position of any planet of either sytem, there will result for the time being either an interchange of suns instead of the change from night to day, or else double sunlight during the day, and a corresponding intensified contrast between night and day. Where the two suns are very unequal, or very differently coloured, or where the orbital path of each is very eccentric, so that they are sometimes close together, and at others far apart, the varieties of condition in the worlds circling around either, or around the common centre of both, must be yet more remarkable. 'It must be confessed,' we may well say with Sir John Herschel, 'that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in.'

If it be supposed that in some instances the smaller component of a double system shines either wholly or to a considerable degree by reflecting the light of its primary, we shall find yet further reason for wonder at the diversity of structure within the star-depths. For unquestionably the largest of all the planets which circle around the Sun would not be visible even under the most powerful telescopic scrutiny, at the distance of the nearest fixed star. Nay, an opaque orb as large as our Sun, if placed where Jupiter is, would not reflect a tenth part of the light necessary to render it visible at such a distance.

But such considerations as these become perfectly bewildering when extended to triple, quadruple, and generally to multiple star-systems. It will afford some idea of the amazing variety of arrangement observable

among such systems, that even among the host of triple stars already observed by astronomers, not two have been found which closely resemble each other in arrangement, while, so soon as we pass to more complex systems, we find that each fresh object of the class differs utterly from all which have been previously observed.

In considering the actual condition of the region of space occupied by a triple, quadruple, or multiple starsystem, we find ourselves surrounded on all sides by sources of perplexity-so long, at least, as we compare the worlds in such regions with our own earth, or with any member of the Solar system. All the most marked characteristics of life on our earth must be wanting in those worlds which circle around the component suns of multiple systems. There can be no year, strictly so called, no orderly succession of seasons, no regular alternation of day and night, in many cases no night at all, save for brief periods at exceedingly long intervals. Placed at such a distance from any star of one of these systems that that star appears as a sun, the others also must supply an amount of light sufficient of itself to banish night. More commonly, indeed, each star of such a system, while above the horizon, would be a sun shining more brightly than our sun does to the inhabitants of many planets of his scheme. But where there are three or four such suns, the simultaneous absence of all from the sky must be an uncommon event-as uncommon, for instance, as those occasions when none of Jupiter's satellites can be seen. The inhabitants of worlds such as these can but

seldom witness the spectacle of the starlit sky; and the study of any orbs beyond their own system must be a task of infinite difficulty, since it can only be pursued for a few occasional hours of darkness, separated by many months of persistent daylight.

The consideration of these multiple systems leads us on, step by step, to the various orders of star-clusters. For we can point to multiple systems of greater and greater richness, and as it were compactness, until we arrive at orders which we are compelled to regard as veritable starclusters. Yet it is to be noticed that we might have approached the study of star-clusters in a different direction. For we have already had occasion to consider the various degrees of stellar aggregation in different parts of the heavens; and some of the regions of aggregation, while indubitably features of the galaxy regarded as a whole, are yet so well defined, and so clearly separated from the surrounding more barren regions, that we cannot refuse to regard them as vast star-clusters. We can thus either proceed from the smaller star-clusters-to which we have been led by the study of multiple star-systems— onwards to larger and yet larger groups of stars, until we have arrived at the aggregations just mentioned; or we can pass from these through the successive orders of a diminishing scale until we arrive at multiple star-systems.

Yet here it is well to remark that a difficulty presents itself which can only be removed by the theory, to which we have been already led by other considerations, that the great aggregations of stars are not (or at least, all of

them are not) to be regarded as formed of orbs necessarily comparable with Sirius and Arcturus, Capella, Vega, and Aldebaran, in real magnitude and splendour. For we can pass from the single suns onwards to double suns, multiple systems, star-clusters, and stellar aggregations, thence to less and less condensed and more and more extended aggregations, until we arrive at unaggregated star-groups, consisting, in fact, of the single orbs with which we set out. Now it is to be observed that we seem to be brought to the single stars by a course which is not a mere retracing of our steps. For supposing we regard star-clusters as intermediate between the least condensed aggregations on the one hand, and single stars on the other, we pass onwards from these least condensed aggregations to single stars, without going through any of the steps of our progress towards the aggregations. Obviously there can have been no true progression here. And we are compelled to believe that by following the course indicated, we arrive at quite another order of star-groups than those which form our constellations, although in appearance the less densely aggregated star-groups may resemble systems of true suns, like Sirius and Arcturus, Aldebaran and Capella.

But the fact is that, again and again, as we contemplate the wonders of the star-depths, we find ourselves thus tracing out perplexing sequences, one class of objects merging into another class seemingly altogether distinct, and this class into yet another, until we are bewildered by the multiplicity of analogies whereof some at least must be deceptive.

For instance, I have spoken of the various orders of star-clusters by which we may be led to the vast stellar aggregations; and there can be no question whatever that an apparently perfect sequence can be traced from sharplydefined clusters such as the magnificent object '13 Messier' in the constellation Hercules, to such groups as the Pleiades, or Præsepe in Cancer, or the splendid star-clusters near the sword-hand of Perseus, these groups being undoubtedly mere condensations in rich star-regions. It also cannot be doubted that we can pass from such a cluster as the one in Hercules, to others less rich in numbers, but equally compact, and so to clusters continually poorer and poorer (numerically) until we arrive at mere multiple systems, and so to quadruple, triple, double, and single stars. But it is equally certain that we can pass from the globular star-clusters to others oval in shape, and more and more closely set, until at length we arrive at the nebulæ, properly so called-that is, spots of cloudy light which have not been resolved into separate stars by any telescopic power yet applied. Here, then, a progression as real as either of the preceding seems to lead us to objects which have been commonly regarded as wholly distinct from any portions of the galactic system, and probably analogues of the whole of that system.

*

*The connection between shape and closeness of star-setting is certain, though most perplexing. Sir John Herschel writes:-'It may be generally remarked, as a fact undoubtedly connected in some very intimate manner with the dynamical conditions of their subsistence, that the elliptic nebulæ are, for the most part, beyond comparison more difficult of resolution than those of globular form.