  A new expansion is established for the Green's function of the
electromagnetic field in a medium with arbitrary $\epsilon$ and $\mu$. The
obtained Born series are shown to consist of two types of interactions - the
usual terms (denoted $\cal P$) that appear in the Lifshitz theory combined with
a new kind of terms (which we denote by $\cal Q$) associated with the changes
in the permeability of the medium. Within this framework the case of uniform
velocity of light ($\epsilon\mu={\rm const}$) is studied. We obtain expressions
for the Casimir energy density and the first non-vanishing contribution is
manipulated to a simplified form. For (arbitrary) spherically symmetric $\mu$
we obtain a simple expression for the electromagnetic energy density, and as an
example we obtain from it the Casimir energy of a dielectric-diamagnetic ball.
It seems that the technique presented can be applied to a variety of problems
directly, without expanding the eigenmodes of the problem and using boundary
condition considerations.
