246 M. H. Fizeau on the Effect of the Motion of a Body 
to satisfy the conditions of the aberration of light and of a cele- 
brated experiment of Arago’s, which proved that the motion of 
the earth does not affect the value of the refraction suffered by 
the light of a star on passing through a prism. Although these 
two phenomena may be explained with admirable precision by 
means of this hypothesis, still it is far from being considered at 
present as an established truth, and the relations between ether 
and matter are still considered, by most, as unknown. The 
mechanical conception of Fresnel has been regarded by some as 
too extraordinary to be admitted without direct proofs; others 
consider that the observed phenomena may also be satisfied by 
one of the other hypotheses ; and others, again, hold that certain 
consequences of the hypothesis in question are at variance with 
experiment. 
The following considerations led me to attempt an experiment 
the result of which promised, I thought, to throw light on the 
question. 
It will be observed that, according to the first hypothesis, the 
velocity with which light traverses a body must vary with the 
motion of that body. Ifthe motions of the body and the ray 
are like-directed, the velocity of light ought to be increased by 
the whole velocity of the body, 
If the ether be perfectly free, the velocity of light ought not 
to be altered by the motion of the body, 
Lastly, if the pony when moving only carries with it a portion 
of the ether, then. the velocity of hght ought to be increased by 
a fractional part of the velocity of the body and not by the whole 
velocity, as in the first case. This consequence is not as evident 
as the two preceding ones, though Fresnel has shown that it is 
supported by mechanical considerations of a very probable nature. 
The question then resolves itself to that of determining with 
accuracy the effect of the motion of a body upon the velocity 
with which light traverses it. 
It is true that the velocity with which light is propagated is 
so immensely superior to any we are able to impart to a body, 
that any change in the first velocity must in general be inappre- 
ciable, Nevertheless, by combining the most favourable cir- 
cumstances, it appeared to be possible to submit to a decisive 
test at least two media, air and water, to which, on account 
of the mobility of their particles, a great velocity may be im- 
arted. 
. We owe to Arago a method of observation, founded on the 
phenomena of interference, which is well suited to render evident 
the smallest variation in the index of refraction of a body, and 
hence also the least change in the velocity with which the body 
is traversed by light; for, as is well known, this velocity is in- 
