COMETS. 93 



nebulous stars. Their appearance is that of circular nebulae 

 shining with a faint light concentrated towards the centre. 

 Tliis is the simplest type ; but not, on that account, a rudi- 

 mentary type, for it might equally be the type of a cosmical 

 body grown old and exhausted by exhalation. We can dis- 

 tinguish in the larger comets, the " head," or " nucleus/' 

 and the "tail," either single or multiple, which the 

 Chinese astronomers more characteristically denominate 

 by the word " brush" (sui). The nucleus usually pre- 

 sents no definite outline, although in some rare instances 

 it appears like a star of the first or second magnitude ; 

 and in the larger comets of 1402, 1532, 1577, 1744, 

 and 1843, has even been clearly seen in bright sunshine ( 43 ). 

 This latter circumstance appears to indicate, in particular 

 individuals, a denser mass, capable of reflecting light with 

 greater intensity. Even in Herschers large telescope, only 

 two comets showed well-defined disks ( 44 ), viz. the comet dis- 

 covered in Sicily in 1807, and the fine comet of 1811, the 

 one having an angle of 1" and the other of 0"'77, whence he 

 inferred their true diameters to be respectively 534 and 428 

 miles. The measurement of the less well-defined nuclei of 

 the comets of 1798 and 1805, gave diameters of only 24 to 

 28 miles. In several comets which were examined with 

 great care, and particularly in the above-named comet of 

 1811 which was so long in sight, the nucleus and its nebu- 

 lous envelope were entirely separated from the tail by a 

 darker space. The intensity of light in the nucleus of a 

 comet does not increase in a uniform manner towards the 

 centre, but bright zones alternate with concentric nebulous 

 envelopes, which are less luminous. The tail sometimes 

 appears single, more rarely double, and in two instances (in the 



