VELOCITY OF LIGHT. 241 
parent body. If this velocity increase, the refraction - 
will be less, and, reciprocally, a diminution of velocity 
that it would, on any obvious a@ priori grounds, enable us to predict 
the result of such an experiment one way or the other. There is 
indeed involved the difficult and complex consideration of the propa- 
gation of vibrations through ether, while the earth and transparent 
media upon it are moving through that zther; a problem which ex- 
ercised the ingenuity of Fresnel, and which, after a long investigation, 
he decided by concluding that the effects would be exactly the same 
as if the earth were at rest. This, however, may be still regarded as 
one of those points connected with what is the most difficult part of 
the wave theory, viz: the primary conception of zther and its prop- 
erties. 
But apart from this consideration, and looking only to the abstract 
problem of light (suppose emitted on the surface of the earth) falling 
on a refracting body with different velocities, there is nothing appar- 
ently in theory to determine whether the refraction will be affected, 
or in what way, by this difference. 
On the undulatory principle, it is true, velocity is intimately con- 
nected with refraction; retardation and refraction being coextensive 
and almost equivalent terms; but it must be borne in mind that it is 
not absolute but relative velocity which is thus connected with refrac- 
tion. It is the relative retardation in the denser medium, whatever 
the absolute velocity may be, which causes refraction. If in theory it 
were shown that the ratio would be constant for all velocities, it 
would give a constant refraction for the medium. But this is the 
very point in question; and there appears nothing antecedently to 
show, on any distinct theory of the nature of ther or of waves, that 
the relative velocities must necessarily be in a constant ratio. There 
is, however, nothing in any conception of waves at variance with the 
idea; and it must be admitted as in itself a rational and probable sup- 
position, fairly admissible in the first instance to ground any reason- 
ing upon. When therefore the fact was established by Arago’s 
experiment, while it completely subverted what was a necessary 
consequence of the emission theory, it offered no contradiction to the 
undulatory; but the proposition it established being one already prob- 
able, and consistent with that theory, was now to be recognized as an 
essential part of it. Yet the result of Arago’s experiment has been 
represented by some able writers as of a very startling and unex- 
pected nature, and, at first sight, equally perplexing on either 
hypothesis. 
SEC. SER. 11 
