require a high degree of optical power to perceive it, of which instances will presently be given.

(836.) The following may be taken as specimens of each class. They are all taken from among the lucid, or conspicuous stars, and to such of our readers as may be in possession of telescopes, and may be disposed to try them on such objects, will afford him a ready test of their degree of efficiency. Class I., 0" to 1".

(837.) Among the most remarkable triple, quadruple, or multiple stars (for such also occur), may be enumerated,

a Andromedæ. € Lyræ. Cancri.

Orionis. μ Lupi.

μ Bootis.

Scorpii.
11 Monocerotis.
12 Lyncis.

Of these, a Andromeda, μ Bootis, and Lupi appear in telescopes, even of considerable optical power, only as ordinary double stars; and it is only when excellent instruments are used that their smaller companions are subdivided and found to be, in fact, extremely close double stars. * Lyræ offers the remarkable combination of a double-double star. Viewed with a telescope of low power it appears as a coarse and easily divided double star, but on increasing the magnifying power, each individual is perceived to be beautifully and closely double, the one pair being about 21", the other about 3" asunder. Each of the stars Cancri, & Scorpii, 11 Monocerotis, and 12 Lyncis consists of a principal star, closely double, and a smaller and more distant attendant, while Orionis presents the phænomenon of four brilliant principal stars, of the respective 4th, 6th, 7th, and 8th magnitudes, forming a trapezium, the longest diagonal of which is 21"-4, and ac

companied by two excessively minute and very close companions (as in the annexed figure), to perceive both which is one of the severest tests which can be applied to a telescope. (838.) Of the "delicate" sub-class of double stars, or those consisting of very large and conspicuous principal stars, accompanied by very minute companions, the following specimens may suffice:

a 2 Cancri.

a 2 Capricorni. a Indi,

a Lyræ.

a Polaris.

B Aquarii.
y Hydræ.

Ursa Majoris.

K Circini.

K Geminorum.
μ Persei.
7 Bootis.

Virginis.

x Eridani.

16 Auriga. 94 Ceti.

(839.) To the amateur of Astronomy the double stars offer a subject of very pleasing interest, as tests of the performance of his telescopes, and by reason of the finely contrasted colours which many of them exhibit, of which more hereafter. But it is the high degree of physical interest which attaches to them, which assigns them a conspicuous place in modern Astronomy, and justifies the minute attention and unwearied diligence bestowed on the measurement of their angles of position and distances, and the continual enlargement of our catalogues of them by the discovery of new ones. It was, as we have seen, under an impression that such combinations, if diligently observed, might afford a measure of parallax through the periodical variations it might be expected to produce in the relative situation of the small attendant star, that Sir W. Herschel was induced (between the years 1779 and 1784) to form the first extensive catalogues of them, under the scrutiny of higher magnifying powers than had ever previously been applied to such purposes. In the pursuit of this object, the end to which it was instituted as a means was necessarily laid aside for a time, until the accumulation of more abundant materials should have afforded a choice of stars favourably circumstanced for systematic observation. Epochal measures however, of each star, were secured, and, on resuming the subject, his attention was altogether diverted from the original object of the inquiry by phænomena of a very unexpected character, which at once. engrossed his whole attention. Instead of finding, as he expected, that annual fluctuation to and fro of one star of a double star with respect to the other, that alternate annual increase and decrease of their distance and angle of position, which the parallax of the earth's annual motion would produce, he observed, in many instances, a regular progressive change; in some cases bearing chiefly on their distance, -in others on their position, and advancing steadily in one direction, so as clearly to indicate either a real motion of the stars themselves, or a general rectilinear motion of the sun and whole solar system, producing a parallax of a higher order than would arise from the earth's orbitual motion, and which might be called systematic parallax.

(840.) Supposing the two stars, and also the sun, in motion independently of each other, it is clear that for the interval of several years, these motions must be regarded as rectilinear and uniform. Hence, a very slight acquaintance with geometry will suffice to show that the apparent motion of one star of a double star, referred to the other as a center, and mapped down, as it were, on a plane in which that other shall be taken for a fixed or zero point, can be no other than a right line. This, at least, must be the case if the stars be independent of each other; but it will be otherwise if they have a physical connexion, such as, for instance, real proximity and mutual gravitation would establish. In that case, they would describe orbits round each other, and round their common center of gravity; and therefore the apparent path of either, referred to the other as fixed, instead of being a portion of a straight line, would be bent into a curve concave towards that other. The observed motions, however, were so slow, that many years' observation was required to ascertain this point; and it was not, therefore, until the year 1803, twenty-five years from the commencement of the inquiry, that any thing like a positive conclusion could be come to respecting the rectilinear or orbitual character of the observed changes of position.

(841.) In that, and the subsequent year, it was distinctly announced by him, in two papers, which will be found in the Transactions of the Royal Society for those years*, that there exist sidereal 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 eye; whereas the individuals of a binary star are, of course, equidistant from the eye, or, at least, cannot differ more in distance than the semi-diameter of the orbit they describe about each other, which is quite insignificant compared with the immense distance between them and the earth.

*The announcement was in fact made in 1802, but unaccompanied by the observations establishing the fact.

n

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 Coronæ, & Bootis, Cassiopeia, y Leonis, Herculis, & Cygni, μ Bootis, e 4 and 5 Lyræ, à Ophiuchi, 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.

(842.) Subsequent observation has fully confirmed these results. 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, received large accessions. Upwards of a hundred double stars, certainly known to possess this character, were enumerated by M. Mädler in 1841*, and more are emerging into notice with every fresh mass of observations which come before the public. They require excellent telescopes for their effective 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.

(843.) 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 Dorpat Observations, vol. ix. 1840 and 1841.