  The interrelation between the condensation energy and the optical sum rules
has been investigated. It has been shown that the so called 'partial' sum rule
violation is related mainly to a temperature dependence of the relaxation rate
rather than to the appearance of superconductivity itself. Moreover, we
demonstrate that the experimental data on the temperature dependence of the
optical sum rule can be explained rather well by an account of strong
electron-phonon interaction.
