96 CELESTIAL PHENOMENA. 



iliis rare case have caused a double continuous current of 

 nebulous matter ( 47 ). Aristotle, in his Natural Philosophy, 

 brings the phsenomena of comets through the medium of 

 these effusions into a singular connection with the existence 

 of the Milky Way. He supposes that the countless multi- 

 tude of stars which form the galaxy give out a luminous 

 or incandescent matter. The nebulous belt which traverses 

 the vault of the heavens is therefore regarded by the Stagi- 

 rite as an immense comet incessantly reproducing itself ( 48 ). 

 The passage over the fixed stars of the nucleus of a 

 comet, or of its innermost vaporous envelopes, might throw 

 light on the physical character of these wonderful bodies, but 

 we are deficient in observations of the kind in which we 

 can be assured that the passage was perfectly central ( 49 ) : 

 for in the immediate vicinity of the nucleus, as I have 

 already noticed, dense coatings alternate with others of great 

 tenuity. On the other hand, there is no doubt that the light 

 of a star of the tenth magnitude passed through very dense 

 nebulous matter on the 29th of September, 1835, at a 

 distance of 7"' 7 8 from the centre of the nucleus of Halley's 

 comet, according to Eessers most careful measurement, 

 without experiencing any deflection in its rectilinear course 

 at any moment of its passage ( 50 ). Such an absence of re- 

 fracting power, if actually extending to the centre of the 

 nucleus, makes it difficult to regard the substance of comets 

 as of a gaseous nature; or, is the absence of refracting 

 power, a consequence of the almost infinite rarity of a 

 fluid of that description ? or, does the comet consist of 

 " detached particles," forming a eosmical cloud, which no 

 more affects the ray of light passing through it, than do the 

 clouds of our atmosphere, which in like manner have no 



