THE TWO COMETS OF THE YEAR 1868. 153 



We might doubt whether these observations were 

 entitled to credit were it not that, quite recently, 

 Biela's comet has been seen to separate into two dis- 

 tinct comets, each having a nucleus, coma, and tail, 

 and each of which pursued its course independently 

 until distance concealed both from view. 



It is clear that nothing but a long series of careful 

 observations can put us in a position to theorise with 

 confidence, respecting the nature of comets, the pro- 

 cesses of change which they undergo, and the functions 

 which they subserve in the economy of the solar system. 

 We may therefore dwell with particular satisfaction on 

 the fact that every comet which has appeared during the 

 last two years has been subjected to careful observa- 

 tion, and that at length, by means of spectroscopic 

 analysis we are beginning to get hold of positive facts 

 respecting comets, and have promise of shortly being 

 able to form consistent theories with regard to these 

 singular members of the solar system. 



I have had occasion in other works to speak of 

 the principles on which spectroscopic analysis depends ; 

 but I think it best briefly to restate the most im- 

 portant points. When the light from a luminous 

 object is received upon a prism, there is formed what 

 is called the prismatic spectrum. According to the 

 nature of the source of light this spectrum varies in 

 appearance. If the source of light is an incandescent 

 body the spectrum is a continuous, rainbow-tinted 

 streak. Where the 1 ight comes from an incandescent mass 

 surrounded with cooler vapours, the streak of rainbow- 



