Mean Solar Time of Observation. h m 8 Right Ascensions and North Polar Distances of Donati's Comet, deduced from Observations made with the Altazimuth. 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 the tail equal to the distance from a Ursa Majoris to a point equidistant from and Ursa Majoris. The tail curved; its convexity to the left.-[G. B. A.] 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 n 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:· Ist. The parabolic envelope and enclosed illumination. 2d. A brighter flat circular disk (with no special ring of brightness) laid upon the inclosed illu mination, 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 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, 7h. 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. The 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.] |