brighter star, that of the less bright one in the same field of view will appear blue; while, if the tint of the brighter star verge to crimson, that of the other will exhibit a tendency to green- —or even appear as a vivid green, under favourable circumstances. The former contrast is beautifully exhibited by Cancri—the latter by y Andromedæ ; both fine double stars. If, however, the coloured star be much the less bright of the two, it will not materially affect the other. Thus, for instance, Cassiopeia exhibits the beautiful combination of a large white star, and a small one of a rich ruddy purple. It is by no means, however, intended to say, that in all such cases one of the colours is a mere effect of contrast, and it may be easier suggested in words, than conceived in imagination, what variety of illumination two suns—a red and a green, or a yellow and a blue onemust afford a planet circulating about either; and what charming contrasts and "grateful vicissitudes,”—a red and a green day, for instance, alternating with a white one and with darkness,—might arise from the presence or absence of one or other, or both, above the horizon. Insulated stars of a red colour, almost as deep as that of blood, occur in many parts of the heavens, but no green or blue star (of any decided hue) has, we believe, ever been noticed unassociated with a companion brighter than itself.

(611.) Another very interesting subject of enquiry, in the physical history of the stars, is their proper motion. A priori, it might be expected that apparent motions of some kind or other should be detected among so great a multitude of individuals scattered through space, and with nothing to keep them fixed. Their mutual attrac tions even, however inconceivably enfeebled by distance, and counteracted by opposing attractions from opposite quarters, must, in the lapse of countless ages, produce some movements some change of internal arrangeresulting from the difference of the opposing actions. And it is a fact, that such apparent motions do exist, not only among single, but in many of the

ment

double stars; which, besides revolving round each other, or round their common center of gravity, are transferred, without parting company, by a progressive motion common to both, towards some determinate region. For example, the two stars of 61 Cygni, which are nearly equal, have remained constantly at the same, or very nearly the same, distance, of 15", for at least fifty years past. Meanwhile they have shifted their local situation in the heavens, in this interval of time, through no less than 4′ 23′′, the annual proper motion of each star being 5"-3; by which quantity (exceeding a third of their interval) this system is every year carried bodily along in some unknown path, by a motion which, for many centuries, must be regarded as uniform and rectilinear. Among stars not double, and no way differing from the rest in any other obvious particular,

Cassiopeia is to be remarked as having the greatest proper motion of any yet ascertained, amounting to 374 of annual displacement. And a great many others have been observed to be thus constantly carried away from their places by smaller, but not less unequivocal motions.

(612.) Motions which require whole centuries to accumulate before they produce changes of arrangement, such as the naked eye can detect, though quite sufficient to destroy that idea of mathematical fixity which precludes speculation, are yet too trifling, as far as practical applications go, to induce a change of language, and lead us to speak of the stars in common parlance as otherwise than fixed. Too little is yet known of their amount and directions, to allow of any attempt at referring them to definite laws. It may, however, be stated generally, that their apparent directions are various, and seem to have no marked common tendency to one point more than to another of the heavens. It was, indeed, supposed by Sir William Herschel, that such a common tendency could be made out; and that, allowing for individual deviations, a general recess could be perceived in the principal stars, from that point occupied by the

starHerculis, towards a point diametrically opposite. This generally tendency was referred by him to a motion of the sun and solar system in the opposite direction. No one, who reflects with due attention on the subject, will be inclined to deny the high probability, nay certainty, that the sun has a proper motion in some direction; and the inevitable consequence of such a motion, unparticipated by the rest, must be a slow average apparent tendency of all the stars to the vanishing point of lines parallel to that direction, and to the region which he is leaving. This is the necessary effect of perspective; and it is certain that it must be detected by such observations, if we knew accurately the apparent proper motions of all the stars, and if we were sure that they were independent, i. e. that the whole firmament, or at least all that part which we see in our own neighbourhood, were not drifting along together, by a general set as it were, in one direction, the result of unknown processes and slow internal changes going on in the sidereal stratum to which our system belongs, as we see motes sailing in a current of air, and keeping nearly the same relative situation with respect to one another. But it seems to be the general opinion of astronomers, at present, that their science is not yet matured enough to afford data for any secure conclusions of this kind one way or other. Meanwhile, a very ingenious idea has been suggested by the present Astronomer Royal (Mr. Pond), viz. that a solar motion, if it exist, and have a velocity at all comparable to that of light, must necessarily produce a solar aberration; in consequence of which we do not see the stars disposed as they really are, but too much crowded in the region the sun is leaving, too open in that he is approaching. (See art. 280.) Now this, so long as the solar velocity continues the same, must be a constant effect which observation cannot detect; but should it vary in the course of ages, by a quantity at all commensurate to the velocity of the earth in its orbit, the fact would be detected by a general apparent rush of all the stars to the

one or other quarter of the heavens, according as the sun's motion were accelerated or retarded; which observation would not fail to indicate, even if it should amount to no more than a very few seconds. This consideration, refined and remote as it is, may serve to give some idea of the delicacy and intricacy of any enquiry into the matter of proper motion; since the last mentioned effect would necessarily be mixed up with the systematic parallax, and could only be separated from it by considering that the nearer stars would be affected more than the distant ones by the one cause, but both near and distant alike by the other.

(613.) When we cast our eyes over the concave of the heavens in a clear night, we do not fail to observe that there are here and there 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 insulated from the rest of the heavens. The constellation called Coma Berenices is another such group, more diffused, and consisting of much larger stars.

(614.) In the constellation Cancer, there is a somewhat similar, but less definite, luminous spot, called Præsepe, or the bee-hive, which a very moderate tele

It is a very remarkable fact, that the center of the visual area is by 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 appreciate it, let the reader look alternately full at a star of the fifth magnitude, and beside it; or choose two, equally bright, and about 30° or 40 apart, and look full at one of them, the probability is, he will see only the other: such, at least, is my own case. 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. -Author.

scope, 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 indivi duals 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 center, 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 nebuleuse sans etoiles), as seen in the 20 feet reflector at Slough).* 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

This beautiful object was first noticed by Halley in 1714. It is visible to the naked eye, between the stars and Herculis In a night-glass it appears exactly like a small round comet.