ABSTRACT
At the end of nineteenth century, it was found that an axially symmetric magnetic field has a focusing effect on an electron beam in a cathode ray oscillograph: it acts as a lens. The concentration of the magnetic field made it possible to produce an electron microscope with magnification higher than the optical microscope. Their idea of the magnetic lens is still the basis of lenses at present. Design of a magnetic lens consists of two parts: design of electron optics and design of magnetic circuits including coils, yokes, and poles. The magnetic flux density distribution in the yoke or in the pole is greatly changed by a small change of its radius. It has long been known that inhomogeneity of magnetic properties of pole-piece material has a severe influence on the onset of astigmatism. A combined electrostatic and magnetic quadrupole lens system is described as an example of a chromatic aberration corrector.
