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Physical Observations of Jupiter. By Sir W. Keith Murray.

"I annex a sketch of Jupiter, showing the position and appearance of remarkable streaks visible below the southern principal belt since the 27th October last. They were first observed by me at 16h 30m G.M.T., on Wednesday 27th October; I have seen them several times since, but not well, till last night, at 9h 48m G.M.T., when they appeared to occupy the same relative position and proportions as when first observed. I have observed another detached spot, and also indications of several white spots, all on the southern hemisphere, but the atmosphere has generally been too unfavourable for delicate observation. The instrument used is an 8-feet Munich telescope.

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Physical Observations of Jupiter. By W. Lassell, Esq. "As a favourable state of atmosphere for the use of the 20-foot equatoreal is in this locality almost as rare as the visit of a comet, I cannot deny myself the pleasure of sending you a

*It is interesting to find that the two streaks here referred to by the author exhibit on the accompanying diagram precisely the same relative magnitude, and the same position on the disk (subject to a slight difference in longitude, caused by the rotation of the planet), as similar streaks observed by Mr. Lassell (see next paper).- ED.

drawing of the planet Jupiter, as seen last night with this instrument- —or rather early this morning.

"I had previously been in the smaller dome, showing the planet with the 9-foot telescope to some visitors, and after their departure to some members of my family, the atmosphere being unusually fine; and, moreover, on turning to Andromedæ, and finding that the picture was very fine, the third star being well and cleanly separated from the second, with a power of 752, so as to show the sky fairly between them; I opened the larger dome and began a series of measures of the equatoreal diameter of Jupiter, with Airy's double-image micrometer.

"When they were nearly completed I was struck with the unprecedented sharpness of the images; and even while they were separated and in process of passing over each other, some details caught my attention which I had not observed with the 9-foot, and induced me forthwith to prepare to observe the planet more attentively and with higher powers. The enclosed drawing I have made as carefully as in my power, from sketches made during observation in my journal, and it represents generally the aspect of Jupiter at about 13h 45m G.M.T. I say generally, because to give every minute spot and shading in its true form, position, and intensity, is far beyond my powers. There is, however, I believe no point of detail here marked down, which had not its counterpart in the original, to which it bears at least a strong resemblance. One of the most novel features I noticed was the existence of a numerous group of white spots in the bright equatoreal region, far more delicate and difficult to see than those in the southern hemisphere, which I first noticed on the 27th March, 1850, and of which there is a sketch in the Monthly Notices, Vol. X., page 134. For several years I failed to see any such spots upon the face of Jupiter at all, but last year they appeared again in the same quarter of the planet, and were attentively and ably observed by Mr. Dawes, who has furnished the Society with a series of elaborate and very beautiful drawings of them. As they are always very delicate phenomena, it may be possible that the southern declination of Jupiter, during a great part of the interval, may account for their not having been observed.

"But the appearance of similar spots in the very brilliant central portion of the planet is to me new. They could only be seen in the most tranquil moments of the atmosphere, while the stantly and steadily visible. formerly noticed spots in the southern region remained con

"The two remarkable oblong spots have existed some time on the planet, but I have never seen them so distinctly before: in point of intensity they are not at all exaggerated in the drawing. The dark belts of Jupiter this year are far narrower, more numerous and far lighter in tint than usual-this accounts perhaps for the great contrast, and suggests the idea that these spots (which have also others in their train) may be an incipient dark belt, such as has formerly occupied this region of the disk.

"This observation has been very interesting to me in another point of view, as establishing the correctness of the opinion I have held for some time, that the last surfaces given to the mirrors of the 20-foot telescope are as perfect both in point of curve and regularity as those of the 9-foot telescope, and only require an atmosphere proportionately fine to establish their proportionate efficiency. No impression was received of the existence of the equatoreal spots with the 9-foot, though defining admirably with 270, while with 565 on the 20-foot they were seen nearly as plainly at the moments of best vision, as the southern spots were seen with the former instrument. The chief difference in the quality of vision was that in the smaller equatoreal, it was almost continuously good, while in the larger equatoreal the fits of finest vision had to be waited for. "Bradstones, Sandfield Park, near Liverpool,

Nov. 19, 1858."

53, 54

ASTRONOMER ROYAL: Note on Variable Star 83.-Mr. HARTNUP: Observations of Comet V. 1858. 54, 55

"Referring to my former communication of the 19th Nov., I send another drawing of the planet Jupiter as seen on the 5th Dec., at 12h 45m with a power of 430 on the 20-foot equatoreal. On comparing this diagram with the former one, it will be seen that the two conspicuous dark spots, or lines, are nearly similarly situated in both, but the largest and plainest bright spot is now evidently in a different position with respect to the dark ones, as if it had gained somewhat in rotation upon them. I observed both the bright and dark spots on the 21st Nov., but they then appeared to me to have the same relative positions as on the 18th. The dark spots have not sensibly changed their relative position either to each other or to any specific locality on the face of the planet since I first noticed them 7 or 8 weeks ago; i.e., if their regular return to conjunction in accordance with the known period of the planet's rotation may be taken as a test. The night of the

5th inst. was not nearly so favourable as that of the 18th ult., and not so many spots in the southern belt could be seen, nor more than indications of bright spots in the bright equatoreal band; yet the two bright spots here delineated were so exactly similar in appearance and in relative position, that I think there is ground for presuming that they are the same as the largest and brightest two seen on the 18th ult., which must lead to the inference of a rate of motion different from that of the dark spots. Further observations, if the sky permit, may possibly settle the question.

"The outline of this diagram is rather carefully drawn from a series of measures made with the double-image micrometer with a view to determine the exact form of the planet, whether truly elliptical or not. The measures were made on the 5th inst., and were as follows-the numbers being simply the readings of the micrometer-magnifying power 467:

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as pale white, and its appearance disk-like; on Nov. 28 the colour was again noted to be white; but on Nov. 29 and Dec. 12 it was slightly red. It was not observed again at Greenwich until the year 1854, and was then estimated to be of the 11th magnitude; since which time it has been repeatedly looked for without being observed. Till the present year this may have arisen from other small stars near it having been observed instead; but this year it has been particularly looked for with an exact knowledge of its place, particularly by Mr. Dunkin on Dec. 2, when the night was remarkably clear, who thus became assured of its total invisibility.

Observations of Comet V., 1858 (Donati's), taken with the
Equatoreal of the Liverpool Observatory. By J. Hartnup,
Esq.

1858.

Sept. 12

G.M.T.

Comet's R.A.

Log P

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Comet's N.P.D.

Log

h m 8

7 18 29.8

h m 8
11 10 24'04

+8.665

53 54 50'7

-9.8859

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A comparison of these measures of the polar and equatoreal diameters gives the equatoreal to the polar as 1: 09441; a former series with power 310 on the 21st Nov., gave the proportion as 1: 09422, another with power 467 on the same evening gave 1: 9458, mean of the whole 1: 09440, or the polar diameter less than the equatoreal by 17th part of the latter. The result shows a less ellipticity than I had expected, but the measures were carefully taken, and the circumstances pretty favourable.

I

17.857

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"Liverpool, 16th December, 1858."

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Note on the Variable Star numbered 83 in the Greenwich Catalogue of 1576 Stars for 1850, whose Variability was discovered by Mr. Hind. By the Astronomer Royal.

This star was discovered to be variable by Mr. Hind on the 4th of November, 1850, as announced in Vol. XI., No. 2, of the Monthly Notices (page 46). He there calls it a fiery-looking star of the seventh magnitude, and states that it was not noticed by Lalande or Bessel, and does not occur in the excellent map of that region recently published by the Berlin Academy. It was observed first at Greenwich on Nov. 8 of the same year, and noted as of the 6.7th magnitude; on Nov. 14, 28, and 29, it was estimated of the 8th magnitude; and on Dec. 5, 7, and 12, of the 8.9th magnitude. Its colour on Nov. 14 was noted

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Do not be alarmed at the sanguine hope expressed in the above proposition, but allow me to detail the observations on which it is founded.

"As long as two years ago (see the supplement to the Augsburger Allgemeinen Zeitung of the 24th March, 1856, first article) I constructed a telescope with silvered glass mirrors. The silvering was effected by Liebig's process, and the extremely thin coat of silver adhered so firmly to the glass that its metal-side could be polished with soft leather without injury. The mirror afforded a surprisingly good image, and I even at that time entertained the hope of arriving by this course at a durable construction of catoptrical instruments, inasmuch as the base of glass maintains the form of the mirror. Quite recently I have taken the matter up again, which in the meanwhile M. Léon Foucault, see the Comptes Rendus, vol. xliv., No. 7, Feb. 1857, p. 339, without being acquainted with my experiments, has laid before the Institute of France as something

novel.

"Very recently Liebig has materially improved his method of silvering, so that one can now obtain faultless mirrors of any size that are coated so thinly with silver that the sun when looked at through the silvering appears like a sharply defined blue disk of light. These mirrors, too, bear polishing on the metal-side quite readily. One has simply to rub the surface with a little cotton pad covered with the finest glove-leather. In a few minutes a high polish is obtained, quite without scratches. I have determined the reflecting powers of these mirrors by carefully measuring their brightness with my eyepiece photometer; and at the same time I observed also therewith other mirrors as well as prisms, and likewise object-glasses, by way of comparison.

"The following amount of brightness was obtained for an angle of reflexion of 45°, that due to direct light being considered 100:-

Decided dark spots were seen in the coma near the nucleus; one on the 8th and two on the 11th of October. The tail was curved towards the north, and broadest at or near the end. It was more luminous and better defined on the convex than on the concave side, and a dark band passed from the nucleus through the centre to the extreme end of the tail. On the 30th September a well-defined conical shadow was visible, the length of which, measured from the nucleus or the base of the cone to the apex, was 18'. The length of this shadow on the 4th October was 21', but the contrast between it and the dark band in the centre of the tail was much less striking than it was on the 30th September. On the 8th October it was rendered invisible by the increased darkness of the band which passed through the centre of the tail. The tail of the comet was more symmetrical and the envelope was brighter and better defined on the 30th September than on any other occasion.

On the Advantages to be derived from the Use of Silver Mirrors for Reflecting Telescopes, and on a Novel Method of Mounting such Instruments. By Dr. Steinheil. (Translation, by W. G. Letsom, Esq., of a Letter from Dr. C. A. Steinheil of Munich, to Professor C. A. F. Peters of Altona, published in No. 1138 of the Astronomische Nachrichten).

"What mainly induces me to address this communication to you is a wish to make you acquainted with the result of my latest experiments,-a result that appears likely to lead to a total modification in the construction of astronomical instruments.

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"It thence follows that silver mirrors reflect more light than rectangular glass prisms, and that after a second reflexion they still have more light than Fraunhofer's object-glasses, and, finally, that after three reflexions they yet have more light than the present mirrors of telescopes after a single reflexion. We are enabled, therefore, by the employment of silver mirrors to construct reflecting instruments that with equal aperture are superior to our achromatics in their amount of light.

"I have examined to what degree the accuracy of the form extends. For although the coat of silver is very thin there, nevertheless in giving the polish it might be more rubbed off at one part than another, and the form be thereby injured. My spherometer, which rests on mirrors of contact without axes, enables me to produce a glass surface of any radius of curvature upon which there is not anywhere a deviation from the sphere amounting to a hundred thousandth of a French line. I see indeed up to two millionths of a line, but I am only certain in my measurement up to about seven millionths. When an object-glass (one of about 6.5-inches aperture, English measure) is finished with this degree of precision, it unites the rays with sufficient accuracy. This cannot, however, be recognised in the image, but only on changing the position of the eye-piece

58, 59 Dr. STEINHEIL: On the Advantages to be derived from the Use of Silver Mirrors for Reflecting Telescopes. 59, 60

towards the focal point of the object-glass. If, namely, the image of the sun upon a highly-polished steel ball about halfan-inch in diameter (the inclosed space being maintained at a uniform temperature) is observed, at the distance of about 60 feet, with a refractor of 4-inches aperture, using a power of about 250, it will have the appearance of a star of the first magnitude when the air is quite steady, and look like a small defined disk of light encircled by fine coloured rings (Schwerd has explained the theory of this in his 'Beugungserscheinungen,' or 'Phenomena of Inflexion'). But on drawing out the eye-piece about half-an-inch, there is formed a disk of light consisting of concentric spectrum-rings. These are due to the interference of the rays proceeding from one point, and each ring corresponds to a determinate zone of the object-glass wherein the rays interfere by one length of wave. These rings, therefore, are the best test of the accuracy of the form, since they at the same time indicate whether the individual portions of a line of wave are situated symmetrically. I have tested my silver mirror by this means; it showed the interference-rings precisely as a good object-glass does. Upon this the mirror was again submitted to another polishing, and that without any further precaution. No alteration of any kind in the effect on the rings was however observable.

"Whether a plane mirror is worked sufficiently true is best seen at small angles of reflexion. If a star at an angle of reflexion of 5° still appears round and sharp, and if the interference-rings. remain rigorously circular the mirror acts satisfactorily. This test is even more sensitive than altering the position of the eyepiece with a direct and a reflected image. I possess silver mirrors that stand these tests perfectly well.

"Now, I infer from the above, firstly, that the forms of silver mirrors can be produced with sufficient accuracy to afford the same precision of the image that is attained with refractors, and I thereby refute practically the opinion that reflectors never give as distinct images as refractors do, and, secondly, that a subsequent polishing of the mirror, which may become necessary, should it tarnish with time, has no appreciable influence on the form of the mirror. When one considers that these mirrors may be protected from tarnishing when they are not in use by a cover of india-rubber, so that a fresh polish will not have to be given to them above once a-year-object-glasses, too, must be cleaned at least once during that time-and that if after the lapse of many years the silver surface of the mirror should have suffered, it is a light matter to have it silvered again, the objection hitherto brought against catoptrical instruments on the score of their want of durability appears also to be overcome.

66

The axis of the

"Hence we now possess reflectors which are in every respect free from the faults of the former instruments. What advantages in the mechanical construction of astronomical instruments may not be attained if one may presuppose two reflexions! I will here only touch shortly upon two arrangements, treating this matter in detail in a monography, that the Bavarian Academy of Sciences will publish in the ensuing autumn on the occasion of its centenary anniversary. Catoptrical meridian circle, or transit. concave mirror of 6-inches aperture and 36-inches focal length, lies east and west, the mirror is fixed on a stone pillar. It receives light from a plane mirror, permanently fixed at an angle of 45°, and which rotates round the prolongation of the optical axis of the concave mirror. This axis of rotation, as also the plane mirror, is traversed by an aperture, so that the image of a point in the meridian falls in the axis of rotation. The cross wires are placed here, and the eye-piece sees them distinctly, as well also as the image of the point in the meridian. An altitude circle fixed firmly on the perforated axis, and lying in the plane of the meridian, measures the nadir distances from a quicksilver horizon. The spherical aberration of the marginal

rays of this mirror amounts to o" 374, and this is strictly corrected by means of a small object-glass, having a long negative focus, which is inserted in the tube 3 inches in front of the cross wires. If both the mirrors are attached to the same stand, pretty much in the manner shown in the figure, we have a reflecting telescope mounted horizontally, since a rotation about A alters the azimuth, while a rotation about B alters the altitude.

"If the axis A be made to coincide with the axis of rotation of the heavens, the telescope is equatoreally mounted, for rotation about A measures the hour-angle, about B the declination. Of course the eye-piece tube is provided with a finder, for the sake of more readily meeting with the objects. This telescope possesses, moreover, another advantageous quality, the cost of an instrument of 6.4 inches aperture English measure

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is only 300 Bavarian florins (about 251. 108.), and in its action it must be superior to a refractor of equal aperture, because it is free from chromatic aberration. For this reason, especially, catoptrical heliometers will possess peculiar recommendations. I am convinced that by the employment of silver mirrors upon glass we shall arrive at obtaining reflecting telescopes of colossal dimensions. Only in that case it will be necessary to adopt a method of mounting them different to that now in use. appears to be indispensable to give the telescope, or rather the two mirrors respectively, for the tube may be dispensed with, a constant position.

It

"If the axis of the concave mirror is brought to coincide with that of the rotation of the heavens, and if the plane mirror, the centre of which is perforated, is so disposed that it transmits to the concave mirror situated parallel to its axis, the parallel rays proceeding from a star, then can the image, formed by the concave mirror be observed through the aperture in the plane mirror, and the elimination of the diurnal rotation of the earth is reduced simply to a rotation of the plane mirror about the axis of the heavens. For other declinations one has only to give the plane mirror another inclination. When thus arranged no special observatory is needed for the erection of a telescope of 2-feet aperture. The plane mirror might be adapted in front of a window of a dwelling-house, the concave mirror being situated below on the ground in the direction of the axis of rotation of the heavens. A tube between the two mirrors would be quite superfluous.

"By means of this application instruments of great optical power, and simple and convenient to manage, are rendered accessible to the admirers of astronomy, and that, too, at a moderate outlay, which is a point that should not be by any means overlooked.

Munich, April 5, 1858."

Catalogue of 317 Stars selected from the B.A. Catalogue (being such as were supposed to have large proper motions), deduced from Observations made at the Honourable E. I. Company's Observatory at Madras in the Years 1853-7. By Capt. W. S. Jacob, Director of the Madras Observatory. "The instruments employed on the observations from which this Catalogue is derived are a 5-foot transit and 4-foot mural circle, both by Dollond, which have been frequently described in the Madras volumes. Nearly the whole work of the Catalogue, both as regards observation and reduction, was executed by the native assistants in the observatory, and will be found creditable to them. In case of any doubt or discrepancy, I have occasionally repeated an observation, and have, of course, maintained a strict watch over the errors and adjustments of the instruments; the whole of the reductions have also been carefully checked by myself.

"During a portion of the time embraced by these observations, the charge of the observatory devolved on Major W. K. Worster, of the Madras Artillery, in consequence of my absence in England for the recovery of my health.

"The large proper motions which had been assigned to many of the southern stars, are for the most part contradicted, and seem to have their origin from instrumental or other errors in the Paramatta observations, they having been dependent chiefly on the Brisbane Catalogue."

Proper Motions of the Stars of the Greenwich Catalogue of 1576 Stars for 1850 not included in the Greenwich TwelveYear Catalogue, deduced by Comparison with the Results of Bradley's Observations as given in the "Fundamenta Astronomia." By the Rev. R. Main, M.A.

This paper is a continuation of a former paper by the author, published in vol. xix. of the Memoirs of the Society, containing a determination of the proper motions of all the stars which are common to the Fundamenta and the Greenwich Twelve-Year Catalogue of 2156 Stars. The last-mentioned Catalogue, and the one referred to in the title of this paper, contain the results of all the star-observations made at Greenwich from the year 1836 to 1853 inclusive, comprising a period of eighteen years. The whole number of stars of which the proper motions are assigned in the two papers amounts to 1437. It was explained in the introduction to the former memoir that all the stars observed at Greenwich have been selected for special purposes, and that the accurate determination of their places forms one of the most important objects of modern sidereal astronomy. In this respect the stars contained in the Catalogue of 1576 Stars are particularly valuable, several new classes having been introduced. Amongst these may be mentioned the results of the observations necessary to complete the list of all stars visible at Greenwich between the first and fourth magnitudes (with very rare and accidental exceptions); of a list of about 140 stars, to be used in addition to those given in the Nautical Almanac, for the determination of clock-error; of stars in the constellation Canis Major compared with Sirius for the purpose of more accurately determining the variability of the proper motion of the latter star; of about thirty stars near the north pole arranged in pairs differing by nearly 12 hours in right ascension, useful for determining azimuthal errors; and, finally, of such stars as have conspicuous proper motions.

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Physical Observations of Comet V., 1858. By
E. B. Powell, Esq.

Donati's Comet was first noticed at Madras on September 30th, when it was seen through breaks in the clouds shortly after sunset. On succeeding evenings it gained rapidly in brilliancy, becoming a truly magnificent object. My equatoreal not having arrived from England, I resolved to try how closely I could determine the comet's path by means of sextant observations, and accordingly took the following measures with a good 8-inch instrument by Troughton and Simms, graduated on platinum to 10"; and I am in hopes that the care I expended upon the work will have prevented the error on any distance from rising above 1'.

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