  It is shown that the low energy spectrum of mesoscopic superconductors
coupled by Josephson interaction can be probed by two-electron tunneling from a
normal electrode. The Andreev reflection in the NS junction of a
normal-superconductor-superconductor double junction (NSS transistor) provides
a unique spectroscopic tool to probe the coherent Cooper pair tunneling and the
energy spectrum of the Josephson (SS) junction. The ground state properties are
reflected in a resonant structure of the linear conductance; excited states
with an energy as low as the Josephson coupling energy lead to a threshold in
the nonlinear $I$-$V$ characteristic.
