great distances from them respectively, in the manner of binary stars: [and it has been recently shown by M. Peters (Ast. Nachr. 748,) that, in the case of Sirius, such a circulation, performed in a period of 50-093 years in an ellipse whose excentricity is 0.7994, the perihelion passage taking place at the epoch A. D. 1701-431, would reconcile in a remarkable manner the observed anomalies, and reduce the residual motion to uniformity.]

(860.) The whole of the reasoning upon which the determination of the solar motion in space rests, is based upon the entire exclusion of any law either derived from observation or assumed in theory, affecting the amount and direction of the real motions both of the sun and stars. It supposes an absolute non-recognition, in those motions, of any general directive cause, such as, for example, a common circulation of all about a common centre. Any such limitation introduced into the conditions of the problem of the solar motion would alter in toto both its nature and the form of its solution. Suppose, for instance, that, conformably to the speculations of several astronomers, the whole system of the Milky Way, including our sun, and the stars, our more immediate neighbours, which constitute our sidereal firmament, should have a general movement of rotation in the plane of the galactic circle (any other would be exceedingly improbable, indeed hardly reconcilable with dynamical principles,) being held together in opposition to the centrifugal force thus generated by the mutual gravitation of its constituent stars. Except we at the same time admitted that the scale on which this movement proceeds is so enormous that all the stars whose proper motions we include in our calculations go together in a body, so far as that movement is concerned (as forming too small an integrant portion of the whole to differ sensibly in their relation to its central point;) we stand precluded from drawing any conclusion whatever, not only respecting the absolute motion of the sun, but respecting even its relative movement among those stars, until we have established some law, or at all events framed some hypothesis having the provisional force of a law, connecting the whole, or a part of the motion of each individual with its situation in space.

(861.) Speculations of this kind have not been wanting in astronomy, and recently an attempt has been made by M. Mädler to assign the local centre in space, round which the sun and stars revolve, which he places in the group of the Pleiades, a situation in itself improbable, lying as it does no less than 26° out of the plane of the galactic circle, out of which plane it is almost inconceivable that any general circulation can take place. In the present defective state of our knowledge respecting the proper motion of the smaller stars, especially in right ascension, (an element for

the most part far less exactly ascertainable than the polar distance, or at least which has been hitherto far less accurately ascertained,) we cannot but regard all attempts of the kind as to a certain extent premature, though by no means to be discouraged as forerunners of something more decisive. The question, as a matter of fact, whether a rotation of the galaxy in its own plane exist or not, might be at once resolved by the assiduous observation, both in R. A. and polar distance, of a considerable number of stars of the Milky Way, judiciously selected for the purpose, and including all magnitudes, down to the smallest distinctly identifiable, and capable of being observed with normal accuracy: and we would recommend the inquiry to the special attention of the directors of permanent observatories, provided with adequate instrumental means, in both hemispheres. Thirty or forty years of observation, perseveringly directed to the object in view, could not fail to settle the question.'

(862.) The solar motion through space, if real and not simply relative, must give rise to uranographical corrections analogous to parallax and aberration. The solar or systematic parallax is no other than that part of the proper motion of each star which is simply apparent, arising from the sun's motion, and until the distances of the stars be known, must remain inextricably mixed up with the other or real portion. The systematic aberration, amounting at its maximum (for stars 90° from the solar apex) to about 5", displaces all the stars in great circles diverging from that apex through angles proportional to the sines of their respective distances from it. This displacement, however, is permanent, and therefore uncognizable by any phænomenon, so long as the solar motion remains invariable; but should it, in the course of ages, alter its direction and velocity, both the direction and amount of the displacement in question would alter with it. The change, however, would become mixed up with other changes in the apparent proper motions of the stars, and it would seem hopeless to attempt disentangling them.

(863.) A singular, and at first sight paradoxical effect of the progressive movement of light, combined with the proper motion of the stars, is that it alters the apparent periodic time in which the individuals of a binary star circulate about each other. To make this apparent, suppose them to circulate round each other in a plane perpendicular to the visual

'An examination of the proper motions of the stars of the B. Assoc. Catal. in the portion of the Milky Way nearest either pole (where the motion should be almost wholly in R A) indicates no distinct symptom of such a rotation. If the question be taken up fundamentally, it will involve a redetermination from the recorded proper motions, both of the precession of the equinoxes and the change of obliquity of the ecliptic.

⚫ Astronomische Nachrichten, No. 520.

ray in a period of 10,000 days. Then if both the sun and the centre of gravity of the binary system remained fixed in space, the relative apparent situation of the stars would be exactly restored to its former state after the lapse of this interval, and if the angle of position were 0° at first, after 10,000 days it would again be so. But now suppose that the centre of gravity of the star were in the act of receding in a direct line from the sun, with a velocity of one-tenth part of the radius of the earth's orbit per diem. Then at the expiration of 10,000 days it would be more remote from us by 1000 such radii, a space which light would require 57 days to traverse. Although really, therefore, the stars would have arrived at the position 0° at the exact expiration of 10,000 days, it would require 57 days more for the notice of that fact to reach our system. In other words, the period would appear to us to be 10,057 days, since we could only conclude the period to be completed, when to us, as observers, the original angle of position was again restored. A contrary motion would produce a contrary effect.

10,005,

CHAPTER XVII.

OF CLUSTERS OF STARS AND NEBULE.

GLOBULAR CLUSTERS.

THEIR

OF CLUSTERING GROUPS OF STARS.
STABILITY DYNAMICALLY POSSIBLE. LIST OF THE MOST REMARK-

IRREGULAR CLUSTERS.

DISTRIBUTION OVER THE HEAVENS.
RESOLVABILITY OF NEBULE. -THEORY OF THE FORMATION OF
CLUSTERS BY NEBULOUS SUBSIDENCE.

OF ELLIPTIC NEBULE.

THAT OF ANDROMEDA. — ANNULAR AND PLANETARY NEBULÆ.,
DOUBLE NEBULE. NEBULOUS STARS. CONNEXION OF NEBULE

WITH DOUBLE STARS. INSULATED NEBULÆ, OF FORMS NOT WHOLLY IRREGULAR. OF AMORPHOUS NEBULA.

THEIR LAW OF DISTRIBUTION MARKS THEM AS OUTLIERS OF THE GALAXY. NEBULÆ, AND NEBULOUS GROUP OF ORION. -OF ARGO. -OF SAGITTARIUS. OF CYGNUS.THE MAGELLANIC CLOUDS. SINGULAR NEBULA IN THE GREATER OF THEM. THE ZODIACAL LIGHT. SHOOTING STARS.

(864.) WHEN we cast our eyes over the concave of the heavens in a clear night, we do not fail to observe that here and there are groups of stars which seem to be compressed together in a more condensed manner than in the neighbouring parts, forming bright patches and clusters, which attract attention, as if they were there brought together by some general cause other than casual distribution. There is a group, called the Pleiades, in which six or seven stars may be noticed, if the eye be directed full upon it; and many more if the eye be turned carelessly aside, while the attention is kept directed' upon the group. Telescopes show fifty or sixty large stars thus crowded together in a very moderate space, comparatively

It is a very remarkable fact, that the centre of the visual area is far less sensible to feeble impressions of light, than the exterior portions of the retina. Few persons are aware of the extent to which this comparative insensibility extends, previous to trial. To estimate it, let the reader look alternately full at a star of the fifth magnitude, and beside it; or choose two equally bright, and about 3° or 4° apart, and look full at one of them, the probability is he will see only the other. The fact accounts for the multitude of stars with which we are impressed by a general view of the heavens; their paucity when we come to count them.

insulated from the rest of the heavens. The constellation called Coma Berenices is another such group, more diffused, and consisting on the whole of larger stars.

(865.) In the constellation Cancer, there is a somewhat similar, but less definite, luminous spot, called Præsepe, or the bee-hive, which a very moderate telescope, an ordinary night-glass for instance, resolves entirely into stars. In the sword-handle of Perseus, also, is another such spot, crowded with stars, which requires rather a better telescope to resolve into individuals, separated from each other. These are called clusters of stars; and, whatever be their nature, it is certain that other laws of aggregation subsist in these spots, than those which have determined the scattering of stars over the general surface of the sky. This conclusion. is still more strongly pressed upon us, when we come to bring very powerful telescopes to bear on these and similar spots. There are a great number of objects which have been mistaken for comets, and, in fact, have very much the appearance of comets without tails: small round, or oval nebulous specks, which telescopes of moderate power only show as such. Messier has given, in the Connois. des Temps for 1784, a list of the places of 103 objects of this sort; which all those who search for comets ought to be familiar with, to avoid being misled by their similarity of appearance. That they are not, however, comets, their fixity sufficiently proves; and when we come to examine them with instruments of great power, such as reflectors of eighteen inches, two feet, or more in aperture, any such idea is completely destroyed. They are then, for the most part, perceived to consist entirely of stars crowded together so as to occupy almost a definite outline, and to run up to a blaze of light in the centre, where their condensation is usually the greatest. (See fig. 1, pl. II., which represents (somewhat rudely) the thirteenth nebula of Messier's list (described by him as nébuleuse sans étoiles), as seen in a reflector of 18 inches aperture and 20 feet focal length.) Many of them, indeed, are of an exactly round figure, and convey the complete idea of a globular space filled full of stars, insulated in the heavens, and constituting in itself a family or society apart from the rest, and subject only to its own internal laws. It would be a vain task to attempt to count the stars in one of these globular clusters. They are not to be reckoned by hundreds; and on a rough calculation, grounded on the apparent intervals between them at the borders, and the angular diameter of the whole group, it would appear that many clusters of this description must contain at least five thousand stars, compacted and wedged together in a round space, whose angular diameter does not exceed eight or ten minutes; that is to say, in an area not more than a tenth part of that covered by the moon.