2l6 LIGHT SCIENCE FOR LEISURE HOURS. 



captured by the disturbing attraction of a planet, and 

 compelled thenceforth to circuit on an oval path of 

 greater or less extent, yet according to all laws of 

 probability, how many times must it have flitted from 

 star to star before it was thus captured ! For the 

 chances are millions to one against so near an approach 

 to a planet as would ensure capture. But if, appalled 

 by the enormous time-intervals thus revealed to us, 

 we turn from that assumption, and find within the 

 solar system itself the origin of the periodic comets, 

 how strange are the theories to which we are led ! 

 Those comets which come very near to the sun may 

 have had a solar origin; and those which approach 

 very near the path of one of the giant planets may 

 have been propelled from out of such a planet when in 

 its sun-like youth. Even then, however, other comets 

 remain which are not thus to be accounted for, unless 

 we regard them as derived from planets outside 

 Neptune, hitherto undetected, and perhaps detectable 

 in no other way. And when we have taken such 

 theories of cometary origin, not, indeed, for acceptance, 

 but to be weighed amongst possibilities, how stu- 

 pendous are the conceptions to which we are thus 

 introduced ! Suns (for what is true of our sun may 

 be regarded as probable of others) vomiting forth 

 cometic matter so violently as to communicate velo- 

 cities capable of bearing such matter to the limits, or 

 beyond the limits of the solar system : planets now 

 passing through later stages of their existence, but pre- 

 sented to us, according to such theories, as once in a sun- 



