  A rigorous formulation of the problem of calculating the electromagnetic
vacuum energy of an infinite dielectric cylinder is discussed. It is shown that
the physically relevant spectrum of electromagnetic excitations includes the
surface modes and photonic modes. The mathematical procedure of summing over
this spectrum is proposed, and the transition to imaginary frequencies is
accomplished. As a result, it is justified the imaginary-frequency
representation for the vacuum energy which has been used in previous Casimir
studies for this configuration.
