  Recent proposals for improved optical tests of Special Relativity have
renewed interest in the interpretation of such tests. In this paper we discuss
the interpretation of modern realizations of the Michelson-Morley experiment in
the context of a new model of electrodynamics featuring a vector-valued photon
mass. This model is gauge invariant, unlike massive-photon theories based on
the Proca equation, and it predicts anisotropy of both the speed of light and
the electric field of a point charge. The latter leads to an orientation
dependence of the length of solid bodies which must be accounted for when
interpreting the results of a Michelson-Morley experiment. Using a simple model
of ionic solids we show that, in principle, the effect of orientation dependent
length can conspire to cancel the effect of an anisotropic speed of light in a
Michelson-Morley experiment, thus, complicating the interpretation of the
results.
