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astronomy say little on the subject, and what they say is for the most part inexact. Herschel began to perceive that there are laws of physical association, binding the stars together in schemes subordinate to the stellar system regarded as a whole. And he came to regard the Milky Way itself as a subordinate system, or as at least distinct in character from the parts of the star system lying around us. He wrote thus in 1802, seventeen years after he had enunciated the cloven-disc theory. Although our sun and all the stars we see may truly be said to be in the plane of the Milky Way, yet I am now convinced by a long inspection and continued examination of it, that the Milky Way itself consists of stars very differently scattered from those which are immediately about us.' And again in 1811 he said, 'When the novelty of the subject is considered, we cannot be surprised that many things formerly taken for granted should, on examination, prove to be different from what they were generally,' but incautiously, supposed to be. For instance, an equal scattering of the stars may be admitted in certain calculations; but when we examine the Milky Way, or the closely compressed clusters of stars, this supposed equality of scattering must be given up.' Now, it will be clear to all that the cloven-disc theory, and the principle

It may be necessary to explain that the word 'generally' here does not refer to the number of those who have adopted the view referred to, but to the general sense in which the view had been adopted. No one can loubt this who has read Herschel's series of papers.

itself of star-gauging, were necessarily based on a belief in the generally equable scattering of the stars. In giving up this belief as a general rule for guidance, Herschel was in effect giving up the hopes he had formed when he thought of the method of star-gauging, as well as all the general results to which he had been led by the application of that method.

When Herschel was a very old man, close on the fourscore years which so few attain without some signs of failing mental powers, he invented another method, commonly confounded in our text-books of astronomy with the method of star-gauging, but in reality quite distinct from it. In star-gauging he had estimated the distance of the boundary of the star system by the number of stars he could count in the telescopic field; in the new method he estimated the distance of star groups by the telescopic power required to resolve them into separate stars. A group like the Pleiades, or like the Beehive in Cancer, which can be resolved into stars with a very small telescope, lies relatively near to us, according to this theory; a group which can only be resolved with one of the mightiest of the telescopes which Herschel used lies very far away; and some groups which none of his telescopes could resolve lie at distances incalculable until some larger telescope can accomplish the resolution of the group.

I shall not say much of this principle. I believe Herschel himself would have abandoned it had he lived

to test it thoroughly. A little consideration will show that it implies a uniformity of structure throughout the stellar system which is very little accordant with what Herschel had himself discovered while making those observations which led to the abandonment of the stargauging method. It will presently appear that other and more decisive evidence of variety of structure has been obtained since Sir W. Herschel's time.

The reader will begin to see the nature of the methods by which alone the astronomer can hope to penetrate the mystery of the star depths. Each of the methods just described must be regarded as a kind of survey of the heavens-not of the heavens presented to ordinary vision, but of the heavens brought into view by the penetrating eye of the telescope. For if the human eye could suddenly obtain the power of telescopic vision, those wealths of star-strewing which it is the province of stargauging to measure would be revealed to our view, not piecemeal, as under telescopic scrutiny, but at once as in a grand celestial panorama. Those varieties of distribution to which Herschel applied his resolution test would be clearly recognised. Here the stars would be seen spread richly over a region of the heavens, but clearly separated from each other; elsewhere would be regions where the stars would more closely cluster, though still separately discernible; but in parts of the heavens veritable star-clouds would be seen, regions where the stars gather so closely together that even the enhanced powers

of vision I have imagined-nay, though the power of the Rosse telescope had been acquired by man-would fail to show discrete stars, the sky in those parts being aglow with condensed star-light, on which, as on a splendid background, brighter stars would be seen spread with inconceivable richness.

Such a scene might not be intelligible at a first view; it might even baffle all attempts at interpretation, all efforts to estimate the relative distances and proportions of its several parts. But our only path to the solution of the noblest problem in science is by presenting to the mind's eye such a picture of the great star-strewn sphere which surrounds us on all sides; when that has been done, we shall begin to know whether the great problem is altogether beyond our mastery.

Herschel's two methods having practically proved insufficient, it remains to be seen by what other methods we may render more distinct our mental picture of the star depths.

It occurred to me very early in my enquiry into the great problem, and before I had fully investigated the long and noble series of researches by which Sir W. Herschel had attempted to master it, that this was a case where the mental vision must be assisted by the bodily vision. It is singular that hitherto, so far as I know (and I think little that has been done on the subject has escaped my reading), the idea of picturing the results obtained by telescopic scrutiny had been altogether overlooked. I do

not here refer to pictures illustrating theories of the universe. Every student of astronomy knows that Sir W. Herschel was careful to give diagrams illustrating his successive conceptions of the structure of the universe. But such illustrations as these, though necessary and useful, cannot throw any light on the structure of the universe, since they illustrate theories already formed, not facts on which theories are to be based. What seemed to me most desirable was that charts should be constructed on which the results of telescopic observation should be carefully mapped down without reference to any preconceived opinions, and solely with the intention of interpreting as satisfactorily as possible whatever laws of stellar distribution may really exist. It appeared to me that mere lists of numbers could afford but unsatisfactory evidence on such points.

Even the mind of a Newton might well shudder before the stupendous problem of attempting to educe a true theory of the stellar universe from a few laws of statistical distribution; while, on the other hand, pictures of the star sphere, or of parts of it, if suitably devised, might at once suggest true views. It is easy to illustrate the difference between the two methods. Suppose we wished to form true ideas respecting the actual distribution of clouds in the air, on some day when the whole sky was flecked with clouds of various form, colour, and shape; but that we had no way of examining the sky except through a movable tube which showed only a small part