streams and clusters seem to be intermingled. These processes seem to have led to an annular or spiral, rather than to a discshaped galaxy; but large portions of the matter, originally distributed perhaps with comparative uniformity, appear to have escaped the influence of these processes. Either because they have been subjected to counteracting attractions, or through the influence of the same principle which makes the centrifugal force near the poles of a rotating globe less than that at the globe's equator, this portion of the universe seems to have been free to form aggregations in regions which lie near to what may be called the polar axis of the galaxy. Nor need we wonder that these aggregations should differ very much in character from those which prevail within the galactic annulus, nor that within the former alone true nebulæ should be found profusely distributed.

The only irregular nebula which has been examined with the spectroscope the great Orion nebula-shines with light whose source is mainly, if not wholly, gaseous. Lord Rosse states that the stars visible in the nebula, when examined with his giant reflector, appear as red points of light upon a blueish-green background of nebulous light; and it is possible, or rather, probable, that these points of light proceed from bodies which are not gaseous. But, however this may be, it is quite clear that there is in the Orion nebula an enormous amount of gaseous matter, forming (it would seem) a connected but irregular system, within which are involved many fixed stars, and notably the second magnitude star e Orionis, and the third magnitude star Orionis. It seems fairly presumable that the other irregular nebulæ consist in like manner of enormous aggregations of the same luminous gas. The annular and planetary nebulæ appear to be, without exception, gaseous masses. Now, we have seen that all the irregular nebula lie within, or close to, the Milky Way. The same is the case with the annular nebulæ, and by far the larger number of the planetary nebulæ.* The Dumbbell nebula, one of the most remarkable gaseous nebulæ in the heavens, also lies on the Milky Way. We see, then, that the gaseous masses revealed to us by the telescope show a marked tendency to aggregate along the galactic zone. Why this should be the case, it would not be easy-in the present state of our knowledge to determine; but it is clearly not a phenomenon which need surprise us when once we have accepted the conclusion that stars

* Of thirty-four planetary nebula recorded in the "General Catalogue," no less than twenty-one lie within 15° of the great circle centrally dividing the Via Lactea.

and nebulæ form but a single system. I do not consider that, in extra-galactic space, the luminous gas which constitutes the common material of all the gaseous nebula (for the spectroscope reveals no variety in this respect *) is in reality wanting. It probably exists, but in a more dispersed form than in the galaxy. It is not a little remarkable that the only comets yet examined with the spectroscope exhibit (as respects the light from the nucleus) the same three lines of light which form the spectrum of the gaseous nebula. Is it not possible that, around some stars or systems of stars, comets are much more thickly congregated than round our own sun; and that, in places, there may even exist systems of comets free from stellar influences? If we suppose the irregular nebula to result from the former arrangement, the annular and planetary nebulæ from the latter, we should be able to understand the permanence of the apparent figures of these objects, since the slow motions of comets in the enormous orbits indicated by our hypothesis would not be appreciable even in hundreds of years.

I cannot but think that there is some significance in the circumstance that so many temporary stars † have made their appearance "in or close upon the borders of the Milky Way, and," as the younger Herschel says, "only within the following semicircle, the preceding semicircle having offered no example of the kind." May there not be a connection between this peculiarity and the circumstance that so many of the more remarkable variables lie near the Milky Way? I have already noted the association of Eta Argûs with a large irregular nebula. Betelgeux, in the neighbourhood of the Orion nebula, is another remarkable variable. Near the nebular region of Cygnus there are also several variable stars.

The Magellanic Clouds remain to be briefly considered. Two arguments have been made use of to show that these mysterious objects are not connected with the galactic system :-First, they contain forms of nebulæ not met with within the Milky Way; and, secondly, there are no traces of any streams of nebulous light leading from the Milky Way towards the Nubeculae. The former

* Some of the gaseous nebula give a spectrum having one bright line; others give a three-line spectrum; and there is one nebula the spectrum of which consists of four lines. But it is presumable that these variations result only from variations in the intensity of the light of these nebula, since the bright lines occupy always the same position.

+ Sir John Herschel says, "all, without exception;" but, since this was written, the temporary star, which appeared in Corona in May, 1866, has formed an exception to the rule we have referred to. It is probable that so-called temporary stars are in reality merely variables of long period and fitful variability.

argument presents no difficulty. It is, indeed, rather a confirmation of our views that they afford an easy explanation of what had been held to be a scarcely explicable phenomenon. That the processes of aggregation in portions of space not falling within the galactic annulus, should, in certain regions, lead to the exhibition of forms seen within that region, can hardly be considered very wonderful. But, in connection with the second argument, there is a circumstance which deserves to be carefully attended to. Herschel dwells forcibly on the exceeding barrenness of the regions which immediately surround the Nubecula. "The access to the Nubecula Minor on all sides is through a desert," he says, in one place; and, among his notes on this district, we find such expressions as “a miserably poor and barren region"; "a region of utter barrenness"; and so on. Now, this peculiarity, so far from confirming Herschel's opinion that the Nubeculæ are disconnected with the sidereal system, is directly opposed to it. One can understand the phenomenon, if one looks on the Nubeculæ as aggregations formed within regions of space belonging to the sidereal system-one would almost expect that the neighbourhood of such regions should be deficient in splendour-drained of stars, so to speak. But, if the Nubeculæ were really distinct systems far beyond the sidereal system, there could be no reason for expecting that their neighbourhood should be more barren than other portions of the sky-still less that it should be oppressively barren. May we not go farther, and say that there is no way of accounting for so remarkable a phenomenon, save on some such hypothesis as we have presented?

But this is not all. It has been well remarked by Sir John Herschel, that the two Nubeculae are so nearly circular as to render the assumption that they are otherwise than globular in figure utterly improbable. It follows, therefore, that the farthest part of either globe is not much farther off proportionately than the nearest part. Hence the Nubeculæ show us that "stars of the seventh and eighth magnitude and irresolvable nebulæ may coexist within limits of distance not differing more in proportion than as nine to ten." Surely this circumstance is of greater force than Sir John Herschel seems to assume. He says that "it must inspire some degree of caution in accepting as certain" the views ordinarily held respecting stars and nebulæ. To me the fact that stars and irresolvable nebulæ appear intermixed in the Nubeculæ seems to afford decisive evidence of the justice of the views which I have been induced to accept on other grounds. In the face of such evidence, the old theories respecting the universe seem to become wholly untenable.

THE LUNAR CRATERS ARISTARCHUS AND LINNÉ.

BY JOHN BROWNING, F.R.A.S.

(With a Tinted Plate.)

THE lunar craters Aristarchus and Herodotus are scarcely half the diameter of Copernicus. Though small by comparison with other craters, they possess features of extraordinary interest. The remarkable serpentine valley running south from Herodotus, and starting from that crater, often spoken of as the dry river bed, is probably, after the wedge-shaped valley of the Alps, the most curious valley on the moon's surface.

In studying Aristarchus, the point to which I directed my attention was its high reflective power. Several authorities, such as Herschel I., Schröter, and the Rev. T. W. Webb, mention this crater as "the most brilliant in the Moon." It seems to me probable, however, that Linné may be more brilliant. Watching the terminator pass over Aristarchus, I have thought that when fully lighted up, the details became much more indistinct than those of any other crater of considerable diameter under similar circumstances. The details of Copernicus, when fully illuminated, can be well made out, with a moderately powerful telescope; while in the case of Aristarchus, the very form is so completely changed that it can no longer be recognized. This leads to the suggestion that a very high reflective power of the surface of the crater Linné, may be the principal cause of the perplexing differences noticed in its appearance when seen under different degrees of illumination. Of course the indistinctness would be greatly increased by the very small size of Linné when compared with Aristarchus. In addition to the loss of definition under full illumination, caused by its high reflective power, Aristarchus seems to alter its form under different angles of illumination more than any other lunar crater, not excepting even Linné.

Every observer of the smallest experience is aware of the changed aspect of the lunar landscapes when viewed at different periods of the Moon's age; but, until lately, no one imagined that a crater could undergo such a complete matamorphosis as is proved to take place in Linné; or as the drawings now given, show to occur in the case of Aristarchus.

The drawings now engraved were made with my 121-inch silvered glass reflector, using powers of 143, 208, 306, and 455. When I observed Linné the air was so unsteady that I could not with the whole aperture gain any advantage by using a higher

power than 306. Having been made without any preparation, the drawings were not done to scale, but one inch will correspond to about fifty miles.

In the first drawing I., A, represents Aristarchus near the terminator; the craters, being near the Moon's limb, are considerably foreshortened east and west. To see this view with the best effect, the engraving should be held with the left side, or that on which the strong shadows appear, turned towards the source of light by which it is illuminated. In Linné, on the same plate, the shadows are necessarily reversed, because the drawing represents it when seen on the evening terminator, that is, when the Sun is setting on the objects near the terminator; while the drawing of Aristarchus is a view of the crater on the morning terminator, that is, when the Sun is rising upon it.

In the second drawing, II., a represents Aristarchus when fully illuminated—that is, at full Moon.

The drawing marked I. is wanting in finish. The Moon was nearly setting when I began it, and I was compelled to draw hurriedly to complete it. Mr. Dawes once expressed to me his mistrust of any drawings that had been altered away from the telescope, with a view to improve their appearance. As I share this feeling, I prefer to have the view engraved just as it was made, to running the risk of sacrificing truth, however slightly, to artistic appearance.

I believe the drawing is substantially correct, for Mr. Slack kindly examined it at the telescope very carefully, on a succeeding occasion, and stated that, in the main features, he could not suggest any alterations.

Of the accuracy of the view II., which shows the same craters fully illuminated, I cannot feel so certain, for the glare of the Moon in the 12-inch aperture so dazzled my eyes, that I could scarcely see the white lines that I was making on almost white paper. Mr. Barnes has, however, examined and described the crater Aristarchus, to which I particularly refer, with his 10-inch equatorial, and his observations coincide with my own.

In the drawing I., it will be seen that Aristarchus, which is to the left, is a large, well-defined crater, nearly oval in form, having walled sides like terraces, and two peaks in the centre. Between Aristarchus and Herodotus there is a raised plateau equal to the width of either of these craters.

In the drawing II., Aristarchus appears only as a nebulous patch of enormously increased diameter, and of a totally different form. From the central peaks, bright lines, like the ridges of