THEOR Y OF LIGHT. 197 



The luminous envelope, the photosphere of the sun, simul- 

 taneously darts its rays in all directions, with an intensity 

 which diminishes with the square of the distance. 



Let us follow those which direct their course towards our 

 planet. 



So long as the rays do not come into collision, or strike 

 against resistant matter, they neither warm nor enlighten. 

 Confirming the Newtonian hypothesis of emission, they travel 

 straight as arrows through the icy space, the shadowy ocean 

 in which the waves of the terrestrial atmosphere are lost. 

 But the moment they encounter a material obstacle, the rays 

 partly penetrate it, and are partly thrown back, in such a 

 manner as to form a series of undulations like those which the 

 falling of a stone into a pond produces. As they recede from 

 these repeated shocks, the rays come under that theory of 

 undulation which Huygens promoted.* 



Let us trace the rays of light back to their origin. These 

 encounter first the globes nearest to the sun : the almost im- 

 perceptible asteroids, which, on account of the solar splendour, 

 can scarcely be detected, and which, as yet, have received no 

 baptismal names. Yonder beams illuminate other revolving 

 globes, Mercury, Venus, the Earth. If we here arrest our 

 gaze, we are influenced by a vague and mournful recollection ; 

 the Earth was our place of sojourn at an epoch when thought, 

 formerly shackled, had become free. It is thus that a seed, re 



* No one, I think, has ever before attempted to reconcile, in this way, 

 the two principal theories which have been put forth on the propagation 

 of light. 



