In this paper, a combined system consists of electrostatic round lens and magnetic deflector with additional superimposed fields is studied. The general power-series expression for magnetic deflection fields, the electron trajectories described in vector form and the Gaussian optical characters of the system are obtained and discussed. Formulae for calculating the third order aberration in such a combined system are given in a compact matrix form, they are appropriate for numerical computational purpose. This paper and the previous work 1 are all concerned with the general theory of a combined electrostatic-magnetic system of electron optics, hence they may be useful for designers of cathode ray tubes and electron optical instruments.