especially voyagers at sea, provided with only good eyes, or moderate instruments, might employ their time to excellent advantage. It holds out a sure promise of rich discovery, and is one in which astronomers in established observatories are almost of necessity precluded from taking a part, by the nature of the observations required. Catalogues of the comparative brightness of the stars in each constellation have been constructed by Sir Wm. Herschel, with the express object of facilitating these researches, and the reader will find them, and a full account of his method of comparison, in the Phil. Trans. 1796, and subsequent years.

(833.) We come now to a class of phænomena of quite a different character, and which give us a real and positive insight into the nature of at least some among the stars, and enable us unhesitatingly to declare them subject to the same dynamical laws, and obedient to the same power of gravitation which governs our own system. Many of the stars, when examined with telescopes, are found to be double, i. e. to consist of two (in some cases three or more) individuals placed near together. This might be attributed to accidental proximity, did it occur only in a few instances; but the frequency of this companionship, the extreme closeness, and, in many cases, the near equality of the stars so conjoined, would alone lead to a strong suspicion of a more near and intimate relation than mere casual juxtaposition. The bright star, Castor, for example, when much magnified, is found to consist of two stars of nearly the third magnitude, within 5′′ of each other. Stars of this magnitude, however, are not so common in the heavens as to render it otherwise than excessively improbable that, if scattered at random, they would fall so near. But this improbability becomes immensely increased by a consideration of the fact, that this is only one out of a great many similar instances. Mitchell, in 1767. applying the rules for the calculation of probabilities to the case of the six brightest stars in the group called the Pleiades, found the odds to be 500000 to 1 against their proximity being the mere result of a random scattering of 1500 stars (which he supposed to be the total number of stars of that magnitude in the celestial sphere1) over the heavens. Speculating further on this, as an indication of physical connexion rather than fortuitous assemblage, he was led to surmise the possibility, (since converted into a certainty, but at that time, antecedent to any observation) of the existence of compound stars revolving about one another, or rather about their common centre of gravity. M. Struve, pursuing the same train of thought as applied specially to the cases of double and triple

1 This number is considerably too small, and in consequence, Mitchell's odds in this case materially overrated. But enough will remain, if this be rectified, fully bear out his argument. Phil. Trans. vol. 57.

combinations of stars, and grounding his computations on a more perfect enumeration of the stars visible down to the 7th magnitude, in the part of the heavens visible at Dorpat, calculates that the odds are 9570 to 1 against any two stars, from the 1st to the 7th magnitude, inclusive, out of the whole possible number of binary combinations then visible, falling, (if fortuitously scattered) within 4" of each other. Now, the number of instances of such binary combinations actually observed at the date of this calculation was already 91, and many more have since been added to the list. Again, he calculates that the odds against any such stars fortuitously scattered, falling within 32′′ of a third, so as to constitute a triple star, is not less than 173524 to 1. Now, four such combinations occur in the heavens; viz. Orionis, o Orionis, 11 Monocerotis, and Cancri. The conclusion of a physical connexion of some kind or other is therefore unavoidable.

(834.) Presumptive evidence of another kind is furnished by the following consideration. Both a Centauri and 61 Cygni are "Double Stars." Both consist of two individuals, nearly equal, and separated from each other by an interval of about a quarter of a minute. In the case of 61 Cygni, the stars exceeding the 7th magnitude, there is already a primâ facie probability of 9578 to 1 against their apparent proximity. The two stars of a Centauri are both at least of the 2d magnitude, of which altogether not more than about 50 or 60 exist in the whole heavens. But, waiving this consideration, both these stars, as we have already seen, have a proper motion, so considerable that, supposing the constituent individuals unconnected, one would speedily leave the other behind. Yet, at the earliest dates at which they were respectively observed, these stars were not perceived to be double, and it is only to the employment of telescopes magnifying at least 8 or 10 times, that we owe the knowledge we now possess of their being so. With such a telescope, Lacaille, in 1751, was barely able to perceive the separation of the two constituents of a Centauri, whereas, had one 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 second those in which it exceeds 1", but falls short of 2′′; the 3d 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"; 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 lucida) 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 sub-class, which he terms residuary (Duplices reliquae). 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 sub-divided 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,

μ

Of these a Andromedae, μ Bootis, and u 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. offers the remarkable combination of a double-double star.

Fig. 111.

Lyræ

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 24", 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 accompanied 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:

(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,