ZODIACAL LIGHT. 133 



explain completely all that takes place at the uncertain and 

 much contested limit of our atmosphere. The wonderful 

 lightness of whole nights in the year 1831, in which, in the 

 latitudes of Italy and Northern Germany, small print could 

 be read at midnight, appears in manifest contradiction to all 

 that we learn ( 10 ) from the most recent and exact researches 

 on the theory of twilight, and on the height of the atmo- 

 sphere. The phenomena of light which astonish us in the 

 variability of the crepuscular limits, and in the changes in 

 the Zodiacal light, must be dependent on conditions which 

 have not yet been successfully investigated. 



Thus far we have been occupied in considering the 

 world of forms governed by our Sun, or the solar 

 system ; comprising planets, satellites, comets of shorter or 

 longer periods of revolution, meteoric asteroids moving either 

 sporadically, or in crowded streams in continuous rings, 

 and, finally, a luminous nebulous ring, revolving round the 

 sun near the orbit of the earth, and for which, from its 

 position, the name of Zodiacal light may be retained, In 

 the movements of all, the law of periodic return everywhere 

 prevails, however different may be the measure of velocity, 

 or the quantity of the aggregated particles : the asteroids 

 alone, as they enter our atmosphere from the regions of 

 space, have their planetary revolution checked, and are them- 

 selves united to the larger planet. In that immense system, 

 of which the limits are determined by the force of attraction 

 of the central body, comets return in their elliptic orbits 

 from distances equal to forty-four elongations of Uranus ; 

 nay, in those very comets in which the nucleus, from the 

 smallness of its mass, appears to us but as a cosmic cloud, 

 it yet retains by its attraction the most remote particles 



