404 SPECIAL RESULTS IN THE URANOLOGICAL 



The molecular state of the head or nucleus, which pre- 

 sents so singular an outline, as well as of the tail of comets, 

 is the more difficult to comprehend, since it does not cause 

 any refraction of rays, and since, by Arago's important dis- 

 covery (Kosmos, Bd. i. S. Ill, 391 and 392, Anm. 1921; 

 English edit. p. 97, xviii. and xix., Notes, 49 51), the 

 light of comets has been shewn to consist partly of polarised, 

 and therefore of reflected solar light. While the smallest 

 stars are seen to shine with undiininished brightness through 

 the vaporous emanations which form the tail, and even 

 through almost the centre of the nucleus itself, or at least 

 exceedingly near to the centre (per cometem non aliter 

 quam per nubem ulteriora cernuntur ; Seneca, Nat. Qusest. 

 vii. 18), the analysis of the light of comets in Arago's ex- 

 periments which I witnessed shows, on the other hand, 

 that the vaporous envelopes are capable, notwithstanding 

 their tenuity, of reflecting or giving back light received from 

 a foreign source ( 664 ) ; that these cosmical bodies have " an 

 imperfect transparency ( 665 ) ; and that light does not pass 

 through them unimpeded." In a group of nebulous bodies 

 of such extreme tenuity, the particular instances of great 

 luminous intensity, as in the comet of 1843, or the star-like 

 brightness of a nucleus, excite the more astonishment, be- 

 cause the reflection of the Sun's light is assumed to be the 

 exclusive cause. But may we not suppose in comets, in 

 addition to this, a light-evolving process of their own ? 



The particles emanating or evaporating from brush-like 

 or fan-shaped comets' tails of many millions of miles in 

 length, disperse themselves in space, and may perhaps either 

 form of themselves the resisting or impeding fluid or me- 

 dium ( 666 ) which gradually contracts the path of Encke's 



