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little, because in point of fact little is known. It includes about thirty of the stars observed by Secchi, chiefly inconspicuous orbs, but remarkable for their deep red colour.

Such is the evidence thus far obtained by the newest instrument of science.

THE DEPTHS OF SPACE.

WHEN we look around us into the regions which surround the solar system, and see the myriads of myriads of stars which are spread through space, it is impossible not to feel strongly the desire to penetrate the mystery of the star-strewn depths. We have learned much respecting the earth on which we live, and not a little of the system to which the earth belongs. We have at least so far solved the problems presented to us by the planetary scheme as to recognise the subordinate position which our earth holds within it, and that the sun is the mighty ruler whose sway guides all the planets in their courses. But the enquiring spirit of man is not satisfied with these discoveries. No sooner has he learned to regard the earth as but one of a system of worlds circling round the sun, and that that system has such and such proportions, and presents such and such forms of motion, than he desires to regard our sun as but one of a system of suns, and to ascertain what may be the nature and the scale of this higher system, what the movements taking place within it. This was the noble problem which the elder Herschel set as the great end and aim of all his labours: 'A

knowledge of the construction of the heavens,' he said, towards the end of his wonderful career as an observer, 'has always been the ultimate object of my observations.'

It is in contemplating this problem that man is most forcibly taught the insignificance of the earth on which he lives-in point of size at least, though we must remember that great and bright infers not excellence So long as the study of external nature is limited within the bounds of the solar system, we are able to measure not merely the proportions, but the dimensions of the objects we study, but so soon as we pass beyond our solar system, the power of measuring is wanting-or at least is so limited as to serve us but for a short distance. We have learned the distances of about half a dozen stars, and even those distances have been only roughly ascertained; all distances beyond are immeasurable, and for the most part must remain so, it would seem, unless some new method of estimating star distances should unexpectedly be discovered.

We have to judge of the star depths around us, then, in some other way than by actual measurement. We must scrutinise them attentively and be on the watch for indications of various nature by which to form some idea of the laws of stellar grouping.

To this problem few astronomers indeed have devoted their energies, probably because it presents difficulties so enormous. The elder and younger Herschel, William Struve, Mädler, and one or two more, are all who can be

named as having actually taken these questions in hand as astronomers, though Kepler, Kant, Lambert of Alsatia, Wright of Durham, and a few others have speculated more or less ingeniously respecting the sidereal system.

It is to the original mind of Sir W. Herschel that science owes the bold idea of gauging the star depths, of actually attempting to apply a measuring-line from our tiny earth by which to determine how far the stellar system extends in this direction and in that, until its whole figure should have been determined. The process, he suggested, might be compared to that by which the nautical surveyor charts the sea bottom, marking its depths and shallows, its hills and depressions, its peaks and mountains, its valleys and ravines. Precisely as the lead line of the seaman passes through more water where the depth is greater, and through less where the shallows lie, so Herschel conceived that the telescopic line of sight would pass through more stars where the stellar system has its greatest extension from us, and through fewer where the boundaries of that system are nearest to us.

He threw out the visual plumbline again and again, now sounding, as he conceived, the profundities of the star system, and now finding that the limits of the system were relatively close to us. He found that when the line of sight was directed towards the zone of the heavens where we see the Milky Way, the telescopic field of view was nearly always rich with stars; but when he turned his telescope away from that zone, and especially

when it was turned nearly square to the general level of the Milky Way zone, few stars could be perceived.

Accordingly, he concluded that the system of stars is flattened in shape, extending farthest where we see the Milky Way, and having boundaries which lie relatively close in, towards the parts of the heavens which lie farthest from the galactic zone. It was on this evidence

that he based what has been called the cloven flat disc' theory of the sidereal system. For the Milky Way has two branches through a considerable portion of its circuit, so that the greatest extension of the star system lies towards two planes, where the Milky Way is cloven. A good general idea of the shape of the stellar universe according to these results may be obtained by taking two wafers, and after wetting one-half of one of them—that is, a semicircle of its surface-applying the other thereto, so that we have a double wafer; but one-half of the double wafer has its two leaves disjoined. Now, if these two semicircular portions be slightly separated from each other with the point of a knife, so that they slope away from each other, we have a figure something like the cloven flat disc of Herschel's theory. Only it will be understood, of course, that he did not suppose there was anything like regularity in the shape which at this time he ascribed to the stellar system.

But as he went on with his observations, Herschel gave up the principle of star-gauging and its results. It is well to notice this carefully; because the text-books of

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