xvn COMET IS NOT PLANET 291 



when they come down toward us, and show larger 

 from a nearer point, smaller when they depart 

 from us, and dimmer when they retire to a greater 

 distance. 



XVIII 



THE reply is ready to this last statement, that the i 

 same thing does not happen in comets as in the 

 other stars. Some comets attain their maximum on 

 the very first day of their appearance. But, according 

 to the argument, they ought to increase the nearer 

 they approach. As it is, their first aspect remains 

 until they begin to fade. Besides, what has 

 been said in reply to former authorities applies here 

 too : If the comet had an erratic orbit, and were a 

 true planet, it would move within the limits of the 

 zodiac, within which all the planets confine their 

 orbits. Again, a star is never seen through another 2 

 star. Our sight cannot pierce through the centre 

 of a planet so as to view through it what lies be- 

 yond. But through a comet the further regions are 

 discerned as through a cloud. Whence it is evident 

 that it is no planet but an insubstantial, irregular 

 fire. 



XIX 



THE following is the opinion of our Stoic sage Zeno. 

 He is convinced that the stars act in concert, and 

 unite their rays with one another a partnership in 

 light which creates the image of a more elongated star. 

 Therefore some persons suppose that comets have 

 no real existence, and that it is only the appearance 

 of them that is reproduced through the reflection 



