II, 12 ASTRONOMER ROYAL: Obs. of Small Planets.-Appearance of Comet V. 1858, as seen at the Royal Obs. 12, 13 Remarks on the Appearance of Comet V. 1858, (Donati's Comet), accompanied by Drawings; as seen at the Royal Observatory, Greenwich, chiefly with the Telescope of the Sheepshanks' Equatoreal. 1858, Sept. 27, 7h. With a doubly refracting prism applied to the unarmed eye, one image of the tail nearly disappeared when the images were placed side by side, and the other when they were placed end to end. I found, upon comparing these effects with those produced on an image reflected from unsilvered glass, that the light is polarised in the plane of the comet's tail. On trying the light of the head in the same manner, as seen in the Sheepshanks' telescope, there were feeble signs of polarisation. [G. B. A.] Sept. 30, 7h. The comet was imperfectly seen. Length of Length of the tail equal to the distance from a Ursa Majoris to a point n equidistant from and Ursa Majoris. The tail curved; its convexity to the left.-[G. B. A.] n Oct. 2, 7h. The night favourable. The comet's head much brighter than Arcturus. The tail, when it left the head pointed at first not quite to a Draconis, but to a place between a Draconis and y Ursa Minoris, but nearer the former; the curvature then made the extremity of the tail point to the right (or to the lower side) of Ursa Majoris; its length being equal to the distance from a to ʼn Ursa Majoris. I could almost fancy that a more brilliant part of the tail proceeded straight from the head in the first-mentioned direction, and a fainter part was then attached to its side. The signs of polarisation were quite distinct, though much less strong than on a previous evening. On viewing the head with the Sheepshanks' telescope it had this appearance :· :- - 1st. The parabolic envelope and enclosed illumination. 2d. A brighter flat circular disk (with no special ring of brightness) laid upon the inclosed illumination, just touching the parabola at its vertex. 3d. A still brighter, flat, circular disk, concentric with the last, about one-fourth of its diameter; no bright ring. 4th. The nucleus, concentric with the last, about one-third of its diameter, well defined, and looking very hard. From the nucleus a dark shadow diverged, with an angle at first of 30° or more, diminishing afterwards to 10° or 15°, cutting off the light of the circular disks and everything except the nucleus itself. [G. B. A.] The nucleus has a very bright stellar point, and is surrounded by a brightness similar to a planetary disk of 5" or 6" in diameter; the circle of the disk is not complete, its lowest part being cut off, through an angle of about 60° or 70°, by a well-marked shadow commencing immediately below the stellar point, continuing downwards, as seen inverted, in the direction of the train, and inclined at an angle of several degrees to the axis of the envelope. Surrounding the planetary disk is an annulus of light, about four times its diameter, considerably fainter, but brighter than the envelope generally, and about two-thirds complete, the lower part being obscured by the same shadow which cuts off a portion of the inner disk; its circumference is exceedingly well defined and of uniform brightness, excepting when it is immediately in contact with the inner annulus, where, perhaps from the effect of contrast, it appears dark. [R. M.] Oct. 3, 7h. The appearance of the comet is generally similar to that of last evening. The inner disk, however, appears larger; and on its surface is a well-defined dark spot, about the same size as the nucleus, situated a little to its left, in the direction of the comet's diurnal motion.-[W. C.] The centre of curvature of the southern edge of the comet's tail was about 3° below 12 Canum Venaticorum (Cor Caroli). -[G. B. A.] Oct. 4, 7. The night unfavourable, and the comet was seen for a few minutes only. The outer annulus of light was narrower and less brilliant; the inner disk was evidently broader than on previous evenings; the spot on its surface was distinctly seen in the same position as before.-[W. C.] Oct. 5, 7h. The nucleus was much as before; but I am not certain whether the small surrounding circle was visible. I fancied that I saw it, much contracted. The large surrounding circle was very plain, and was distinctly separated from the parabolic envelope; the black shadow more obtuse; the tail about the same length. The general light of the comet, I think, was diminished. Its head now is not so bright as Arcturus; whereas, in the last two views I thought it much brighter than Arcturus.-[G. B. A.] The diameter of the inner disk immediately surrounding the nucleus, which has been increasing for the last three days, is now about double its apparent size on Oct. 2. The spot on its surface appears to be developing itself in the form of a dark 14, 15 Comet V. 1858: at the Royal Observatory; at the Cambridge Observatory. concentric ring, as it is considerably elongated, and a little to its right are slight traces of its continuation. The outer circle of light has entirely disappeared. This may, perhaps, be owing to the general faintness of the comet as compared with its brightness on the last few evenings. It has been continually diminishing both in lustre and width. The shape of the envelope appears rather sharper than a parabola. Arcturus was in the middle of the tail at 7h 15m M.T.; its distance from the nucleus being as nearly as possible equal to twice the width of the tail measured through the point bisecting that distance.-[W. C.] Oct. 9, 7h. The bright point exhibits more of a planetary character than when I last saw it. The inner disk is of a silvery whiteness, and is complete on the apparent upper side of the nucleus; the lower part being cut off by the dark shadow, which is much more diffused, but very dark immediately under the nucleus, spreading out into a parabolic form, leaving two branches of the envelope of considerable brightness on each side. The outer annulus is visible and well defined; but the lower part of it assumes more of a parabolic character, and seems to throw out streams, merging in the two branches of the envelope.-[R. M.] Oct. 11, 7. The nucleus is still of a planetary character, and seems to be cut off on the under side by something like a phase of darkness. The inner disk now extends considerably below the bright point, and has decidedly assumed the parabolic character, throwing two small streams of light to a small distance, where they abruptly terminate. There is a dark curve passing through the extremities of this bright parabola and the central bright point, and going off into a parabolic form into the envelope, leaving a lune of light on each side. The outer bright parabolic annulus is not essentially different from that seen on Oct. 9. It first appeared well defined in the head; but later in the evening its definition was not so good, owing, perhaps, to a thin haze, indicating that its light is not strong. There is apparently a darkish band between the two parabolas of light. A peculiar feature in the comet's appearance this evening is a decided ray that shoots upwards (apparently), or towards the head of the comet, making an angle of 45° with the axis of the envelope towards the apparent left, as seen inverted, and extending about the length of the semidiameter of the nucleus beyond it.-[R. M.] Oct. 15, 6h 30m. The outer parabolic disk much in the same state as on Oct. 11, but a considerable change has taken place with respect to the inner one. The nucleus is seen at the apparent right-hand extremity of the inner disk, which appears to be thrown off from it towards the left in a kind of jet. shadow beneath the nucleus is not very strong. The envelope is seen to extend beyond the outer disk, excepting immediately at the extremity of the parabola. [R. M.] The Oct. 16, 6h 15m. The nucleus is in the same position as last evening, but appears less circular than hitherto. The peak, or vertex of the parabolic shadow seems-as far as the excessive faintness of the comet admits of correct observation-to issue from the centre of the disk in which the nucleus is eccentrically situated. The outer parabola of light is ill defined.-[W. C.] The following measures of distance from the centre of the bright point to the extreme edge of the inner disk and outer annulus of the head of Donati's Comet, in the direction of the axis of the envelope, were made with a wire micrometer upon the Sheepshanks' equatoreal. October 11, 7h. 15, 16 = 26 Distance of bright point to outer edge of the inner disk (4 measures) October 15, 7h. = 34 Observer, R.M. October 16, 6h. Distance of bright point to outer edge of the inner disk (2 measures) Observer, W.C. The initials G.B.A., R.M., W.C., are those of the Astronomer Royal, Mr. Main, and Mr. Christy. In the telescopic examination of the head of the Comet the remarks refer to the appearance of the Comet as seen inverted in the telescope, though in the drawings the representations are as seen in the heavens. Mr. Carpenter's drawings (which were exhibited at the meeting) were made mainly from the descriptions given above, and are intended to embody all the phenomena therein detailed; while Mr. Christy's are intended to represent the different appearances of the Comet as seen by himself with the Sheepshanks' equatoreal telescope. Physical Phenomena of Comet V. 1858, as observed with the In the following account of the physical appearances of Donati's Comet observed by myself and my Senior Assistant, Mr. Breen, I have first given the phenomena as recorded in rough notes, or represented by sketches, at the times of observation, and have then added some explanatory and supplementary remarks. The observations were all made with a power of 166, the same that was used in taking positions of the comet by reference to stars, and most probably several details which could only be seen with high powers, have on this account escaped observation. At the same time the advantage was gained of judging with a greater degree of certainty of the variations of brightness and form in the larger features of the phenomenon. Mr. Breen's observations were as follows: 66 1858, Aug. 19. Comet observed through dense cloud; bright and easily seen in the finder, although the field was too cloudy for seeing any stars." The aperture of the object-glass of the finder is 24 inches, focal length 29 inches, and diameter of the field of view 115'. 66 Aug. 23. Bright, but no trace of a tail; the sky clear, but the moon nearly at full. 66 "Aug. 27. Condensed nucleus. Aug. 30. Tail to north. Sept. 7. Very conspicuous to the naked eye, being nearly of mag. 2. Long and broad tail pointed directly north. "Sept. 13. The nucleus beautifully sharp and stellar; the tail slightly curved and pointing nearly north. "Sept. 15. Tail 3° in length; a fan-shaped tail to the nucleus opposite to the tail of the comet." A sketch exhibited the nucleus round, with two projections turned from the tail and equally inclined to its axis. "Sept. 16. Bright and easily observed; power 166 as usual. The tail curved to the preceding direction and nearly north. "Sept. 17. The nucleus very distinct and well defined. The tail is now slightly curved towards the east." A sketch exhibited the curvature and an excess of brightness on the convex side. "Sept. 21. The nucleus like a star of the first mag. The tail about 5o in length, as well as I could judge. "Sept. 24. Comet very brilliant; nucleus well defined; the luminous sector well seen and brightest at the edges.' A sketch represented the nucleus as nearly round, surmounted by a luminous sector, which on the side towards the tail was bounded by two arcs tending to form a cusp at the nucleus, and on the other side by a semicircular arch, brighter than the space between it and the nucleus. The coma extended beyond this arch, and in the direction of the tail was considerably fainter near the axis than at the sides. "Sept. 25. Nucleus large and bright; no dark space be tween it and the arch." A sketch exhibited in other respects the same appearances as on Sept. 24. "Sept. 27, 7. The luminous sector very bright and beautifully seen; the nucleus is now extremely small and bright like a star, almost dissipated, except a brilliant point,- quite changed since Sept. 25. The tail is getting broader at the end; it could be traced for 71° in the finder." According to a sketch, the nucleus was round, and attached to it on the upper side (as seen in the telescope) was a luminous sector, depressed towards the tail on the right side. The coma extended beyond to a distance greater than the radius of the sector, and was terminated by a bright border between which and the sector was a darker space. "Sept. 30, 8h. The nucleus again large and planetary; no fantail, but a little rainbow over the nucleus as on Sept. 27. Dark band sharply defined down the tail; the outer boundary of the tail sharp on the right side, and ill-defined on the opposite side." By measurement with the position circle, the axis of the tail was inclined southward from the great circle through the pole by 13°. "Oct. 5, 7. A hole in the fantail adjoining the nucleus." A sketch represented a dark patch on the left side of the nucleus, and within it near the nucleus a bright spot; also a less conspicuous patch above the nucleus. "Oct. 6. The same appearance as on Oct. 5, but the dark part in the nucleus (?) was not so well or so largely seen, and the left side of the arch was not so well defined." Oct. 8 and 9. Sketches on these days represented a wingshaped envelope attached to the nucleus, the arch, as usual, farther off, with coma beyond, and the space dark under the nucleus. Oct. 11. A sketch indicated two projections from the upper side of the nucleus, turned from the tail, and equally inclined to its axis. The attached envelope and the arch were nearly as before. Oct. 15 and 16. According to the sketches, the near envelope was broader, and more depressed towards the tail on the left side than on the right side. The arch was marked at about the same distance from the nucleus as before. "The comet had now much faded." My own observations and sketches were commenced when I perceived that the comet was exhibiting phenomena of a very remarkable character, and unlike any which it had happened to me to witness in any previous instance. "Sept. 27, 8h. A drawing which I took at this time exhibited a small fan-shaped centre, the broader part turned from the tail, and the brightness increasing towards the narrow part which had a small, circular, and very bright termination [which is properly the nucleus]. This central part was surrounded, except for about 90° on the side towards the tail, with a kind of hood or envelope, less bright and much larger than the central part, but bright enough to be in definite contrast with the coma which extended from it to a breadth about equal to its radius. The hood was more depressed towards the tail on the right side (as seen in the telescope) than on the left side. The right side of the tail was also considerably the brighter; the intermediate part was comparatively dark. A star of mag. 8, seen through this part at the distance of a few minutes from the nucleus, exhibited no unusual phenomena. By measurement with a position-circle the axis of the tail was inclined southward from the great circle through the pole by 8°. Sept. 30, 7-8h. In the finder a narrow dark band well defined was observable along the axis of the tail." My drawing represented the central brightness as larger and more oval than on Sept. 27, and having a round blunt termination at the lower or brightest part. Above the upper part was a less illumined space, increasing in brightness towards a luminous semicircular arch pretty definitely bounded on the outside, beyond which the coma was faintly visible to some extent. This arch descended towards the tail a little more on the right side than on the left. The tail appeared to stream both from the arch and from the exterior and interior coma, and the right-hand stream was considerably the brighter. By a drawing of the appearance to the naked eye, the curvature of the tail was now very considerable, the convexity turned southward, the convex side much the brightest, and the extremity of the tail broad and diffused. Its estimated length was 20°. "Oct. 2, 74. About the very bright and small nucleus on the upper side was a bright fan or hood, and beyond this a larger hood, with a less illumined space between them. The coma was traceable beyond the second hood. Under the cusp of the nucleus the space was remarkably dark, and a dark and rather broad band divided the tail into two parts which, at a considerable distance from the head, overlapped each other." A drawing represented the right-hand branch of the tail considerably the brighter, but the form and appearance of the head was now symmetrical about its axis. "Oct. 5, 7h. There were two dark openings in the space included by the external arch, the larger on the left side of the nucleus, and the smaller above it. Also between the larger and the nucleus was a bright point, but not so bright as the nucleus." A sketch exhibited these openings as faint patches irregular in form and distribution of light, and the bright point as a small spot of light near the nucleus. The right-hand stream of the tail was the brighter; the nucleus and fan-shaped attachment appeared nearly as on Oct. 2. This evening I looked at the light of the tail with a Nichol's prism and a tourmaline, and found that not far from the nucleus there was some polarisation. To the naked eye the tail was much more sharply defined on the southern than on the northern side, and was widely spread out at the extremity. Its length was 33°, as measured on a celestial globe, by reference to Arcturus, which was very close to the nucleus, and to stars of Ursa Major. "Oct. 6, 7h. The cusp of the nucleus not so sharp to-night; the left-hand irregular patch seen, but the one above scarcely discernible. There was much cloud about." A sketch exhibited the bright spot on the right-hand border of the irregular patch, and very near the nucleus. "Oct. 8, 64. Mottled appearance about the envelope [i. e. the space between the exterior arch and the bright central fan]. The left-hand patch seen, but not the bright spot. Apparently a greater divergence of the coma; the right-hand stream still the brighter." The central fan, as shown in a drawing, was spread out farther than on Oct. 6. To the naked eye the tail appeared more diffused than on Oct. 5, especially at its extremity. The greatest apparent curvature was at the distance from the nucleus of one-fourth its length. The length was but little diminished. "Oct. 9, 7. The angular divergence of the streams of 19, 20 Comet V. 1858: Mr. LASSELL, and Rev. T. W. WEBB. coma greater, and the light more diffused; the comet altogether less bright. The bright part of the nucleus and the hood appeared smaller. Clouds about and the sky unfavourable. The axis of the cusp and nucleus not coincident with the axis of the tail." [This appearance was probably owing to the left side of the envelope being now extended farther than the right in the direction of the tail.] To the naked eye the tail was fainter and still more diffused than on Oct. 8, but nearly of the same length. "Oct. 11, 61. Greater angular separation of the streams of the tail; the hood not so definitely bounded, the central brightness rounder, probably owing to the state of the atmosphere, the comet being low." A sketch exhibited a determination of the hood towards the left side, but the two streams of the tail of equal intensity. To the naked eye the tail was much spread out laterally, especially at the extreme part, and the nucleus was less bright. The length of the tail was judged by reference to stars to be 30°. "Oct. 13, 64. Comet scarcely to be seen for mist. I endeavoured to estimate the radius of the exterior arch in the direction transverse to the axis of the tail. I think it was 6' of the micrometer, or 52"." [This estimation is very uncertain.] Oct. 15, 61. The length of the radius of the exterior arch in a direction inclined from its vertex by about 65° was found by measurement to be 4' of the micrometer, or 35". "The coma was distributed very unsymmetrically about the nucleus, being much more apparent on the lower, or northern side, than on the other. It was too faint on the latter side for measurement of the radius of the arch; there was a good deal of daylight. The boundary of the interior envelope was very definite; that of the exterior arch very indistinct and uncertain. The radius of the former on the apparent lower side was about one-third that of the other, or 12"." The nucleus and attached envelope presented in a sketch the form of an inverted comma. The interior boundaries of the two streams of the tail were confused, the coma spreading over the intermediate space. By measurement with a position circle the axis of the tail was inclined northward from the equatoreal direction by 4°. "Oct. 16, 6h. The comet had nearly the same appearance as on the preceding evening; the comma form of the brightest part was seen in strong daylight; the boundary of the arch was still visible on the apparent lower side. By estimation from the teeth of the comb, its radius transverse to the axis was 47, or 41′′. According to a drawing, the radius of the brighter part was three-sevenths of this measure, or about 18". axis of the tail was nearly in the equatoreal direction." The Professor Challis has appended to the foregoing observations a few explanatory notes, but as they will probably be published at full length in another form, we shall merely quote the following remark: "In the course of the observations my attention was especially directed to the following particulars, respecting which I can speak with confidence. The brightness contiguous to the nucleus preponderated on the right side (as seen in the telescope) till October 2; and on October 9 the excess had passed to the left side. The excess of brightness of the right-hand stream of the tail above that of the other, attained its maximum about October 2; after which there was a gradual diminution, till, on October 11, 15, and 16, the two streams were not sensibly unequal. The dark band separating the two portions of the tail was of uniform width and definite boundary on September 30 and October 2; and in proportion as the boun daries afterwards became indefinite, and the intervening space was gradually filled with luminosity, the angular divergence of the two streams also increased." 21, 22 The following additional details respecting the comet have been communicated: Observed at Bradstones, near Liverpool, by W. Lassell, Esq. The time of observation was 1858, September 12, from 8 to 9 P.M. The comet was first viewed with the 20-foot equatoreal, with a power of 155, having a field of 19'1 in diameter. Nucleus estimated to be 10" diameter, remarkably well defined. Its appearance reminded me of the disk of the planet Uranus in this telescope with a power of 400 when the atmosphere was unfavourable enough to make the edge of the disk soft. A slight bifurcation was suspected at about 10' from the nucleus, but was not fully ascertained. Breadth of the tail about 12'. Edge of the tail on the side towards which the comet was advancing obviously brighter than the opposite edge. The comet was also viewed with a refractor of 2.6 inches aperture and power 35, with a field of 76'. The tail was about twice the diameter of the field in length, or 2 degrees. The tail seemed narrower in proportion than in the 20-foot, but that might arise from the small portion of the tail visible at one time in the latter instrument. The substance of the tail appeared streaky in the direction of its length. It was almost exactly parallel from within a very short distance of the head and had a slight curvature, the convex side being that which, speaking relatively to the comet's motion, was the preceding side. The nucleus in this telescope seemed bright, but rather stellar than planetary. Owing to the general cloudiness of the sky here I did not see this comet at all until the evening of the 11th September, and then only for a short time between clouds, too short to allow of placing the large telescope upon it.* Observed at Tretire, Hereford, by the Rev. T. W. Webb. but a Having lately received an object-glass from Mr. Alvan Clark, of 5 inches' clear aperture, of which he speaks highly, but which I have as yet had no opportunity of testing, I was anxious to turn it upon the recent magnificent comet; disappointment relative to the tube obliged me to content myself with a very rough temporary mounting, so that my observations were made under altogether unfavourable circumstances as to convenience; their results are, however, I believe, worthy of confidence. 1. The Nucleus. This appeared invariably circular, several seconds in diameter, of uniform light throughout; its definition decreasing with the increase of the power employed. As far as I could judge in the use of an excellent little 24-inch objectglass by Bardon, I think its distinctness on Oct. 11 had increased, as compared with its aspect on Sept. 21. Its colour was a clear yellow, not as deep as that of Arcturus. 2. The Envelope. This was of considerable extent, much fainter than the nucleus, but brighter than the exterior haze and train. Instead of being a hemispherical cap, serving as a base to the tail, as in the comet of 1811, its outline was distinctly continued through an arc of much more than 180°, round to the edges of the central darkness of the train. Sept. 30, when I perceived it on the first trial of the 5-inch object-glass, the portion next the sun was least luminous, and seemed to consist chiefly of a narrow are of light, indicating a hollow structure; this bright ring became more evident at a greater distance from the sun, so that the portions adjoining the central darkness were most conspicuous, especially, perhaps, that on the preceding side, according to the comet's orbital motion. Oct. 4, the whole circumference of the envelope, or photosphere, as it *The details of his subsequent observations of the comet may be expected from Mr. Lassell.-Editor. |