaps originally thrown forth from them. It is not matter of light alone, for it is sometimes capable of opacity, though usually luminous, and a source of light. When this subtle material exists in irregular loose masses he calls it a nebulosity, or luminous cloud. Its residence he supposes to be within the range of the remotest stars, though beyond those of the second and third magnitudes; yet similar diffusions, he conceives, may also exist at distances where they cannot be seen. Agreeably to the laws of gravitation or the centripetal force, he supposes the different particles to have a tendency of approaching to each other, and that some circumstances, unknown to us, may occasion a preponderating influence, usually in the centre of a diffused mass, but sometimes in other quarters. Hence, a luminous nucleus will be produced by the concentration that must necessarily follow, which will progressively attract and determine the circumjacent luminous matter to itself, and diminish the extent of the general range; and in this case what was a nebulosity will become a nebula. The nuclei may sometimes be double, or triple, or still more complicated in the same nebulosity, and whenever this occurs, the nebulosity will be broken into different nebulæ, or smaller luminous clouds; and if some of them be much minuter than others, they may ultimately attend upon them as satellites upon a planet. If the nebulæ have a hazy or superficial lustre, he calls them planetary; if they be uniformly luminous, he denominates them stellar. Dr. Herschel conceives that stars, planets, and comets, may all occasionally originate from such a source; that a rotatory motion must ensue from the preponderancy of action of a greater mass of particles on one side of the nucleus than on other sides: and that, as many of these particles are probably elastic, an apparent haze or chevelure, coma or hair, must often appear to surround the nucleus. When the whole of the luminous circumferent matter is consolidated by gravitation into the nucleus, the planetary nebula becomes then a real star; some of which stars, before they become perfectly consolidated, have visibly faint chevelures, or else burs, or prickles. The nebulous matter contained in a cubical space seen under an angle of ten degrees, will admit of a condensation of two trillion and two hundred and eight thousand billion times, before it can be so consolidated as to constitute a globe of the diameter of our sun. The ground-work of these opinions, or the principles from which these results are drawn, will be found at length in the preceding chapter, and they are too curious and of too much consequence for us to diminish their force by an abridgment. Yet the younger and the less inquisitive among our readers, will feel obliged to us for presenting them, in the chapter before us, with a miniature view of the general information they contain, Editor. |