CHAPTER II.
PHYSICAL DESCRIPTION OF COMETS.
It was long a question whether comets are self-luminous, shining with some intrinsic light of their own, or whether, as in the case of the planets, they shine with light reflected from the Sun. Whilst it cannot be doubted that they do exhibit independent light of their own, yet it is now generally believed that to a certain extent some of the light which they yield is received by them from the Sun. It cannot, however, be said that astronomers are agreed upon the point; and further evidence from advocates on both sides of the controversy is much to be desired. The spectroscope negatives the idea that comet light is sunlight, whilst the polariscope seems to indicate the presence of reflected light. Like the instruments named, observers of high repute have taken opposite sides. Sir W. Herschel, from his observations of the Comets of 1807 and 1811 (i.), was in favour of the idea that comets were self-luminous,[1] but the observations of Airy and others on Donati's Comet in 1858 point to exactly the opposite conclusion as regards the tail of that comet.[2] If we know little about the heads of comets we know still less about their tails, for they are such strange ethereal structures. If the existence of phases in the case of a comet could be certainly known
