Abstract: A corpuscular beam microscope for ring segment focusing is provided with a ondenser and an objective magnetic lens system which is disposed substantially axially symmetrically about the microscope axis for the purpose of generating two field maxima separated by a distance not exceeding five times the arithmetic means of half the half height widths of the component magnetic fields forming the maxima. The provision of such a magnetic lens system facilitates the elimination of aperture aberrations of the first and second order and the elimination of chromatic aberrations of zero order and partly of the first order as well as certain extra-axial defects.