130 CELESTIAL PH/ENOMENA. 



Telleriano-Remensis ( 93 ), in the Royal Library at Paris) , was 

 the Zodiacal light. 



TMs phenomenon, doubtless of primeval antiquity, but 

 first discovered in Europe by Childrey and Dominic 

 Cassini, is not the luminous atmosphere of the sun 

 itself, which, according to the laws of mechanics, cannot be 

 more oblate than in the ratio of 2 : 3, and could not there- 

 fore extend to a greater distance than nine-twentieths of the 

 distance of Mercury from the Sun. The same laws deter- 

 mine that the height of the extreme limit of the atmosphere 

 of a rotating cosmical body above its equator, or the point 

 at which gravity and the centrifugal force are in equili- 

 brium, can only be that at which a satellite would complete 

 its revolution in the same time that the central body rotates 

 around its own axis ( 94 ). This restricted limit of the 

 solar atmosphere, in its present concentrated condition, 

 is particularly striking when we compare the central 

 body of our system with the nucleus of other nebulous 

 stars. Herschel discovered several in which the semi- 

 diameter of the nebula surrounding the star subtends an 

 angle of 150". Assuming a parallax of not" quite one 

 second, we find the outermost nebulous stratum of such 

 a star to be 150 times farther from its center than the dis- 

 tance of the Earth from the Sun. If, therefore, such a 

 nebulous star was in the place of our Sun, its atmosphere 

 would not only include the orbit of Uranus, but would ex- 

 tend eight times as far ( 95 ). 



The solar atmosphere being thus limited in extent, we 

 may with great probability attribute the Zodiacal light to 

 the existence of an extremely oblate ring ( 96 ) of nebulous mat- 

 ter, revolving freely in space between the orbits of Venus 



