systems, composed of two stars revolving about each other in regular orbits, and constituting what may be termed binary stars, to distinguish them from double stars generally so called, in which these physically connected stars are confounded, perhaps, with others only optically double, or casually juxtaposed in the heavens at different distances from the cye; whereas the individuals of a binary star are, of course, equidistant from the eye, or, at least, cannot differ more in distance than the semidiameter of the orbit they describe about each other, which is quite insignificant compared with the immense distance between them and the earth. Between fifty and sixty instances of changes, to a greater or less amount, in the angles of position of double stars, are adduced in the memoirs above mentioned; many of which are too decided, and too regularly progressive, to allow of their nature being misconceived. In particular, among the more conspicuous stars, Castor, y Virginis, Ursæ, 70 Ophiuchi, and Corone, Bootis, Cassiopeiæ, Leonis, Herculis, & Cygni, μ Bootis, ε 4 and ≤ 5 Lyræ, à Ophiuchi, μ Draconis, and Aquarii, are enumerated as among the most remarkable instances of the observed motion; and to some of them even periodic times of revolution are assigned, approximative only, of course, and rather to be regarded as rough guesses than as results of any exact calculation, for which the data were at the time quite inadequate. For instance, the revolution of Castor is set down at 334 years, that of y Virginis at 708, and that of y Leonis at 1200 years.

μ

(605.) Subsequent observation has fully confirmed these results, not only in their general tenor, but for the most part in individual detail. Of all the stars above named, there is not one which is not found to be fully entitled to be regarded as binary; and, in fact, this list comprises nearly all the most considerable objects of that description which have yet been detected, though (as attention has been closely drawn to the subject, and observations have multiplied) it has, of late,

begun to extend itself rapidly. The number of double stars which are certainly known to possess this peculiar character is between thirty and forty at the time we write, and more are emerging into notice with every fresh mass of observations which come before the public. They require excellent telescopes for their observation, being for the most part so close as to necessitate the use of very high magnifiers, (such as would be considered extremely powerful microscopes if employed to examine objects within our reach,) to perceive an interval between the individuals which compose them.

(606.) It may easily be supposed, that phænomena of this kind would not pass without attempts to connect them with dynamical theories. From their first discovery, they were naturally referred to the agency of some power, like that of gravitation, connecting the stars thus demonstrated to be in a state of circulation about each other; and the extension of the Newtonian law of gravitation to these remote systems was a step so obvious, and so well warranted by our experience of its all-sufficient agency in our own, as to have been expressly or tacitly made by every one who has given the subject any share of his attention. We owe, however, the first distinct system of calculation, by which the elliptic elements of the orbit of a binary star could be deduced from observations of its angle of position and distance at different epochs, to M. Savary, who showed *, that the motions of one of the most remarkable among them (Ursa) were explicable, within the limits allowable for error of observation, on the supposition of an elliptic orbit described in the short period of 58 years. A dif. ferent process of computation has conducted Professor Encket to an elliptic orbit for 70 Ophiuchi, described in a period of seventy-four years; and the author of these pages has himself attempted to contribute his mite to these interesting investigations. The following may be stated as the chief results which have been hitherto obtained in this branch of astronomy:+ Berlin Ephem. 1832.

* Connoiss. des Temps, 1830.

(607.) Of these, perhaps, the most remarkable is y Virginis, not only on account of the length of its period, but by reason also of the great diminution of apparent distance, and rapid increase of angular motion about each other, of the individuals composing it. It is a bright star of the fourth magnitude, and its component stars are almost exactly equal. It has been known to consist of two stars since the beginning of the eighteenth century, their distance being then between six and seven seconds; so that any tolerably good telescope would resolve it. Since that time they have been constantly approaching, and are at present hardly more than a single second asunder; so that no telescope, that is not of very superior quality, is competent to show them otherwise than as a single star somewhat lengthened in one direction. It fortunately happens, that Bradley, in 1718, noticed, and recorded in the margin of one of his observation books, the apparent direction of their line of junction, as being parallel to that of two remarkable stars, a and 8 of the same constellation, as seen by the naked eye; and this note, which has been recently rescued from oblivion by the diligence of Professor Rigaud, has proved of signal service in the investigation of their orbit. They are entered also as distinct stars in Mayer's catalogue; and this affords also another means of recovering their relative

situation at the date of his observations, which were made about the year 1756. Without particularizing individual measurements, which will be found in their proper repositories*, it will suffice to remark, that their whole series (which since the beginning of the present century has been very numerous and carefully made, and which embraces an angular motion of 100°, and a diminution of distance to one sixth of its former amount) is represented with a degree of exactness fully equal to that of observation itself, by an ellipse of the dimensions and period stated in the foregoing little table, and of which the further requisite particulars are as follows:

Perihelion passage.

August 18. 1834.

Inclination of orbit to the visual ray

22° 58'

Angle of position of the perihelion projected on the heavens

36° 24

Angle of position of the line of nodes, or intersection of the plane of the orbit with the surface of the heavens

97° 23'

(608.) If the great length of the periods of some of these bodies be remarkable, the shortness of those of others is hardly less so. η Coronæ has already made a complete revolution since its first discovery by Sir William Herschel, and is far advanced in its second period; and Ursæ, Cancri, and 70 Ophiuchi, have all accomplished by far the greater parts of their respective ellipses since the same epoch. If any doubt, therefore, could remain as to the reality of their orbitual motions, or any idea of explaining them by mere parallactic changes, these facts must suffice for their complete dissipation. We have the same evidence, indeed, of their rotations about each other, that we have of those of Uranus and Saturn about the sun; and the correspondence between their calculated and observed places in such very elongated ellipses, must be admitted to carry with it proof of the prevalence of the Newtonian law of

* See them collected in Mem. R. Ast. Soc. vol. v. p. 35.

gravity in their systems, of the very same nature and cogency as that of the calculated and observed places of comets round the central body of our own.

(609.) But it is not with the revolutions of bodies of a planetary or cometary nature round a solar center that we are now concerned; it is with that of sun around sun-each, perhaps, accompanied with its train of planets and their satellites, closely shrouded from our view by the splendour of their respective suns, and crowded into a space bearing hardly a greater proportion to the enormous interval which separates them, than the distances of the satellites of our planets from their primaries bear to their distances from the sun itself. A less distinctly characterized subordination would be incompatible with the stability of their systems, and with the planetary nature of their orbits. Unless closely nestled under the protecting wing of their immediate superior, the sweep of their other sun in its perihelion passage round their own might carry them off, or whirl them into orbits utterly incompatible with the conditions necessary for the existence of their inhabitants. It must be confessed, that we have here a strangely wide and novel field for speculative excursions, and one which it is not easy to avoid luxuriating in.

(610.) Many of the double stars exhibit the curious and beautiful phænomenon of contrasted or complementary colours.* In such instances, the larger star is usually of a ruddy or orange hue, while the smaller one appears blue or green, probably in virtue of that general law of optics, which provides, that when the retina is under the influence of excitement by any bright, coloured light; feebler lights, which seen alone would produce no sensation but of whiteness, shall for the time appear coloured with the tint complementary to that of the brighter. Thus, a yellow colour predominating in the light of the other suns, perhaps,

With their attendant moons thou wilt descry,

Communicating male and female light,

(Which two great sexes animate the world,)

Stored in each orb, perhaps, with some that live."

Paradise Lost, viii. 148.