Page:The story of the comets.djvu/193

X. "During the following days the tail has been constantly diminishing, but it is remarkable that it has always passed near to α Herculis, and that it reached to the Milky Way up to July 6. It would seem that the two tails were nearly independent, and that on July 5 the length and straightness had gone off from the large one, and that this bent itself to the southern side. Last night (July 7) the long train was hardly perceptible. The light was polarised in the plane of the tail."

Observations on the polarisation of the light of the comet were also made by Poey at Passy, near Paris. He found that "the plane of polarisation seemed to pass sensibly perpendicular to the axis of the tail". Poey had in 1858 observed Donati's Comet for polarisation, and found that its light was polarised in a plane passing through the Sun, the comet, and the observer.

The Comet of 1862 (iii.), though not one of first-class magnitude or brilliancy, was nevertheless a very interesting object on account of the fact that a jet of light, frequently altering in form, was observed for a long time to emanate from its nucleus. Jets of light shooting forth under such circumstances are not uncommon, as we have already seen, but in the case of this particular comet there seemed an unceasing supply thrown out from the nucleus without any material deterioration of the luminosity of the source of supply. It need hardly be added that the late Professor Challis, to whom we owe the annexed drawings, was a skilful and experienced observer. This comet had a tail which on Aug. 27 was 20° long.

The Comet of 1874 (iii.), discovered by Coggia at Marseilles, was one of considerable interest. The drawing from which Plate XX. has been engraved (and of which Fig. 69 is a skeleton outline) was made with an achromatic telescope of 8½ inches aperture and 11½ ft. focal length, on July 13, the most favourable night of all for a careful scrutiny of the comet, when its position in the heavens, its proximity to the earth, and the absence of twilight, 3 favourable circumstances, are jointly taken into consideration. The southerly motion of the comet was so rapid that on July 14 the presence of twilight greatly interfered with the visibility of the details shown in the drawing. The following description is from the pen of F. Brodie:—