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.] 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. 1858, October 9, 7h. Distance of bright point to outer edge of the inner disk (4 measures) outer annulus (2 measures) = 38 Inclination of axis of the envelope to the meridian 64° to the East, the train diverging upwards. October 11, 7h. Distance of bright point to outer edge of the inner disk (4 measures) Observer, R.M. = 26 = 42 Observers, R.M. and W.C. October 15, 7h. outer annulus (2 measures) Inclination of axis of the envelope to the meridian 71° to the east. Distance of bright point to outer edge of the outer annulus (2 measures) = 34 Inclination of axis of the envelope to the meridian 90° to the east. October 16, 6h. Observer, R.M. 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 Northumberland Telescope at the Cambridge Observatory. By Professor Challis. 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 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. 66 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. 66 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. 66 Sept. 25. Nucleus large and bright; no dark space between it and the arch." A sketch exhibited in other respects the same appearances as on Sept. 24. 66 Sept. 27, 7h. 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. 66 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, 7h. 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. comet had now much faded." "The 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. B |