hibited no trace of that curdling up or resolution into separate star-points which was presented to us by the other class ; it remained perfectly homogeneous under the highest telescopic power which, till within the last two years, was available to us. Two of these nebulæ, for example, are distinctly discernible by the unaided eye; yet to Sir William's four-feet reflector, approaching us to a two hundredth of our natural distance, and even to Lord Rosse's three-feet one-a still more efficient instrument—they exhibited simple extension of form and greater fulness of light, but not only no trace of separate constituent stars, but no beginning appearance, or promise of it,—no trace of that hardening of aspect which began to present itself in the resolvable nebulæ, under the slightest advance of telescopic power beyond that which first brought them into view.

Such were the undoubted facts upon which Sir William Herschel proceeded. Two explanations of the remarkable peculiarities characterising these unresolved nebulæ were admissible; and, so far as we can even yet see, two only. The one assumed that the bodies presenting themselves under these peculiarities of aspect were really identical with the resolvable nebulæ, and that the marked distinction of phenomenal appearance was simply a result of comparative distance: that these also were firmaments of suns, so far removed from us in space, that not only did their aggregated light only reach our earth as a faint and homogeneous haze, but that the abridgment of this distance to a hundredth or less of its original amount, left us still comparatively unbeginning to approach them; farther removed still from them than from the first we were from any known firmamental nebula. There was more than mere immensity of distance to be accounted for. There was visibility at this immensity of distance, which, as to its principal feature, had not become more visible when but a small fraction of the original distance continued to intervene. This first explanation would in the circumstances have been wholly unwarranted and unsupported by facts. It inferred the existence of systems entirely unlike those which were known: their orbs so massed and crowded together; the space occupied by the aggregate of these so vastly surpassing all we had any example of; and the forms assumed by some of these aggregations such entire departures from the prevalent simplicity of firmamental arrangement, as then recognised, and so apparently irreconcilable with subjection to known principles of force and of motion, that there had been direct violation of every principle of inductive science in assuming identity where every recognised feature told of contrariety. Another resource remained: and the alternative it presented,

HERSCHEL'S DEDUCTION FROM THEIR APPEARANCES.

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preferable as in the circumstances it certainly was, was adopted by Sir William Herschel. These unresolved and apparently irresolvable nebulæ present appearances peculiar to themselves of all we know: are we not called on to regard them as in themselves peculiar, and as constituting among the hosts of heaven a class distinct and alone-as distinct and alone, at least, as are the comets in our own home system of planets and moons ? The answer which presented itself to the distinguished inquirer's mind was affirmative.* Peculiar in the aspects they wear, and the phenomena they seem to exhibit, these stellar appearances are peculiar in their own constitution and character: and the character which their recognised phenomena seemed most distinctly to assign to them was that of vast diffusions of self-luminous matter, somewhat, though very remotely and generally, resembling that of our comets, occupying the same region of space as the fixed stars, and irregularly scattered through our own firmamental system; this matter in a state of very high dilatation, gaseous, or possibly ultra-gaseous, in a condition peculiar to itself, and susceptible of condensation and solidification. Other inferences subsequently followed. On examining and roughly classifying these unresolved nebulæ, marked differences of form were observable. Many-none more so than one of those two we have referred to as lying apparently so near to us—were characterised by the utmost conceivable irregularity. In others, this irregularity was less strongly exhibited: an approach to the sphere or spheroid—the ultimate type, so far as is yet known, of stellar form—was apparent, and sensible condensation of light toward the centre distinguishable; while that of the more irregular and abnormal masses had been generally uniforin, and even appeared, in some cases, deeper towards the circumferential portions.

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And in a still higher class, all irregularity bad disappeared: they presented themselves as regular discs; in some of which the intensity of the central light distinctly intimated spherical mass. Upon these observations the illustrious observer founded his hypothesis of a process of change progressing in these nebulæ: a condensation of the diffused and highly dilated matter composing them around a central nucleus, proceeding age after age, and cycle after cycle, until from the apparently chaotic disorder and misrule of such a mass as the nebula in the constellation Orion a fair and well ordered sun should arise. * If, indeed, such a process were actually in progress, its steps must be too majestic, and its phases of result too slow of evolution, that observation could hope to take cognisance of them during centuries heaped on centuries, or to discover the slightest waxing or waning of phase. Yet a process of indirect observation seemed not only possible, but spontaneously to force itself upon the observer's regard. These nebulæ appeared to present among themselves almost every stage and phase of this progression. While change could not be hoped to be appreciable in any one individual orb during thousands or millions of years, its all-prevalent sway, and the results of this seemed visibly to stand displayed. These

• Under the general name of nebular hypothesis, two perfectly distinct and independent speculations have been currently included : that of Herschel, which extended no farther than we have now defined ; and that of Laplace, which aimed to eliminate a system of cosmogony applicable to our earth and its associated planets and satellites, of which a nebula in the act of aggregating and condensing formed the starting point, and afforded the materials from which the French mathematician endeavoured to show elaborating, through means of the operation of prevalent laws, the solar system. It was only, however, after Laplace's speculation had, as regarded all its principal details, been fully wrought out by its originator on the ground of facts exclusively associated with the solar system, that he was enabled, by the publication of Sir William's observations on the nebulæ and speculations regarding them, to call in the aid of their appearances to the support of his own hypothesis. And while the assumed existence of these masses, and the change they seemed to be undergoing, afforded powerful support to the planetogeny of Laplace, they neither originated nor directed it. Without that support, it must of course stand or fall by its own inherent and self-approving probabilities, and by its involving higher explanation of the peculiarities of the system, or the reverse.

Viewing, then, Laplace's speculation as in a great measure an independent one, we do not enter into any examination of it. Those who do not know it, and may wish to do so, will find a full sketch of it by Pontecoulant, in Professor Nichols present work: or, which we greatly prefer, Laplace's own Memoir in a previous work by our author, the Architecture of the Heavens. In Pontecoulant's Memoir there seem to us far too many substitutions of the imperative must for the mildest possible may: far too much in the minor details is assumed as certain which is only possible, for an hypothesis whose strongest reasoning is from the present to the past, and from the known to the unknown. And we refer our readers to the original Memoir, rather than to any paraphrase or expansion of it, because some of these paraphrasts, including conspicuously Mr. Nichol and the author of the Vestiges of the Natural History of the Creation, have thrown much more of suspicion on the speculation than of right belongs to it, by attempts to fill up its details characterised by greater zeal than knowledge.

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nebulæ, classified, presented examples of its every phase in almost unbroken series. At one extremity of the chain appeared the formless diffusion, spread through extents of space which the most moderate estimate made far more than enough to fill

up

the voids between stars; shooting its great streamers north and south, east and west; looming vast, but pale and shadowy, before the mightiest instruments; and baffling conception as to how or by what laws order should arise out of seeming disorder so immense and chaotic. At the other, was seen a bright star-point, enveloped in a faint and limited halo; or even two such, closely adjoining, and connected by a narrow streak of nebulous light, the apparent gerin of a double star. And between these two extremes of the chain — the chaos, and the sun with but the lingering moisture of its birth-process clinging around it—the intervening links were perfect, or nearly perfect. The homogeneous diffusion showing traces of local aggregation and condensation; the great streamers drawing in toward the body; the shortening ellipsoid; the circular or nearly circular disc; that disc with brightening centre, and contracting and brightening still, till spherical or spheroidal form became undoubted and conspicuous :-such was the well-defined and perfect series, which seemed to bring before us in its successive stages of result one great process of stellar organisation; and to present visibly before us, in cotemporaneous picture, the long past of the sun, and the far future of the nebula, its germ.

A few years more, and all this was as a told tale and a done dream. A mightier and more perfect instrument came into operation, and the irresolvable nebulæ yielded before it. Once again science was taught to distinguish between the truth of appearance and the truth of reality, and to know that, even with regard to its investigations, the maxim holds good, Judge not according to the appearance.' And once again has it been admonished that its spirit cannot be too child-like even in its daring, and too open to receive the light and life of truth. That which Sir William Herschel, on what were at the time and in the circumstances just and legitimate grounds, pronounced the true nebulæ not to be, they have now visibly become; and the apparently broad and clear distinctions which broke down the whole class of these nebulæ into two strongly-marked and definite orders, have resolved themselves into mere incidents of distance

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and space.

To the examination of the nebulæ generally, the first labours of Lord Rosse and his coadjutors, on the completion of his two larger instruments, were devoted. With regard to the first class indicated by Herschel, some of the results obtained were of the

most unexpected kind. To one or two of these we shall hereafter refer: and, in the meantime, we turn to the still more unexpected and greater achievements of these instruments,--the resolution of many, and the distinct indication of the resolvability of all the so-deemed true nebulæ. This resolution began with the smaller, or three-feet speculum. Several of these appearances surrendered before its search the secret of their stellar constitution, and presented themselves as firmamental clusters, differing in little save their far greater remoteness, and, on the whole, the seemingly more capricious and fantastic aspects of their aggregation, from the clusters previously recognised.

But most of those which thus yielded before this minor instrument belonged to the number which lay near the limits of previous telescopic reach : and with respect to some at least of them, resolution had been regarded as quite possible, or even probable, whenever the sphere of observation should be enlarged by the possession of mightier means of reaching forth into

th into space. The inferences of Sir William Herschel remained comparatively unshaken, so long as clusters or nebulous aggregations, apparently so near to us as those in the constellations Orion and Andromeda, remained refractory beneath the search of an instrument whose power of vision so far outstripped all that had gone before it, and gave up to its survey only vaster extent of diffusion, and new strangenesses of form. The six-feet speculum was completed, and, towards the end of 1845, was first directed toward the mysterious Orion nebula. Even before these examinations, it lay still obdurate and unchanged; its light still brightening; its dimensions expanding ; its form becoming yet more irregular and grotesque, as the fainter arms and streamers came into view; but its aspect still wholly nebulous and starless. These earlier observations, however, were not regarded by Lord Rosse at the time as decisive. To an examination which could be considered crucial, there were requisite the utmost attainable perfection of the telescope, the most favourable possible atmospheric conditions, and also, we are strongly inclined to think, some measure of habituation of the observer's eye to the rerelations of such an instrument-a habituation which time and practice alone could give. But the mighty instrument was still farther perfected; clearer skies, and a more congenial air, admitted of more searching examination ; the eye ceased to be strange to the character of the disclosures brought to it: and in March, 1846, the problem was solved at last, and the resolution of the Orion nebula announced. With its manifestation as a firmament, or cluster of stars, Herschel's lofty speculations fall to the ground, while Laplace's more specific planetogeny is left self-standing, if it