188 TERRESTRIAL PHENOMENA. 



the direction in which the evolution of light most frequently, 

 but by no means always, commences. The fact which gives to 

 the phenomenon its greatest importance is, that the Earth 

 becomes self-luminous ; that besides the light which, as a 

 planet, it receives from the central body, it shews a 

 capability of sustaining a luminous process proper to itself. 

 The intensity of the "terrestrial light," or rather of the 

 degree of illumination which it diffuses at the surface of 

 the Earth, when the rays are brightest, are coloured, and 

 ascend to the zenith, is a little greater than that given 

 by the Moon in her first quarter. Sometimes (as on the 

 7th of January, 1831) it has been possible to read print by 

 it without effort. This terrestrial luminous process, going 

 on almost uninterruptedly in the polar regions, leads us by 

 analogy to the remarkable phenomenon presented by Venus, 

 when the portion of that planet not illumined by the Sun 

 is seen to shine with a phosphorescent light of its own. It 

 is not improbable that the Moon, Jupiter, and the comets, 

 radiate a light generated by themselves, in addition to the 

 reflected light which they receive from the Sun and which 

 is recognised by means of the polariscope. Without speak- 

 ing of the enigmatical but not uncommon kind of lightning, 

 which, unaccompanied by thunder, is seen flickering 

 throughout the whole of a low cloud for minutes together, 

 we have yet other examples of the production of terrestrial 

 light. To these belong the celebrated mists, luminous at 

 night, seen in the years 1783 and 1831 ; the steady luminous 

 appearance in great clouds observed by Eozier and Beccaria; 

 and even, as Arago ingeniously remarks, the faint diffused 

 light which guides our steps in densely clouded moonless 

 and starless autumn or winter nights, and when no snow is 



