Variation of the Axial Aberrations of Electron Lenses with Lens Strength

The variation of refractive power and spherical aberration with electrode voltages and field strengths is studied for two characteristic unipotential lenses, an immersion lens, and a magnetic lens. Conclusions are drawn herefrom regarding the variation, with lens strength and applied voltage, of the resolving power obtainable with the lens as an electron-microscope objective. Scattered measurements by other authors agree satisfactorily with the results. The ``relativistic aberration'' of the electrostatic unipotential lenses, i.e., the effect on the image of fluctuations in the over-all applied voltage, is calculated and shown to be of significance in the electrostatic electron microscope. Furthermore, the axial chromatic aberrations are computed for the four systems and the question of upper limits of the last two aberrations is discussed.