  It has been argued by R. T. Cahill and others that a Michelson interferometer
in "gas mode" - in which the light paths are through an included gaseous medium
- are able to detect and have detected an absolute frame of reference. It is
shown here that this argument supposes incorrectly that the reduced velocity of
light in gas is an observer-independent invariant. This velocity is not
invariant, but given in a frame with respect to which the interferometer moves
with velocity v by the usual relativistic formula for the addition of
velocities, these being in this case the velocity v and the reduced velocity of
light in the inertial frame of the interferometer. It is suggested nonetheless
that though the absolute frame urged by Cahill may be undetectable, there are
persuasive grounds for considering the alternative Lorentzian Relativity that
did suppose the existence of such a frame.
