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 
Yun et dans l’autre des deux systémes,) the original velocity of a ray 
determines its refraction, there seems to be a certain degree of con- 
