OBSERVATIONS OF A COMET. J 3 



but have no knowledge of floating particles, we ought cer* 

 tainly not to ascribe an effect to a hypothetical cause, when 

 the existence of one, quite sufficient to explain the pheno- 

 mena, is evident. 



If we admit, that the observed full illumination of the disk Other circurn- 

 of the comet cannot be accounted for from 'reflection, we the same. 

 may draw the same conclusion, with respect to the bright- 

 ness of the head, coma, and tail, from the following consi- 

 deration. The observation of the 2d of February mentions, 

 that not only the head and coma were still very bright, but 

 that also the faint remains of the tail were still visible; 

 but the distance of the comet from, the Earth, at the time of 

 observation, was nearly 240 millions of miles*, which 

 proves, I think, that no light reflected from floating particles 

 could possibly have reached the eye, without supposing the 

 number, extent, and density of these particles far greater 

 than what can be admitted. 



My last observation of the comet, on the 21st of Feb- 

 ruary, gives additional support to what has been said ; for 

 at the time of this observation, the comet was almost 2*9 

 times the mean distance of the sun from the Earthf . It 

 was also nearly 2*7 from, the sun J. What chance then could 

 rays going to the comet from the sun, at such a distance, 

 have to be seen after reflection, by an eye placed at more 

 than 275 millions of miles § from the comet? And yet the 

 instant the comet made its appearance in the telescope, it 

 struck the eye as a very conspicuous object. 



The immense tails also of some comets that have been ob- Tails of comets 

 served, and even that of the present one, the tail of which, not vapour, 

 on the 18th of October, was expanded over a space of more 

 than 9 millions of miles||, may be accounted for more satis- 

 factorily, by admitting them to consist of radiant matter, 

 guch as, for instance, the aurora borealis, than when we un- 

 necessarily ascribe their light to a reflection of the sun's il- 



* 839894939* 



•J* The suns mean distance being i, that of the comet was a 89797. 



% The comet's distance from the sun was z-6$3T96, 



§ 275077889, 



11 9i 6 °5i*- . 



lumination 



