CHAP. IX.

ACCOUNT OF DR. HERSCHEL'S PAPER ON THE CHANGES THAT HAVE HAPPENED DURING THE LAST TWENTYFIVE YEARS, IN THE RELATIVE SITUATION OF DOUBLE STARS; WITH AN INVESTIGATION OF THE CAUSE TO WHICH THEY ARE OWING.

DR. Herschel devotes this paper principally to the consideration

of the second class of the systems into which he has divided the sidereal world. After cursorily remarking, with respect to the solar system, as a specimen of the first class, which, among the insulated stars, comprehends the Sun, that the affections of the newly discovered celestial bodies extend our knowledge of the construction of this insulated system, which is best known to us; he proceeds to support, by the evidence of observation, the opinion which he has before advanced, of the existence of binary sidereal combinations, revolving round the common centre of gravity; Dr. Herschel first considers the apparent effect of the motion of either of the three bodies concerned, the two stars, and the Sun with its attendant planets; and then states the arguments respecting the motions of a few only out of the fifty double stars, of which he has ascertained the revolutions. The first example is Castor, or alpha Geminorum: here Dr. Herschel stops to show how accurately the apparent diameter of a star, viewed with a constant magnifying power, may be assumed as a measure of small angular distances; he found that ten different mirrors, of seven feet focal length, exhibited no perceptible difference in this respect. In the case of Castor no change of the distance of the stars has been observed, but their angular situation appears to have varied somewhat more than 45° since it was observed by Dr. Bradley, in 1759; and they have been found by Dr. Herschel, in intermediate positions at intermediate times. Dr. Herschel allows that it is barely possible that a separate proper motion, in each of the stars and in the Sun, may have caused such a change in the relative situation, but that the probability is very decidedly in favour of the existence

of a revolution. Its period must be a little more than 342 years, and its plane nearly perpendicular to the direction of the Sun. The revolution of gamma Leonis is supposed to be in a plane considerably inclined to the line in which we view it, and to be performed in about 1200 years. Both these revolutions are retrograde; that of epsilon Bootis is direct, and is supposed to Occupy 1681 years, the orbit being in an oblique position with respect to the Sun. In zeta Herculis, Dr. Herschel observed, in 1802, the appearance of an occultation of the small star by the larger one: in 1782 he had seen them separate; the plane of the revolution must therefore pass nearly through the Sun; and this is all that can at present be determined respecting it. The stars of delta Serpentis appear to perform a retrograde revolution in about 375 years: their apparent distance is invariable, as well as that of the two stars which constitute gamma Virginis, the last double star which Dr. Herschel mentions in this paper, and to which he attributes a periodical revolution of about 708 years.

[Young's Nat. Philosoph. Vol. II.-Journals of the Royal Institution, Vol. II.]

CHAP. X.

DR. HERSCHEL'S OBSERVATIONS OF THE COMET OF THE YEAR 1811, WITH REMARKS ON THE CONSTRUCTION OF ITS DIFFRENET PARTS.

THE Comet which has lately visited the solar system has moved in an orbit very favourably situated for astronomical observations. I have availed myself of this circumstance, and have examined all the parts of it with a scrutinizing attention, by telescopes of every degree of requisite light, distinctness, and power.

The observations I have made have been so numerous, and so often repeated, that I shall only give a selection of such as were made under the most favourable circumstances, and which will serve to ascertain the most interesting particulars relating to the construction of the comet.

As my attention in these observations were every night directed to as many particulars as could be investigated, it will be most convenient to assort together those which belong to the same object; and in the following arrangement I shall begin with the principal part, which is

The planetary Body in the Head of the Comet.

By directing a telescope to that part of the head where with the naked eye I saw a luminous appearance not unlike a star, I found that this spot, which perhaps some astronomers may call a nucleus, was only the head of the comet; but that within its densest light there was an extremely small bright point, entirely distinct from the surrounding glare. I examined this point with my 20 feet, large 10 feet, common 10 feet, and also with a 7 feet telescope; and with every one of these instruments I ascertained the reality of its existence.

At the very first sight of it, I judged it to be much smaller than the little planetary disk in the head of the comet of the year 1807; but as we are well assured that if any solidity resembling that of the planets be contained in the comet, it must be looked for in this bright point; I have called it the planetary body; in order to distinguish it from what to the naked eye or in small telescopes appeared to be a nucleus, but which in fact was this little body with its surrounding light or head seen together as one object.

With a new 10 feet mirror of extraordinary distinctness, I examined the bright point every fine evening, and found that although its contour was certainly not otherwise than round, I could but very seldom perceive it definedly to be so.

As hitherto I had only used moderate magnifiers from 100 to 160, because they gave a considerable brightness to the point, it occurred to me that higher powers might be required to increase its apparent magnitude; accordingly the 19th of October, having prepared magnifiers of 169, 240, 300, 400, and 600, I viewed the bright point successively with these powers.

With 169 it appeared to be about the size of a globule which in the morning I had seen in the same telescope and with the same magnifier, and which by geometrical calculation subtended an angle of 1",39.

I suspected that this apparent size of the bright point was only such as will spuriously arise from every small star-like appearance;