Abstract
The chief factors limiting the performance of electron microscopes are discussed: spherical and chromatic aberration, diffraction, imperfections in lens construction. Methods of approach to the correction of aberrations are reviewed. It appears preferable to proceed from a consideration not of the final aberration equations, but from the fundamental equations of electron motion, with the use of relaxation methods. It is shown that, even if the mechanical difficulties in lens construction are overcome, a very great reduction in spherical aberration is required to improve the resolution to below 10 A. Further reduction in chromatic aberration is unnecessary in present circumstances.