  The Maxwell operator in a 3D cylinder is considered. The coefficients are
assumed to be scalar functions depending on the longitudinal variable only.
Such operator is represented as a sum of countable set of matrix differential
operators of first order acting in $L_2({\mathbb R})$. Based on this
representation we give a detailed description of the structure of the spectrum
of the Maxwell operator in two particular cases: 1) in the case of coefficients
stabilizing at infinity; and 2) in the case of periodic coefficients.
