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of escaping from the conclusion accepted by Sir John Herschel, that we are here" looking through a sheet of stars nearly of a size, and of no great thickness compared with the distance which separates them from us.'

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In other cases a double phenomenon of the kind just described is presented, leading to the conclusion "that in such cases we look through two sidereal sheets separated by a starless interval."

It is hardly necessary to point out that the uniformity evidenced by these peculiarities is of a very different kind from that which the elder Herschel took as the basis of his star-gaugings. His results were obtained on the supposition that there are no such starless intervals as must necessarily be assumed to lie on either side of the "star-sheets" of Sir John Herschel.

Another law has been detected in the aggregation of stars, a law which tends rather to modify than to negative the results of Herschel's star-gauging. Among the possible explanations of the existence of a galactic zone, there is clearly this one, that in place of a great extension of our system in the direction indicated by that zone, there exists a real condensation of stars in that direction. To determine whether such a condensation, or a condensation of any sort, exists in the neighbourhood of the Milky Way, it is necessary to compare not merely the number of stars visible in different fields of view, but the number of stars of the same apparent magnitude. That is, it is necessary to take into account the relative distance of the stars as supposed to be indicated by their relative lustre. This has been done for all orders of stars down to the ninth, inclusive, by the elder Struve. He has shown that there is a marked condensation of stars towards the plane of the galactic circle; a condensation, however, which, as might be anticipated, is far from being sufficient, per se, to account for the observed increase of light in that direction. In other words, we are still bound to accept a considerable extension of our system in the galactic plane as fairly deducible from the gauges of Sir W. Herschel, but we have a secondary cause for the law observed in those gauges which is not to be neglected when we are attempting to realize the actual constitution of the sidereal system. It may be noticed in passing that the condensation detected by Struve is more marked among the smaller stars; but is not wholly wanting among the stars of the first four or five magnitudes. Sir John Herschel's gauges in the southern hemisphere gave a different result, and he holds that it is only as we approach the sphere of the telescopic stars that we obtain any appreciable indications of condensation. The difference between

the two hemispheres in this respect is worth noticing, and may be looked upon as connected with another peculiarity mentioned by Sir John Herschel, viz., the greater richness of the southern hemisphere in stars of all orders.

We see, then, that in many important respects Herschel's first supposition, that there exists an approach to uniformity in the magnitudes of stars, and in their distribution throughout our system, is found not to be consistent with observed facts. Let us next consider his second supposition, or we believe we should be more correct in saying his tacit assumption, that there is no appreciable extinction of light in traversing interstellar space. The considerations connected with this question are of great importance and

interest.

It may seem at first sight, and has been maintained by many eminent scientific men, that if infinite space is occupied by an infinite number of stars, whose average intrinsic lustre is equal to that of our own Sun, then, unless light suffer extinction in its progress through celestial regions, the whole sky would everywhere appear as brilliant as the solar disc. Indeed it may be proved that an uniform distribution of such suns, at whatever common distance, and whatever their mean magnitude, must inevitably lead to an illumination of this sort unless light be gradually extinguished as supposed. Or, again, if we suppose that the system of stars whereof our Sun is a member is limited on every side, but that an infinite number of such systems exist uniformly distributed, at whatever interval, throughout space, the same result would inevitably follow. Now, wholly incomprehensible as the idea of infinite space is to our finite minds, we cannot escape the admission of its existence. The only question for us is between an infinity of occupied space and an infinity of vacant space, surrounding a finite universe. Either idea is equally incomprehensible; but the former is merely beyond, the latter seems contrary to reason. It would seem, therefore, as if we were compelled to accept the gradual extinction of light as the only possible explanation of the darkness of the background on which the stars appear to be projected.

But there is another way in which we may explain the darkness of the sky at night, without assuming either the extinction of light, or that occupied space is an infinitely minute speck amidst an infinity of vacant space. So far as I am aware, it is new, at least as regards its application to the difficulty we are now considering: In the solar system we are presented with a finite number of bodies placed within a region of unoccupied space, bearing a very large ratio to

the dimensions of the solar domain. In the sidereal system we seem to be presented with a finite number of such suns, forming a cluster, which is surrounded on all sides with unmeasured depths of space. Assuming our system to form one of a finite number of similar systems, separated from each other by distances bearing a very large ratio to the dimensions of each, and that thus a system of a higher order is formed, which again forms one of a finite number of similar systems, and so on continually,-the dimensions of each system of whatever order being always very small in comparison with the distance separating it from its neighbours. There would no longer result as a necessary consequence even an appreciable illumination of the whole heavens. I do not present these views as hypothesis which I wish to support, all I seek to show is that by assuming the existence of such systems in successive orders, the subject of the extinction of light is left an open one. Of course the question is also left open if we assume that occupied space is finite. Such an assumption appears to me, however, to be inconsistent with the conception of an omnipotent Creator.

The arguments for and against the extinction of light have an important bearing on the theory of our sidereal system. Few subjects have been more acutely and closely reasoned.

Olbers was the first, I believe, to put forward a definite theory asserting the extinction of light, though Halley, and many other philosophers, had held somewhat similar views. In support of Olbers' theory the elder Struve put forward the following considerations :

Sir W. Herschel's twenty-feet reflector should have a spacepenetrating power of 74.83, the range of the naked eye being taken as unity. Now Struve found by a computation based on the law of condensation of the stars in the plane of the Milky Way, that such a power as this should result in giving an average of 3021 stars for the telescope's field of view, 15' in diameter. But, instead of this number, Herschel's observations give only 122 stars per field. And Struve calculated that Herschel should have seen as many stars as this with a space-penetrating power of only 25.67, if there is no extinction of light. It seemed to follow, therefore, that the minutest stars visible in Herschel's telescope send no more light to us than stars nearly three times as far off would send if there were no extinction. Therefore the smallest stars visible to Herschel shine -according to Struve-with little more than one-ninth part of their proper lustre; or, more exactly, the diminution of brightness is no less than ths of the whole quantity.

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VOL. III.-NO. II.

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But it is answered by Sir John Herschel that the results on which Struve founds his calculations may be otherwise explained. If there is a diminution in the density of stellar aggregation towards the limits of the Milky Way, corresponding to the observed diminution at right angles to the plane of that zone, the same effects would be perceived as would appear on Struve's hypothesis of an extinction of light.

To this Struve answered that the diminution of density mustin order to account for the results of observation-be assumed to take place on every side of the Sun, which therefore must be supposed to occupy a central position in the sidereal system. But we have no evidence whatever-said Struve-in support of this supposition. He considered that the galactic system is unfathomable-so far as we are concerned. He therefore dismisses the supposition of a diminution in the density of star-aggregation, as altogether inadmissible.

He pointed out another objection to the hypothesis of a diminution of density in stellar aggregation. He found that on the supposition of a stratum of stars of indefinite extent-that is, unfathomable to us-and uniformly diminishing in density on either side of its central plane, there results from Herschel's gauges a law of diminution according almost exactly with the law which results from an examination of the numerical relations among stars down to the ninth magnitude. He considers that this agreement cannot be accidental. It must be remarked, however,-before proceeding to positive objections to Struve's view-that, a single agreement of this sort, though antecedently improbable, is not sufficient to establish such a theory as that for which Struve was contending.

But now an argument was brought forward which appears to me absolutely conclusive against the views of Struve and Olbers. It was pointed out by Sir John Herschel that the Milky Way is fathomable in almost every part by the twenty-feet reflector. Therefore, on the one hand, the fact that in nearly every direction the richest portions of the Milky Way are resolved into discrete stars perfectly free from nebulosity, affords decisive evidence that the sidereal stratum is not infinite in extent. On the other hand, the fact that in certain directions the stratum has an increased range which prevents the twenty-feet telescope from completely resolving it, affords equally decisive evidence that light coming from a distance far beyond that which Struve regarded as the limiting range of such a telescope is rendered perceptible to us by its means. "We are not at liberty," Herschel justly points out, "to argue that at one part

of its circumference our view is limited by this sort of cosmical veil which extinguishes the smaller magnitudes, cuts off the nebulous light of distant masses, and closes our view in impenetrable darkness; while at another, we are compelled by the clearest evidence the telescope can afford to believe that star-strewn vistas lie open exhausting their powers and stretching out beyond their utmost reach, as is proved by that very phenomenon which the existence of such a veil would render impossible,—namely, infinite increase of number and diminution of magnitude terminating in complete irresolvable nebulosity."

It seems, therefore, that we have as yet no evidence that light suffers extinction at all; and certainly light does not appear to suffer appreciable extinction within the bounds of our sidereal system.

Hence it follows that we must either accept Herschel's view that there is a rapid diminution in the density with which stars are distributed, as we leave the central regions of our system; or else we must suppose that there is a sensible diminution in the magnitude of stars around the borders of the sidereal disc. The latter view, which I believe to be new, seems at least as probable as the other. In either case we must assume that our Sun is not very far from the centre of the system.

There are other features of the sidereal system which are well deserving of a careful scrutiny. The existence of variable stars, and of binary and multiple systems, the sudden blazing forth of temporary stars, some of which have excelled even Sirius in splendour, the reputed existence of dark orbs,-all these and many other evidences suffice to show how much variety there is amongst the denizens of stellar regions. That there should be such variety is what our contemplation of the solar system leads us to expect. And, indeed, when we remember how largely the resources of astronomy have been taxed for the detection of the known peculiarities of the solar system, and that, for aught we know, not a tithe of the various orders of bodies which form that system have yet been revealed to us, surely it seems little likely that what we know of the sidereal system affords the faintest conception of the wondrously varied forms of creation which doubtless exist within the stellar spaces.

But we have yet to consider those mysteries of mysteries-the nebulæ.

The discovery that the vault of heaven is bestrewn with multitudes of objects, which differ wholly in character from the fixed stars, may be said to belong to the last hundred years. In 1768,

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