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A theoretical and experimental investigation of contour phenomena observed in electron microscope images near focus has been carried out. An explanation for the apparent reversal of the Fresnel fringes at exact focus is given and a method is described for the determination of the degree of asymmetry from the fringe patterns. A procedure for empirically correcting the asymmetries usually present in magnetic electron microscope objectives is outlined and some of the results obtained with a compensated lens are shown. A number of instrumental defects which may prevent the attainment of the ultimate resolving power of an instrument, together with methods for their elimination, are listed.

Both the spherical and the chromatic aberration of electron microscope objectives may, in principle, be corrected with the aid of a uniform retarding field acting as a mirror. Such an arrangement has the drawback of requiring a conducting film in the ray path and the insertion of the specimen in a region of high field strength. The employment of concave electron mirrors with concentrated field distribution, forming a real image of approximately unity magnification, is free from this drawback. The formulas for spherical and chromatic aberration, presented in a form suitable for calculation, are applied to a characteristic electron mirror field of this type (Φ = C+tanh(sinhz)). It is found that the aberration coefficients of the mirror are so large, however, that this method of aberration correction encounters serious practical difficulties.