 Electrostatic immersion objective lens is used to form a real image of the sample by way of a 2/3-magnification virtual image behind the sample.The uniformity of the electrostatic field between the objective lens and specimen, limited by spherical and chromatic aberrations larger than those of any other lenses, ultimately determines the overall performance of the instrument.Contrast aperture is located in the center on the projector lens side of the beam separator.