1 64 LIGHT SCIENCE FOR LEISURE HOURS. 



hypotheses which have been put forward to account for 

 these peculiarities must now for a brief space claim our 

 attention. Although we are far from being in a posi- 

 tion to theorise with any confidence respecting the 

 nature of comets, and still less as to the purposes which 

 they subserve in the economy of nature, yet the observa- 

 tions made upon the second comet of the year 1868 have 

 resulted in a positive discovery which may serve as a 

 stand-point, so to speak, whence we can examine some- 

 what more confidently than of old, the various theories 

 which have suggested themselves to those who have 

 studied cometic phenomena. 



In considering these hypotheses we have to distinguish 

 between the views which have been entertained respect- 

 ing the nucleus and coma, and those which regard the 

 less intelligible phenomena presented by the tail. This 

 remark may seem trite and obvious, but in reality 

 the two classes of hypotheses are found singularly con- 

 founded together in many works on popular astronomy. 

 Let it be understood then, that when, in speaking of 

 an hypothesis respecting comets no special mention is 

 made of the tail, it is to be assumed that the hypothe- 

 sis applies solely to the head of the comet. The same 

 holds, by the way, with reference to the phenomena 

 presented by comets. For instance, when we said in 

 the paper on Comet I. that comets grow smaller as 

 they approach the sun, the remark was to be under- 

 stood to apply to the volume of the head, not to the 

 whole space occupied by the head and tail. In fact, it 

 would have been impossible to assert anything with 



