VELOCITY OF LIGHT. 239 



fallen. Insurmountable objections have been found in 

 various phenomena of whose very existence that philos- 

 opher was necessarily ignorant. This great advance in 

 the science belongs to the physicists of our own day, and 

 is due in a great measure to the labours of Fresnel. 

 This consideration alone obliges me to point them out in 

 detail, even if the interest of the question did not oblige 

 me to do so. 



If light is a wave, the rays of different colours, similar 

 in that respect to the sounds employed in music, are 

 composed of vibrations unequally rapid ; and the red, 

 green, blue, and violet rays, are transmitted through the 

 ethereal spaces, as are all the notes of the gamut through 

 the air, with velocities exactly equal. 



If light be an emanation, the rays of different colours 

 are formed of molecules necessarily different, either as to 

 their nature, or their mass, and which besides are en- 

 dowed with different velocities. 



An attentive inspection of the borders of the shadows 

 produced by the satellites of Jupiter in their passage 

 across the luminous disk of the planet, and better still, 

 the observations on changeable stars, have proved that 

 all the rays of light move equally fast. Thus a charac- 

 teristic feature of the system of waves is found verified. 



In each of the two systems of light * the original 



is a series of felicities; and if not true, eminently deserves to be true." 

 And the increasing proof which it continues to receive by its readi- 

 ness in meeting nearly every new experimental case as it arises, aug- 

 ments in the same proportion our conviction that it will sooner or 

 later be equally successful in the solution of those few phenomena, 

 which still appear to stand out as exceptional instances to its appli- 

 cation. Translator. 



* When the author affirms that in each of the two theories, (dans 

 1'un et dans I'autre des deux systemes,) the original velocity of a ray 

 determines its refraction, there seems to be a certain degree of con- 



