of them only been affected with the observed proper motion, they should then have been 6' asunder. In these cases then some physical connexion may be regarded as proved by this fact alone.

(835.) Sir William Herschel has enumerated upwards of 500 double stars, of which the individuals are less than 32" asunder. M. Struve, prosecuting the inquiry with instruments more conveniently mounted for the purpose, and wrought to an astonishing pitch of optical perfection, has added more than five times that number. And other observers have extended still further the catalogue of "Double Stars," without exhausting the fertility of the heavens. Among these are a great many in which the distance between the component individuals does not exceed a single second. They are divided into classes by M. Struve (the first living authority in this department of Astronomy) according to the proximity of their component individuals. The first class comprises those only in which the distance does not exceed 1"; the 2nd those in which it exceeds 1" but falls short of 2"; the 3rd class extends from 2" to 4" distance; the 4th from 4" to 8"; the 5th from 8" to 12"; the 6th from 12" to 16"; the 7th from 16" to 24", and the 8th from 24" to 32". Each class he again subdivides into two sub-classes, of which the one under the appellation of conspicuous double stars (duplices lucidae) comprehends those in which both individuals exceed the 8 magnitude, that is to say, are separately bright enough to be easily seen in any moderately good telescope. All others, in which one or both the constituents are below this limit of easy visibility, are collected into another subclass which he terms residuary (Duplices reliquæ). This arrangement is so far convenient, that after a little practice in the use of telescopes as applied to such objects, it is easy to judge what optical power will probably suffice to resolve a star of any proposed class and either sub-class, or would at least be so if the second or residuary sub-class were further subdivided by placing in a third sub-class" delicate" double stars, or those in which the companion star is so very minute as to 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.

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

a Andromeda.
& Lyræ.
Cancri.

• Orionis.

μ Lupi. μ Bootis.

Scorpii.
11 Monocerotis.
12 Lyncis.

Of these, a Andromedæ, & 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. Lyra 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 5 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 phenomenon 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 accompanied by two Fig. 111.

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:

(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 phenomena 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 centre, 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 centre 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 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

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