244 FRESNEL. 
ble extent, if not the absolute, yet the relative velocity 
of aray; it is to make observations on it when, in the 
annual course of the earth, its motion is directed towards 
the star from which the ray proceeds, and again when it 
is diametrically in the opposite direction. In the former 
case, it is as if the velocity of the ray was increased by 
the whole of that of our globe; in the second, the nu- 
merical difference has the same amount, but the velocity 
is diminished. Now no one is ignorant that the velocity 
of the earth’s revolution is quite comparable with that of 
light ; being in fact about a ten-thousandth part of it. 
And again; to observe a star, towards which the earth 
is moving, and then one from which it is receding, is to 
operate upon rays whose velocities differ from each other 
by one five-thousandth part. Such rays ought to be 
unequally refracted; the theory of emission furnishes 
the means of expressing in numbers the amount of 
the inequality; and we may easily see that it will far 
exceed the small errors of observation. Now precise 
measures have completely negatived such calculation ; 
the rays proceeding from all stars, in whatever region 
they are situated, undergo precisely the same refraction. 
The disagreement between this theory and experience, 
could not be more manifest, and from that moment the 
system of emission seemed to be overturned from its very 
foundations. Nevertheless, this definitive sentence has 
been suspended by the aid of a supposition which I can 
explain in two words ;—it consists in admitting that 
incandescent bodies emit rays with all sorts of velocities, 
but that a special and determined velocity is necessary 
to make them rays of light. If a ten-thousandth part of 
increase or diminution in their velocity takes away from 
rays their luminous properties, the observed equality of 
