The first great comet of the period we are now considering was the celebrated 1858 comet of Donati, the finest comet within living memory, though not considered quite so bright as that of 1811. Visible to the naked eye for sixteen weeks, and to the telescopes of the time for nine months, this comet was exhaustively observed. Its bright ness, which at maximum surpassed that of Arcturus, was a notable instance of the great increase in actual brightness near the sun, which far exceeded that predicted by theory from its relative distances from the earth and the sun. Its tail, or rather tails, were well placed for observation, and as one after another became visible, the main portion curved, with two straight tangents, the idea suggested by Olbers, that these tails were different kinds of matter unequally repelled by the sun, seemed almost to require no further proof. The head of the comet was also in violent agitation, throwing off veils of gauzy matter towards the sun to curve back like fountains after a comparatively short time, the general appearance being that of a hollow cone or cones seen brightest at two opposite edges. This comet was too early for the spectroscope; its brightness was analysed only by the polariscope, an instrument often dis appointing in its results. In this case the analysis showed no evidence that the brightness, although only about 3 per cent, of it could be accounted for by its position, was at all intrinsic, such as might be caused by a great increase of temperature in careering with increasing velocity through a medium even of great tenuity, or possibly by the friction of
its agitated parts, or, as was commonly suggested, by ignition in the heat of the sun’s rays. Simple reflected light was the unsatisfactory verdict of the polariseope, and there was then no higher court of appeal. So all that issued from the mass of observa tions was a very accurate determination of the orbit, giving a period of 2000 years.