Page:The story of the comets.djvu/36

14 The unsubstantial and flimsy nature of comets is shown by the numerous recorded instances of comets passing in front of stars without dimming their light, much less obliterating them. Sir J. Herschel once watched Biela's Comet pass in front of a cluster of stars without any obliterating effect being noticed; and observations of this kind have so often been recorded since that it is not worth while to cite instances in detail. There are, however, some observations to the contrary on record. A partial stoppage of light seems suggested by what Sir W. Herschel stated respecting the Comet of 1807. He says that stars seen through the tail lost some of their lustre, and that one near the head was only faintly visible by glimpses. Again, on Sept. 13, 1890, an 11th mag. star is said to have completely disappeared during the passage in front of it of Denning's Comet. It un- fortunately happens that we possess no clearly expressed record of the nucleus of a comet having been seen to occult a star, and therefore the extent of the solidity which is to be regarded as an attribute of cometary nuclei is at present indeterminate. According to Max Wolf the Comet of 1903 (iii.) seemed to absorb some of the light of stars which it passed over. These citations suggest that the comets in question were more dense than the general run of comets.

A question of great interest which is often raised is, "Do comets ever break up and disperse and disappear?" The question must certainly be answered in the affirmative, but the cases on record are not numerous, and except in a few instances the evidence is not very definite. Seneca mentions on the authority of Ephorus, a Greek author, that the Comet of 371 B.C. separated into two parts which pursued different paths. Seneca seems to distrust the statement which he repeats, but Kepler accepted it after what he himself had