, NOMOS. 153 



sun ; and this is what we may understand, though at 

 first glance we should have been disposed, perhaps, 

 to expect the contrary. It is what we may under- 

 stand, because a large quantity of the substance of 

 the comet is dissipated in invisible vapour by the 

 increasing heat of the sun as the distance dimi- 

 nishes, and because this vapour is again condensed 

 into visible substance as the comet moves away 

 into colder regions. 



Nor is it difficult to account for the ejection of 

 nebulous matter from the side facing the sun at the 

 time when the tail is in process of forma- 



r The solar 



tion ; for it is to be expected that the part heat may 



,-, .,, , , -. cause the 



nearest to the sun will be most heated ejection of 



j j 3 nebulous 



ana expanded. matter from 



It is more difficult to understand the 

 changes in the nucleus and the formation sun - 

 of the tail ; but this difficulty is not insuperable, and 

 the same explanation will apparently apply in both 

 cases. 



What is this luminous nucleus ? It can scarcely 

 be a solid substance in the ordinary sense of the 

 word, for the smallest stars were seen 

 through it without any perceptible dimi- heat may he 



nution of lustre. It can scarcely be 

 more solid than the most ethereal light. 

 What, then, is the luminous nucleus? 

 Is it the focus in which the rays of the sun luminous 



i 1.1 nucleus. 



are made to converge by the lens-like ac- 

 tion of the comet ? Is it the analogue of the " central 

 fire " which is hid from sight within the bowels of the 

 earth the " central fire " seen through a transparent 



