the lesser circles of the sphere, which would be more or less contracted according to the distance of the eye; and if this distance were exceedingly increased, the whole stratum might at last be drawn together into a lucid spot of any shape, ac cording to the position, length, and height of the stratum. In order to determine those points, Sir W. Herschel put in practice a method which he calls gauging the heavens, which consists in repeatedly counting the number of stars in the fields of view very near each other, by which he obtained a mean of the number of stars in that part of the heavens. He then proceeds on the supposition that the stars are equally scattered, and, from the number of stars in any part of the heavens, he deduces the length of his visual ray, or the distance through which his telescope had penetrated, or, in other words, the distance of the remotest stars in that particular region of the heavens. To illustrate this, let us suppose the Milky Way a nebula, and that the sun is not placed in its centre. Then, on the supposition that the stars are nearly equally scattered, it is evident that the part of the Milky Way where the stars are most numerous must extend farthest from the sun, and the parts where they are less numerous must extend to a less distance. Proceeding on these grounds, Sir W. Herschel found the length of his visual ray for different parts of the heavens. In some cases he found it equal to 497 times the distance of Sirius, supposed to be the nearest star, as formerly stated. The following is a representation of a section of the nebula of the Milky Way according to his delineation. This section is one which makes an angle of thirty-five degrees with our equator, crossing it in 124 and 304 degrees. A celestial globe adjusted to the latitude of fifty-five degrees north, and having a Ceti near the meridian, will have the plane of this section pointed out by the horizon. If the solar system (fig. 39) be at S, the brightness of the Milky Way will be greatest in the directions Sa, S b, Sp, where the stars that intervene are most numerous, or where the visual ray is longest. In the lateral directions Sn, Sm, the nebulosity will not appear, from the small number of interposing stars, and the stars, though numerous, will appear more scattered. In the direction Sc, on account of the opening between a and b, there will be an empty space contained between these two branches, where the nebulosity is not observed, as is the case in the Milky Zone between μ Scorpio in the south and y Cygni in the north, a length of about 102 degrees. The stars in the border, which are marked larger than the rest, are those pointed out by the gauges; the intermediate parts are filled up by smaller stars arranged in straight lines between the gauged ones. The circle described around S represents an extent about forty times the distance of the nearest fixed stars, which may be considered as comprehending all those which are visible to the naked eye. From this figure," says Sir W. Herschel, "we may see that our nebula is a very extensive, branching, compound congeries of many millions of stars, which most probably owes its origin to many remarkably large, as well as pretty closely scattered small stars, that may have drawn together the rest." Again: "If it were possible to distinguish between the parts of an indefinitely extended whole, the nebula we inhabit might be said to be one that has fewer marks of antiquity than any of the rest. To explain this idea more clearly, we should recollect that the condensation of clusters of stars has been ascribed to a gradual approach; and whoever reflects on the number of ages that must have passed before some of the clusters that are to be found in my intended catalogue could be so far condensed as we find them at present, will not wonder if I ascribe a certain air of youth and vigour to very many regularly scattered regions of our sidereal stratum. There are, moreover, many places in it in which, if we may judge from some appearances, there is the greatest reason to believe that the stars are drawing towards secondary centres, and will in time separate into clusters, so as to occasion many subdivisions. Our system, after numbers of ages, may very possibly become divided so as to give rise to a stratum of two or three hundred nebulæ ; for it would not be difficult to point out so many beginning or gathering clusters in it. This throws considerable light upon that remarkable collection of many hundreds of nebula which are to be seen in what I have called the nebulous stratum in Coma Berenices. It appears, from the branching and extended figure of our nebula, that there is room for the decomposed small nebulæ of a large reduced former great one to approach nearer to us in the sides than in any other parts." "Some parts of our system seem, indeed, already to have sustained greater ravages of time than others; for instance, in the body of the Scorpion is an opening or hole, which is probably owing to this cause. It is at least four degrees broad, but its height I have not yet ascertained. It is remarkable that the 80th nebula of the Connoissance des Temps, which is one of the richest and most compressed clusters of small stars I remember to have seen, is situated just on the west border of it, and would almost authorize a suspicion that the stars of which it is composed were collected from that place, and had left the vacancy." The remarks in the above paragraph I present to the reader merely as the opinions of an illustrious astronomer and an indefatigable observer of celestial phenomena, without vouching for the accuracy or probability of such speculations and hypotheses. To determine the reality of such changes in bodies so numerous and so distant, would require an indefinite lapse of ages; yea, perhaps the revolutions of eternity are alone sufficient for determining the sublime movements and changes which happen among the immense assemblages of material existence which constitute the universe. There is a high degree of probability that everything within the material system is liable to change of one kind or another, and that there is no sun nor world, among all the myriads of globes which replenish. the sidereal heavens, but what is actually in motion, and moving, too, with a velocity which the inhabitants of such a world as ours can scarcely appreciate; and such motions, in the course of ages, may be productive of a vast diversity of scenery in different regions of the universe. And if so, it presents to view another instance of that variety which the Creator has introduced into his universal kingdom to, gratify the unbounded desires of intelligent beings. I shall conclude this chapter with the following description of the Milky Way, which Sir John Herschel has published since his residence in the southern hemisphere: "The general aspect of the southern circumpolar region-including in that expression sixty or seventy degrees of south-polar distanceis in a high degree rich and magnificent, owing to the superior brilliancy and larger development of the Milky Way, which, from the constellation of Orion to that of Antinous, is in a blaze of light, strangely interrupted, however, with almost starless patches, especially in Scorpio, near a Centauri and the Cross; while to the north it fades away pale and dim, and is, in comparison, hardly traceable. I think it is impossible to view this splendid zone, with the astonishingly rich and evenly distrib uted fringe of stars of the third and fourth magnitudes-which form a broad skirt to its southern border, like a vast curtainwithout an impression amounting almost to a conviction that the Milky Way is not a mere stratum, but an annulus; or, at least, that our system is placed within one of the poorer or almost vacant parts of its general mass, and that eccentrically, so as to be nearer to the parts about the Cross than to that diametrically opposed to it.' CHAPTER XI. ON GROUPS AND CLUSTERS OF STARS. On a cursory view of the heavens, the stars appear to be very irregularly scattered over the concave of the firmament. In some places a considerable interval appears between neighbouring stars, while in others they appear so crowded that the eye can with difficulty perceive the spaces between them. Even to the unassisted eye, there are certain groups of this description which strike the attention of every observer, and lead to the conclusion that the stars of which they are composed have been brought together by some general law, and not by mere casual distribution. Of these, the group called the Pleiades, or Seven Stars, is the most obvious to common observers. This group is situated in the constellation Taurus, about 14° to the westward of the star Aldebaran (see Plate I.), and may be seen every clear evening from the end of August till the middle of April.* It is generally reckoned that only six stars can be distinctly counted in this group by common eyes, but that originally they consisted of seven, which every one could easily perceive, and it has therefore been conjectured that one of them has long since disappeared. To this circumstance Ovid, who lived in the time of our Sa viour, alludes in these lines: "Now rise the Pleiades, those nymphs so fair, * A telescopic view of the Pleiades is exhibited in the Appendix. |