SIGNIFICANCE OF THE MILKY WAY.

15

there is alike abundant evidence of this especial daring and sure presages of his fame: without difficulty or pretence, he there casts aside an idea which had not been questioned before, unless in a few of those obscure, indefinite speculations, which, strangely enough, often prelude important discoveries. It seems to have been fancied, prior to the systematic inquiries of this great astronomer, that the sky, as we see it, is infinite; or that stars, in collocation similar to what meets our eye, stretch outwards through all space; but the justice of this conclusion is impugned by the existence of a MILKY WAY manifested as ours. Had our planetary system occupied a place within some unlimited, and therefore formless bed of stars, we would most probably have possessed a Milky Way, of greater or less brilliancy, as a background of the whole heavens; for multitudes of stars, invisible by any increase of power derivable from artificial contrivances, must then have lain in every direction; and, from the intermingling of their rays, an illumination would probably have proceeded, sufficient to efface absolute darkness. Such an illumination, however, being descried only along one splendid but comparatively narrow zone, spanning the sky almost as a great circle of the sphere; does it not seem, even at first sight, that, only in the direction of that zone, do the orbs around us stretch outwards in countless multitudes-that there alone are our heavens of an immeasurable depth? The conception is,

16

SIGNIFICANCE OF THE MILKY WAY.

indeed, an extraordinary one; but once again, look thoughtfully on the face of the Night. All regions of the sky are assuredly full of these stellar glories; but if one compares the superb belt of the Milky Way with spaces at the side of it, how faint are these, and how dazzling the former!

Nay, does it sound surprising

that through its depths alone, can the telescope wander without fearing any end to discovery; and that when directed elsewhere, farther stars begin as if to hesitate to appear even at its most potent invocations? Singular though it be, the idea speedily commends itself, that along the dazzling circuit of this zone, our stellar heavens must have depths far more profound than those of other regions; so that, almost unwittingly, we become inclined to connect the conception of SHAPE with that scheme of orbs which men still generally regard as the most august material emblem of the INFINITE.

I intend, for the most part, to follow HERSCHEL in farther elucidating the truth now presented, as I shall suppose, for the first time, for my reader's acceptance. Let us think of the appearances which would be noticed by an observant man, as characteristic of his position, if he were in the midst of a forest, infinite or very extensive. In his immediate vicinity the surrounding trees would be well defined, and of the largest proportion; behind these, he would see another range, smaller, but

THE GALAXY FINITE.

17

also well defined; and so on through many gradations of size and distinctness, until individual trees could no longer be distinguished, and the view terminated in an unnamed and vague appearance, which I may be permitted to call a diffused woodiness. But if this peculiar background were not seen in every direction-the light of the sky appearing through the trees in different places -the conclusion would be just and manifest, that the forest has not the characteristics of one stretching out indefinitely, or even equally, on all sides-that in some directions its edges are nearer than in others, or that it is merely a group of trees, having boundaries, and of a particular and ascertainable shape. Now, recollecting, in the case of the heavens, that on looking around any one of its circuits, it is only at two opposite points that the perspective melts into this diffused milkiness, is not the opinion forced on us, that we are placed amid what is merely a group or cluster of stars of peculiar configuration, any section of which must be greatly elongated in the direction of the Milky Way? A simple diagram will explain how existing appearances are, at all events, roughly accounted for by this hypothesis; and the same illustration indicates the chief modifications required, to bring it into a perfect and minute consistency with every characteristic of the skies. If a spectator were placed in a world, S, in the midst of a stratum or bed of stars, bounded, as beneath,

C

18

THE GALAXY OBLONG.

by the lines C and D, i. e., narrow, or, at least, measurable in breadth, as in the directions SC, SD,

D

but indefinitely prolonged towards A and B; he would manifestly be engirt by heavens having the general aspect of ours: for, on looking along any line, from S towards Cor D, he might see through the cluster, and the regions in that quarter would, therefore, all appear as a comparatively dark ground, bespangled with multitudes of distinguishable luminaries; while in the directions SA and SB, before and behind, his eyes would fail, as ours do when turned to the Milky Way, in separating the individual bodies, or in recognising the existence of the remoter masses, otherwise than in the silvery twilight coming from their aggregation. If our Milky Way were a simple continuous belt, I think it evinced by these considerations, that all the more prominent phenomena of the sky could be explained by the supposition that we are in the midst of an oblong stratum of stars of a certain regularity: but our zone is not simple as supposed. On any favourable night, a cursory inspection of it shows that through one-third of its course, it divides into two branches; which, after

PECULIARITIES OF ITS SHAPE.

19

flowing somewhat apart-leaving between them a comparatively dark space-reunite, and again form a single stream: so that a new hypothetical figure is needed to explain why, in looking towards the region where the stream is divided, the prospect terminates, not in one milky spot, but in two, separated by a considerable breadth of space, which has no greater number of stars, and not much more general illumination, than the sides of the cluster. To effect this, it is enough to suppose the cluster oblong as before, but divided or split towards one of its extremities: for it will readily be seen that if the Sun, S, were in a cluster similar in shape to the annexed figure, a spectator in it would necessarily discern one bright spot towards B; two equally bright ones at A' and A′′ in the opposite direction; and towards C and D, as before,

the background of the heavens would appear comparatively dark; while a third but limited dark space, of precisely the same character, towards the vacuity A", would separate the divergent branches.

Further, and very memorable elucidations of such views were soon yielded to HERSCHEL by his great