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II. this would furnish an unquestionable proof that the comet exhibiting such phases shone by reflected sunlight. It has been asserted from time to time that such phases have been seen, but the evidence is very far from satisfactory. Delambre mentions that the Records of the Paris Observatory afford undoubted evidence of the existence of phases in the Comet of 1682; but neither Halley nor any other astronomer who observed that comet has left any intimation of phase phenomena having been noticed by them. James Cassini mentions the existence of phases in the celebrated Comet of 1744; on the other hand, Heinsius and De Chéseaux, who paid particular attention to that comet, positively deny having seen anything of the kind. More recently Cacciatore of Palermo expressed a decided conviction that he had seen a crescent in the Comet of 1819. There were 4 comets in that year and apparently the second is the one referred to. Arago sums up by saying that Cacciatore's observations only prove that the nuclei of comets are sometimes very "irregular", by which word I suppose he means that they conform to no regular laws. Sir W. Herschel states that he could see no signs of any phases in the Comet of 1807 although he fully ascertained that a portion of its disc was not illuminated by the Sun at the time of his observation. Pons's Comet of 1812 was found at its return in 1883-4 to be brighter than the theory of its orbit led one to expect; indeed, it underwent during its visible career various ups and downs of brilliancy instead of varying gradually as its distance from the Earth varied. Niesten suggested that this fact was a proof of the comet being endued with some inherent light of its own. This surmise may be applied to Holmes's Comet of 1892, and Morehouse's Comet of 1908, both of which underwent remarkable fluctuations of brilliancy, in accordance apparently with no definite law. As to both these comets more will be said hereafter.

A critical reader might suggest that the foregoing paragraph conveys an uncertain sound, and the complaint would be well-founded. I should therefore like to take leave of the