When difficulty is mentioned, it is always to be understood that a considerable time as well as attention was required in the examination before a decisive opinion could be formed *. III. 7 is " A nebulous star, but doubtful of the nebulosity. "With 240 the same doubtful appearance continues." Fig. 42. With this object the doubt which remained could only relate to the nature of it; for being at first sight taken to be a nebulous star, its existence could not be a subject for examination; but the unresolved doubt, whether an object is a nebula or a star, must certainly be allowed to be as great a proof of identity as we can possibly expect to see. 35. Concluding Remarks. The total dissimilitude between the appearance of a diffusion of the nebulous matter and of a star, is so striking, that an idea of the conversion of the one into the other can hardly occur to any one who has not before him the result of the critical examination of the nebulous system which has been displayed in this paper. The end I have had in view, by arranging my observations in the order in which they have been placed, has been to show, that the above-mentioned extremes may be connected by such nearly allied intermediate steps, as will make it highly probable that every succeeding state of the nebulous matter is the result of the action of gravitation upon it while in a foregoing one, and by such steps the successive condensation of it has been brought up to the planetary condition. From this the transit to the stellar form, it has been shown, requires but a very small additional compression of the nebulous matter, and several instances have been given which connect the planetary to the stellar appearance. The faint stellar nebulæ have also been well connected with all sorts of faint nebulæ of a larger size; and in a number of the smaller sort, their approach to the starry appearance is so advan ced, that in my observations of many of them it became doubtful whether they were not stars already. It must have been noticed, that I have confined myself in every one of the preceding articles to a few remarks upon the appearance * See Third assortment, containing four objects that could not be verified. III. 7, 176, 263, 293. 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 Nebulous matter consists of substances that give out light, which may have many other properties 68 Nebulous diffusions contain both milky nebulosity, and such as from its faint appearance may be mistaken for resolvable The range of its visibility is confined to very moderate 69, 70 69 70 70 Either greater depth or greater compression of the nebulous matter may occasion greater brightness Condensation will best account for greater brightness 73 2233 72 The condensation of the nebulous matter ascribed to gravita tion 73 When a nebula has more than one preponderating seat of attracting matter, it will probably in time be divided 73 This conception is supported by the appearance of double nebulæ Their double appearance can be no deception Their nebulosity is derived from one common stock This opinion is supported by the remarkable situation of nebulæ Which may be seen in Mr. Bode's Atlas Cœlestis 74 74 74 77 78 The real form of the nebulous matter of which nebulae 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 84 The central brightness of nebulæ points out the seat of attraction 86 The effect of attraction on the form of the nebulous matter depends on its original expansion, or the time of the action, and on the quantity of the attracting matter 86 Three different stages of condensation pointed out 88 89 Progressive condensation takes place 90 Concentric brightness as well as globular form indicates the 91 91 Nebulous matter is probably capable of being consolidated; 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 95 condensation will be impeded 95 Three indications of the compression of the nebulous matter 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 The nebulous matter in a cubical space of 10' will admit of a The nebulous star in Orion is fictitious 99 Two out of three nebulous stars in Orion have lost their former nebulous appearance 100 100 Their fictitious appearance was owing to a dispersion of their 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 106 107 107 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 unnoticed. 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.] 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 nebulo. sity, 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. |