Page:The story of the comets.djvu/190

144 the reverse. At 66° of elongation from the Sun (which was that of the comet on the occasion in question), the blue light of the sky is very considerably polarised. The constitution of the comet, therefore, is analogous to that of a cloud; the light reflected from which, as is well known, at that (or any other) angle of elongation from the Sun, exhibits no signs of polarity."

Williams's drawing of the Comet of 1861, reproduced in Plate XVIII, gave a much more extensive and complex character to the comet's tail than any of the other drawings published.

A very interesting point was raised by Hind, and developed, so to speak, by E. J. Lowe, the well-known meteorologist. Hind stated that he thought it not only possible, but even probable, that in the course of Sunday, June 30, the Earth passed through the tail of the comet at a distance of perhaps $2⁄3$rds of its length from the nucleus. The head of the comet was in the Ecliptic at 6 p.m. on June 28, at a distance from the Earth's orbit of about 13,000,000 miles on the inside, its heliocentric longitude (its longitude seen from the centre of the Sun) being 279°. The Earth at that moment was rather more than 2° behind that point, but would arrive there soon after 10 p.m. on June 30. The tail of a comet is seldom an exact prolongation of the radius vector, or imaginary line joining the nucleus with the Sun; towards its extremity a tail is almost invariably curved; or, in other words, the matter composing it lags behind what would be its position if it travelled with the same speed as the nucleus. Now judging from the amount of curvature on June 30, and the direction of the comet's motion, Hind thought that the Earth very probably encountered the tail in the early part of that day; or, at any rate, that it was certainly in a region which had been swept over by cometary matter a short time previously. He added that on the evening of June 30 there was a peculiar phosphorescence or illumination of the sky which he attributed at the time to an auroral glare. It was remarked by other persons as something unusual; and it seems scarcely open to doubt that the Earth's proximity to the comet had something to do with it. Lowe confirmed Hind's statement of the sky