24 LIGHT SCIENCE FOR LEISURE HOURS. 



covery is undoubtedly not an isolated fact, but must be 

 associated in the most intimate manner with a variety 

 of important cosmical relations. To speculate as to 

 the nature of the as yet undiscovered interpretation 

 of Angstrom's researches would at present be an idle 

 task, perhaps. But one feature of the solar scheme 

 with which we cannot doubt that it will be found to 

 be associated, must be mentioned before we conclude. 



Of all the phenomena presented to the contempla- 

 tion of astronomers, the tails of comets are undoubted- 

 ly the most perplexing. Their rapid formation, their 

 swift motions (if, indeed, we could believe that their 

 changes of position are due to a real transmission of 

 their material substance), and the enormous variety of 

 configuration and of structure which they present to 

 our contemplation, render them not merely amazing, 

 but altogether unintelligible. 



!Nbw, there is one feature of comets' tails which has 

 long since attracted attention, and will remind the 

 reader of the peculiarities common to the zodiacal and 

 the auroral light. "We refer to the sudden changes 

 of brilliancy, the flickerings or coruscations, and the 

 instantaneous lengthening and shortening of these 

 mysterious appendages. Olbers spoke of " explosions 

 and pulsations which in a few seconds went trembling 

 through the whole length of a comet's tail, with the 

 effect now of lengthening, now of abridging it by 

 several degrees." And the eminent mathematician 



