  This paper aims at demonstrating that: 1/ Assuming the equality of the
two-way transit time of light in vacuo, along the two perpendicular arms of
Michelson's interferometers (modern versions of Michelson's experiment), and
the anisotropy of the one-way speed of light in the Earth frame, two facts
supported today by strong experimental arguments, length contraction (in
Lorentz and FitzGerald's approach) should no longer be regarded as an ad hoc
hypothesis, it appears necessary and can be easily deduced. 2/ Builder and
Prokhovnik had the great merit of having shown that, as a result of to length
contraction, the two-way transit time of light along a rod is the same in all
directions in space (and not only in two privileged directions). We agree with
these authors up to this point, but, contrary to what is often believed, their
approach failed to reconcile aether theory with the invariance of the apparent
(measured) two-way speed of light. Yet, as we shall show, due to the systematic
measurement distortions entailed by length contraction and clock retardation
assumed by aether theory, the two-way speed of light, although anisotropic and
dependent on the absolute speed of the frame where it is measured, is always
found equal to C. The reasons of this paradoxical but important result will be
developed here. They confirm Lorentz-Fitzgerald's contraction and lend support
of the existence of a preferred aether frame.
