Page:The story of the comets.djvu/188

142 but before I could bring a telescope to bear on it clouds intervened, and continued till morning twilight. On the 2nd (Tuesday), being now much better situated for observation, and the night being clear, its appearance at midnight was truly magnificent. The tail, considerably diminished in breadth, had shot out to an extravagant length, extending from the place of the head above of the Great Bear at least to  and  Herculis; that is to say, about 72°, and perhaps somewhat further. It exhibited no bifurcation or lateral offsets, and no curvature like that of the Comet of 1858, but appeared rather as a narrow prolongation of the Northern side of the broader portion near the comet than as a thinning off of the latter along a central axis, thus imparting an unsymmetrical aspect to the whole phenomenon.

"Viewed through a 7-feet Newtonian reflector of 6-inches aperture the nucleus was uncommonly vivid, and was concentrated in a dense pellet of not more than 4" or 5" in diameter (about 315 miles). It was round, and so very little woolly that it might almost have been taken for a small planet seen through a dense fog; still so far from sharp definition as to preclude any idea of its being a solid body. No sparkling or star-light point could, however, be discerned in its centre with the power used (96), nor any separation by a darker interval between the nucleus and the cometic envelope. The gradation of light, though rapid, was continuous. Neither on this occasion was there any unequivocal appearance of that sort of fan or sector of light which has been noticed on so many former ones.

"The appearance of the 3rd was nearly similar, but on the 4th the fan, though feebly, was yet certainly perceived; and on the 5th was very distinctly visible. It consisted, however, not in any vividly radiating jet of light from the nucleus of any well-defined form, but in a crescent-shaped cap formed by a very delicately graduated condensation of the light on the side towards the Sun, connected with the nucleus, and what may be termed the coma (or spherical haze immediately surrounding it), by an equally delicate gradation of light, very evidently superior in intensity to that on the opposite side. Having no micrometer attached, I could only estimate the distance of the brightest portion of this crescent from the nucleus at about 7' or 8', corresponding at the then distance of the comet to about 35,000 miles. On the 4th (Thursday) the tail (preserving all the characters already described on the 2nd) passed through α Draconis and Herculis, nearly over  and  Herculis, and was traceable, though with difficulty, almost up to  Ophiuchi, giving a total length of 80°. The northern edge of the tail, from Draconis onwards, was perfectly straight,—not in the least curved,—which, of course, must be understood with reference to a great circle of the heavens.

"Viewed, on the 5th, through a doubly refracting prism well achromatised, no certain indication of polarisation in the light of the nucleus and head of the comet could be perceived. The two images were distinctly separated, and revolved round each other with the rotation of the prism without at least any marked alternating difference of brightness. Calculating on Mr. Hind's data, the angle between the Sun and Earth and the comet must then have been 104°, giving an angle of incidence equal to 52°, and obliquity 38°, for a ray supposed to reach the eye after a single reflection from the cometic matter. This is not an angle unfavourable to polarisation, but