258  M. H. Fizeau on the Effect of the Motion of a Body 
that the change is great for highly refracting bodies, but small 
for feebly refracting ones such as air. 
I was thus led to anticipate a sensible displacement of the 
bands by means of the motion of water, since its index of refrac- 
tion greatly exceeds that of air. 
It is true that an experiment of Babinet’s, mentioned in the 
ninth volume of the Comptes Rendus, appeared to be in contra- 
diction to the hypothesis of a change in the velocity of light in 
accordance with the law of Fresnel. But on considering the 
conditions of that experiment, I detected the existence of a cause 
of compensation whose influence would render the effect due to 
motion insensible. This cause proceeds from the reflexion which 
the light suffers in the experiment in question. It may, in fact, 
be demonstrated that if a certain difference of path exists be- 
tween two rays, that difference becomes altered when these rays 
suffer reflexion from a moving mirror. Now on calculating 
separately the two effects (of reflexion) in the experiment of 
Babinet, their magnitudes will be found to be equal and oppo- 
site In sign. 
This explanation rendered the hypothesis of a change of velo- 
city still more probable, and induced me to undertake the expe- 
riment with water, as being the most suitable one for deciding 
the question with certainty. 
The success of this experiment must, I think, lead to the 
adoption of the hypothesis of Fresnel, or at least to that of the 
law discovered by him, which expresses the relation between the 
change of velocity and the motion of the body; for although the 
fact of this law being found to be true constitutes a strong argu- 
ment in favour of the hypothesis of which it is a mere conse- 
quence, yet to many the conception of Fresnel will doubtless still 
appear both extraordinary and, in some respects, improbable ; 
and before it can be accepted as the expression of the real state 
of things, additional proofs will be demanded from the physicist, 
as well as a thorough examination of the subject from the ma- 
thematician. 
Shortly before the publication of the above interesting memoir 
in the Annales de Chimie, M. Fizeau presented to the Academy 
a second memoir, containing the results of his experiments on 
the effect of the motion of a transparent solid body, such as glass, 
upon the yelocity with which it is traversed by light, The 
Comptes Rendus of November 14th, 1859, contains a brief ex- 
tract from this memoir ; and from it we gather the principal re- 
sults of his experiments, and the principles upon which the same 
were based. 
The method of experiment which was employed in the fore- 
