  Motivated by the critical remarks of several authors, we have re-analyzed the
classical ether-drift experiments with the conclusion that the small observed
deviations should not be neglected. In fact, within the framework of Lorentzian
Relativity, they might indicate the existence of a preferred frame relatively
to which the Earth is moving with a velocity v_earth\sim 200$ km/s (value
projected in the plane of the interferometer). We have checked this idea by
comparing with the modern ether-drift experiments, those where the observation
of the fringe shifts is replaced by the difference \Delta \nu in the relative
frequencies of two cavity-stabilized lasers, upon local rotations of the
apparatus or under the Earth's rotation. It turns out that, even in this case,
the most recent data are consistent with the same value of the Earth's
velocity, once the vacuum within the cavities is considered a physical medium
whose refractive index is fixed by General Relativity. We thus propose a sharp
experimental test that can definitely resolve the issue. If the small
deviations observed in the classical ether-drift experiments were not mere
instrumental artifacts, by replacing the high vacuum in the resonating cavities
with a dielectric gaseous medium (e.g. air), the typical measured \Delta\nu\sim
1 Hz should increase by orders of magnitude. This expectation is consistent
with the characteristic modulation of a few kHz observed in the original
experiment with He-Ne masers. However, if such enhancement would not be
confirmed by new and more precise data, the existence of a preferred frame can
be definitely ruled out.
