136 ON COMETS. 



the stars shining through it. Arcturus was noway dimmed 

 when it shone through the very middle of the brightest 

 part of the tail of that comet. But I have already stated 

 that that part measured 90,000 miles, and as this part of 

 the tail was no doubt round, as thick as broad, the star's 

 light must have shone through 90,000 miles of this mist. 

 Now, every one must have noticed that the steam puff of 

 a railway carriage completely obscures the sun, much 

 more a star. You cannot see the sun through it. Well, 

 then : there must have been less substance in the line of 

 90,000 miles of tail between the eye and star than in the 

 line of a few yards of steam smoke penetrated by the 

 eye in the other case. 



(50.) If you look at a filmy cloud at sunset, though not 

 thick enough to hide a star, you see it bright with vivid 

 golden light by reflection from the sun. How much 

 more then if it were much nearer to the sun, and much 

 more strongly illuminated. Such a cloud is penetrated 

 with light through its whole thickness and reflects it 

 equally from its interior and exterior. Just so in the 

 almost infinitely more thin texture of a comet even in 

 the densest part of the head it cannot be compared to 

 the lightest cloud so far as substance goes. In Biela's 

 comet very minute stars have been seen by myself 

 through a part of the head at least 50,000 miles in thick- 

 ness, which a fog a few yards thick would have extin- 

 guished. A solid body of a round shape would exhibit 

 phases like the moon, and would appear sometimes as a 

 half moon, sometimes as a crescent, and sometimes as a 

 full moon but the heads of comets show no such appear- 



