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these numbers, and compared with that which really exists, shows that there are not in the heavens two stars of the first magnitude so near one another that their nearness may not probably be considered as accidental. On the other hand, the magnitudes which follow the first present examples of the most remarkable proximities. Who does not know the three bright stars of the second magnitude in the belt of Orion, of which the two external ones are distant from the middle star, one by a degree and twenty-six minutes, and the other a degree and eighteen minutes? The calculations show that there are 1400 chances to one that their nearness is not accidental. The constellation of the southern cross is still more remarkable. We find there in a space of fifteen degrees square (which does not include the 2700th part of the celestial vault) one star of the first magnitude, two of the second, one of the third, and one of the fourth; and the probability that such a distribution is accidental is only that of 1 in 20,000. We have thus the best reasons for thinking that these stars depend upon one another.

These conjectures are confirmed when we consider the stars from the sixth to the seventh magnitude, relative to their distribution throughout the celestial vault.

From a calculation of probabilities, founded upon the number of the stars which are in the celestial Atlas of Harding, the case where two of them should be distant from one another from thirty-two seconds to one minute, ought to occur only one-and-a-half time, while fifteen instances of it are known. There ought to be but six or seven pairs of stars from the first to the seventh magnitude, where the two stars forming the pair are distant from one to two minutes; and there are already fifteen cases known. If we consider it for greater distances for stars of the sixth magnitude, we shall find that there ought to be but seven or eight pairs where the stars are distant from one another from two to five minutes; while there are eighteen cases known. Between five and ten minutes of distance, the calculation of probabilities gives twenty-seven to twenty-eight pairs; and we know thirty-six cases. We can find in the heavens still more pairs of stars, distant from each other ten and fifteen minutes, than the calculation gives, viz.

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twenty-five instead of twenty-two. We may then, with much probability, consider a great number of pairs of stars, from the first to the sixth magnitude, where the two stars are distant from each other from one to fifteen minutes, as being systems of stars, of stars really double, visible to the naked eye, and which are consequently the most brilliant and the nearest to us. Such are, for example, the Nos. 16 and 17, and 1 and 2 Draconis, Dragon, Nos. 4 and 5 of Lyra, the two 1a and 2 α Libræ, Ursa Majoris, and the well known star Alcor, &c. &c. A remarkable confirmation of this opinion may be drawn from the circumstance already observed by Bessel, that some of these pairs have a common proper motion, such as, for example, No. 36 Serpentarii and 30 Scorpii, and the two stars above-mentioned in the last of the Great Bear. But what is well worthy of remark, it frequently happens that sometimes one of the stars of these pairs, and sometimes both, are themselves double in the strictest meaning of the word.

We find also very frequently three stars near each other, which would not probably happen if they were distributed by chance. Among the 1386 stars between the first and fifth magnitude which are in the maps of Harding, the case that there are three in one circle of one degree diameter, ought not to take place more than a quarter of a time; that is, it ought not to occur at all. But it actually presents itself seven times, or twenty-five times oftener than would be probable if it were accidental. From what has now been said, therefore, we may hazard the conjecture that the stars, such as the three of Tauri, and the three of Aquarii, which we can recognize with the naked eye, are stars physically, and not optically triple.

ART. XII.-Notice of "The Third Series of Observations with a twenty-feet Reflecting Telescope, containing a Cata logue of 384 new Double and Multiple Stars, completing a first thousand of those Objects detected in sweeps with that Instrument. By J. F. W. HERSCHEL, Esq. F.R.S. and President of the Astronomical Society.

THE valuable paper, the title of which is given above, was

read before the Astronomical Society of London on the 11th January 1828. As it is so intimately connected with the preceding paper by Professor Struve, we have thought it right to place beside it the following abstract from the Annals of Philosophy, till we shall be enabled to publish a more copious analysis of it.

"This paper, as its title imports, is a continuation of the two papers previously communicated by the author on the same subject. The field of discovery in this department of astronomy, though narrowed by the great work recently published by Professor Struve, the author considers as not yet exhausted; since, on an average of the part of the heavens swept by him not above one in four of double stars, sufficiently remarkable to attract attention in sweeping, have been catalogued by the eminent astronomer last named; not to mention the vast number of interesting close double stars, below the ninth magnitude, which a minuter examination than the nature of his sweeps permits would no doubt produce. The double stars of this catalogue, he observes, are considerably more select than those of his two former ones; those whose distance exceeds 32′′ being (except in particular cases) excluded, and the limit of distance being narrowed according to the faintness of the component stars.

"The author prefaces his catalogue with a comparison of the magnitudes habitually assigned to the stars by himself and Professor Struve; from which it appears that on the average his magnitudes have a denomination about one unit lower than those of that astronomer;—a star (for example) which M. Struve would call of the ninth magnitude, being, in Mr Herschel's nomenclature, of the tenth. The limit of vision in the Dorpat telescope, he presumes to lie about his average fourteenth magnitude, though such a determination must necessarily be liable to some latitude. This conclusion he deduces from a series of instances, in which small companions have been seen by him attached to large stars, within the limits of Professor Struve's fourth class, which have escaped the notice of the latter.

"The author then states the principle on which he estimates magnitudes below the sixth, which is that of continual bisection

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of the light; and he cites some experiments, by which it арpears that the light of an average star of the first magnitude is at least 150 times that of the sixth. He then adduces a series of observations of a considerable number of the closer stars of M. Struve's catalogue, by which it appears that the Slough telescope easily defines with its ordinary sweeping power the generality of M. Struve's stars of the first class, and many of those marked by him as vicine, and even pervicinæ ; but those which have the epithet vicinissima, he has not yet succeeded in separating with the highest power (240) usually applied,which indeed was to be expected. In lieu of M. Struve's classification of double stars, which he considers as enlarging beyond due limits the number of those of the first class, he proposes the following system, which in fact very nearly approximates to that originally followed by Sir William Herschel.

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So that the limit of distance of stars of the nth class shall be 2n x 1".

"The author then subjoins a list of stars common to his two former catalogues, and to that of Professor Struve, 86 in number; after which he proceeds to describe some singular phenomena observed in the course of his examination of these objects, which explain certain discrepancies between the results of observations of their angles of position on different nights, and which tend to throw light on some obscure points in the theory of vision. He considers it as rendered very probable by some of the facts adduced, that time is required for light to make an impression on the retina, as well as for the impression made to wear off; and that this time is the less, the brighter the object; and explains by this principle a remarkable degree of unsteadiness and fluctuation observed in the limb of the planet Mars, while small stars in the field remained perfectly tranquil, as well as certain other curious phenomena.

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"He then adds some observations on the contrasted colours so frequently observed in double stars, and regards them as (at least) in many cases referable to the laws of vision; in virtue of which, a strong light having an excess of the less refrangible rays, will cause a feebler one, in which no such excess exists, to appear of the complementary hue; instances of which, in artificial lights, are adduced. He notices especially the extremely intense red colour of a star of the eighth magnitude, R. A. 4 41m. N.P.D. 61° 47′ (1828.)

"These prefatory remarks are terminated by some observations of the fifth star in trapezio nebula Orionis, pointed out by M. Struve. The author adduces evidence, which he considers as satisfactory, that no such star existed in that situation on the 13th March 1826. It was observed, however, by M. Struve to be conspicuous on the 11th November of that year. It is now readily seen in the Slough telescope; and at the time of drawing up the present paper, it was so bright as not to be overlooked with the most ordinary degree of attention. He considers it, therefore, if not as a NEW STAR, at least as a variable one of very singular character.

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"The catalogue which follows is arranged in all respects like the preceding ones published in the Memoirs of this Society, and is followed by a list of about 200 double stars, for the most part found in the same sweeps with the others; but which, occurring in M. Struve's catalogue, cannot now be regarded as new double stars. Their observed places, and estimated angles of position, distances, and magnitudes, are, however given, in order to afford ground of comparison between the two catalogues, of which comparison the results are stated."

ART. XIII.-Description of Erinite, a New Mineral Species. By WILLIAM HAIDINGER, Esq. F. R. S. E., &c. Communicated by the Author.

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MINERALOGY is indebted to Count Bournon for the esta blishment as distinct species, of several of those minerals which contain arsenic acid and copper, some of which are found exEdinburgh, April 21, 1828.

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