subject by quoting from a well-known American writer of great experience his view of the case, to which I think the same criticism applies. Says the late Professor Young:—"There has been much discussion whether these bodies shine by light reflected or intrinsic. The fact that they become less brilliant as they recede from the Sun, and finally disappear while they are in full sight simply on account of faintness and not by becoming too small to be seen, shows that their light is in some way derived from the Sun. The further fact that the light shows traces of polarization also indicates the presence of reflected sunlight. But while the light of a Comet is thus in some way attributable to the Sun's action the spectroscope shows that it does not consist, to any considerable extent, of mere reflected sunlight, like that of the Moon on Planets."[1] The writer adds:—"If a comet shone with its own independent light, like a star or a nebula, then, so long as it continued to show a disc of sensible diameter, the intrinsic brightness of this disc would remain unchanged; it would only grow smaller as it receded from the earth, not fainter." This last remark does not seem sound.
It occasionally happens that a telescopic comet, especially when first discovered, exhibits a round and well-defined disc. History indeed records this as the attribute of several naked-eye comets discovered either before the invention of the telescope or when the telescopes in use were of a very juvenile character. Seneca, speaking of the second comet of 146 B.C., which appeared after the death of Demetrius, King of Syria, says that it was but little inferior to the Sun, being a circle of red fire sparkling with a light so bright as to surmount the obscurity of night. It is to be presumed that he meant that it was but little inferior to the Sun in size. The Comet of 1652, seen by Hevelius, was almost as large as the Moon although not nearly so bright. The Comets of 1665 and 1682 are said to have been as well-defined in their outlines as the Planet Jupiter. It is doubtful whether these statements can be received as literally true: at any rate I am not acquainted
- ↑ C. A. Young, General Astronomy, Ed. of 1898. p. 442.
