This paper is concerned with the evaluation of the optical properties of the portion of a beam of charged particles lying in the plane of symmetry of any toroidal condenser used as an electrostatic mirror. It gives a general treatment that permits the calculation of the optical characteristics and the aberrations when a trajectory and its derivatives relative to the entry energy and angle are known. Results are applied to the case of a cylindrical condenser, resulting in an important extension of the knowledge of its properties. No attention is given to space‐charge effects and to the motion off the plane symmetry where the electric field is generally not central anymore.