upon the Velocity with which it is traversed by Light. 257 
was not accidentally compensated by a difference of density, 
which, though small, might be sufficient totally to mask such 
displacement. 
Notwithstanding these precautions, however, no displacement 
of the bands occurred in consequence of the motion of the air ; 
and according to an estimate I have made, a displacement equal 
to one-tenth of the breadth of a band would have been detected 
had it occurred. 
The calculations with respect to this experiment are as fol- 
lows. Under the hypothesis that the air, when moving, carries 
with it all the zther, we have 
A=2L- m2=0-0002418 millim., 
m* being equal to 1:000567 at the temperature 10° C. 
This experiment having been made in air, the maximum illu- 
mination was due to the yellow rays; and this maximum deter- 
mined the breadth of the bands. Hence the value of A corre- 
sponding to the ray D being taken, we have 
A 
= =0°4103. 
Now so great a displacement could certainly not have escaped 
observation, especially since it might have been doubled by re- 
versing the current. 
The following would be the results of the calculation accord- 
ing to the hypothesis of Fresnel :— 
A=2L = (m?—1) =0:0000001367, 
= =0:0002325. 
Now a displacement equal to ;,!,5th of the breadth of a band 
could not be observed; it might, m fact, be a hundred times 
greater and still escape observation. Thus the apparent immo- 
bility of the bands in the experiment made with moving air may 
be explained by the theory of Fresnel, according to which the 
displacement in question, although not absolutely zero, is so small 
as to escape observation. 
After having established this negative fact, and seeking, by 
means of the several hypotheses respecting wether, to explain it 
as well as the phenomenon of aberration and the experiment of 
Arago, it appeared to me to be necessary to admit, with Fresnel, 
that the motion of bodies changes the velocity with which light 
traverses them, but that this change of velocity varies according 
to the energy with which the traversed medium refracts light ; so 
