242 FRESNEL. 
will manifest itself by an increasing deviation. Refrac- 
tion thus becomes a sure means of comparing the veloci- 
ties of all sorts of rays. In following out this research 
with experimental means so precise that they would 
mark differences of one fifty-thousandth part of the whole 
amount in question, we have been able to ‘ascertain that 
the light of all the heavenly bodies,—of our fires, of 
tapers, and lamps, (with double currents of air,) and 
even more, the feeble rays emitted by glow-worms, all 
go through 77,000 leagues in a second, as well as the 
burning light of the sun. 
It is easy to conceive in what way this result is a 
mathematical consequence of the system of waves, if we 
only remark that all the notes of the musical scale are 
propagated with equal rapidity through the air, whether 
they originate from the voice of a singer, from the metal- 
lic string of a piano, the catgut of a violin, the glass sur- 
face of an harmonica, or the metallic sides of the great 
pipe of an organ. Now, there is no reason why the 
luminous notes (if I may be pardoned the expression) 
should proceed differently in wether. On the hypothesis 
of emission the explanation is not so simple. If light is 
composed of material elements, it would find itself sub- 
The undulatory view of refraction depends entirely on the assump- 
tion that the velocity must be diminished in the denser medium; but 
as the refraction is proved to be constant for all velocities, this dimin- 
ished velocity must be always in a constant ratio to the original veloc- 
ity. This is the condition to which our conception of ether must 
conform. As to the fact of a retardation, that has been directly 
proved by another beautiful experiment devised by Arago, but car- 
ried out by the experimental skill of Foucault, on the principle of 
Wheatstone’s revolving mirror, which, if it received one of two rays 
at the smallest interval of time after another, would reflect it in a dif- 
Serent direction. Thus the existence of any retardation in one of the 
rays would be manifested. 
