COMETS. 219 



elsewhere shown, there is more evidence in favour of 

 the theory of expulsion than of the other two theories 

 just sketched. But we have reason to feel assured, as 

 we contemplate a comet like that which now adorns 

 our skies, that could we learn its history, a practical 

 infinity of time would be brought before us as the 

 aggregate of the time-intervals we should have to deal 

 with. Nor is the marvel of the comet diminished by 

 what we have learned from observation or from mathe- 

 matical analysis. We have found that the tracks of 

 comets are followed by countless millions of meteoric 

 bodies, and thus the strangest thoughts of infinity of 

 space occupied by infinite numbers of cosmical bodies, 

 aggregating towards multitudinous centres during in- 

 finity of time are suggested to us. The telescope 

 has shown us wonderful processes taking place during 

 the comet's approach to the sun, and most wonderful 

 process of all, the repulsion of the vaporous matter in 

 the tail, as though to assure us that the expelling 

 power of suns is even more than matched by the 

 repelling power they exert on portions of cometic 

 matter brought in certain conditions under their 

 influence. Analysis by the spectroscope, that wonderful 

 instrument which astronomy owes to Kirchhoff, has 

 taught us much respecting cometic structure, showing 

 that the light of the nucleus is that of a glowing solid 

 or liquid (or of matter reflecting sunlight), the light of 

 the coma that mainly of glowing vapour, while in the 

 tail these two forms of light are combined. And 

 polariscopic analysis speaks with equal clearness of the 



