Abstract
The spherical and chromatic aberrations, the diffraction error and the total aberration of a magnetic electron lens are discussed, as regards variation with voltage, on the basis of theoretical data for the dependence of the aberration coefficients and focal length on lens power. The total aberration shows a minimum value at a given value of the lens power, in a lens of fixed aperture and maximum field strength. The minimum does not occur at either the minimum focal length or the minimum of the spherical aberration coefficient, but at intermediate values of these quantities. When the maximum field is 10,000 oersteds, the minimum resolution is 10-12 A. at an accelerating voltage of 50 kv., the corresponding focal length being 2 mm. If the aperture is adjusted, in a lens of otherwise fixed dimensions, to the optimum value at each voltage, the total aberration varies very little at high voltages. If the lens dimensions are appropriately increased as well as the aperture, the total aberration can be progressively reduced with increasing voltage.