Page:Proceedings of the Royal Society of London Vol 1.djvu/317

Rh tion between the stomach and spleen is carried on; but the evidence is too strong to leave a doubt on the mind of the author as to their existence.

Dr. Herschel presuming that the motion of the comet would be correctly ascertained at the Royal Observatory at Greenwich, conﬁned his observations to its physical condition, and relates the several circumstances which he remarked concerning its nucleus, its head, its coma, and its tail. The form of its nucleus was particularly attended to on the 4th of October, and remarked to be perfectly circular and equally bright on all sides.

Its magnitude was at the same time estimated at about 3¾°, due care being taken to determine that the visible disc was not spurious, by means described in a former paper. It was examined with a 10-feet reﬂector, and with powers of 200 and 300; but its light was not found sufﬁcient to bear higher powers.

On the 19th at 5" 40' it was again estimated at 2¾", but at 6h 20’ it was compared with J upiter's third satellite, and found to be rather less than it.

The coma or ncbulom appearance surrounding the head was at the same time estimated at about six minutes, and on the 6th of December at 4" 45'.

The tail on the 18th of October measured 3%“, but on the 6th of December it was reduced to 23’. The tail was at various times observed to appear longer and more distinct on the south preceding, than on the north following, side.

Various authors having said that the tails of comets were so rare as not to affect the light of the smallest stars seen through them, Dr. Herschel paid particular attention to that circumstance; but uniformly found thOSe stars which he saw emerge from behind the tail to become brighter than before in comparison to neighbouring stars.

On the 6th of December the head had the appearance of a large brilliant round nebula, suddenly much brighter in the centre. On the 1st and 14th of January it had still more the appearance of a nebula, with an increase of brightness in the middle; but even so late as the 21st of February, the nebulosity was observed to project a little on that side where the tail used to be seen.

The inferences which Dr. Herschel draws from these observations, are, ﬁrst, that the body of the comet consists of solid matter about 538 miles in diameter; secondly, that since the portion of its visible hemisphere which could be illuminated by the sun on the 4th of October did not exceed 120°, and since it did not appear gibbous but perfectly circular, the surface of the comet must itself be luminous,