NUCLEUS. HEAD. TAIL. 



with high optical power, these appearances are changed, and the 

 object seems to be a mere mass of illuminated vapour from its 

 borders to its centre. 



The nebulous haze which always surrounds the nucleus is called 

 the COMA. 



These terms coiTA and COITET are taken from the Greek word 

 KO^ (kerne") hair, the nebulous matter composing the coma and tail 

 being supposed to resemble hair, and the object being therefore 

 called KO/^TTJS (kometes), a hairy star. 



A telescopic view of one of the globular comets without a tail is 

 given at the head of this chapter. This is the comet known as 

 Encke's comet, so called fnom the astronomer who calculated its 

 orbit. 



This may be taken as a general representation of the apparent 

 form of the comets without tails. The real form is evidently 

 globular or spheroidal. 



The comets with tails are infinitely various in form. In fig. 2 is 

 represented the comet known as Halley's Comet, as it appeared on 

 the 3rd October, 1835 ; and this may also be taken as a very general 

 representation of comets with tails. 



Fig. 2. 



The rapidly changing and capricious forms of these singular 

 bodies may be conceived from fig. 3, p. 70, which represents the 

 same comet as it appeared on the 9th October ; and the figure at the 

 head of Chapter II. as it appeared on the 5th November. 



Nothing which attends these extraordinary objects is more asto- 

 nishing than their prodigious dimensions. The head of the great 

 comet which appeared in 1811 was a globular mass, whose diameter 



69 



