  Let $V(t) = e^{tG_b},\: t \geq 0,$ be the semigroup generated by Maxwell's
equations in an exterior domain $\Omega \subset {\mathbb R}^3$ with dissipative
boundary condition $E_{tan}- \gamma(x) (\nu \wedge B_{tan}) = 0, \gamma(x) > 0,
\forall x \in \Gamma = \partial \Omega.$ We study the case when $\Omega = \{x
\in {\mathbb R^3}:\: |x| > 1\}$ and $\gamma \neq 1$ is a constant. We establish
a Weyl formula for the counting function of the negative real eigenvalues of
$G_b.$
