Magnetic objective lenses, from the point of view of pole piece
geometry, can he roughly classified into two types, viz., symmetrical and
asymmetrical. In the case of the former, the optical properties have been
calculated by several authors1-3) and the results would
appear to suggest that, in order to reduce the spherical and chromatic
aberration coefficients, Cs and Cc, it is necessary to decrease the
half-width value of the axial field distribution and to increase the peak
flux density. The expressions for either minimum Cs or minimum Cc were
presented in the form of ‘universal’ curves by Mulvey and
Wallington4).
