This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Figure 3610 shows the simplest system which can eliminate all second-order fundamental rays and thus all the second-order aberrations outside of the corrector. It consists of a round lens doublet and two identical sextupoles. The outer focal points of the corrector are the same as the nodal points N₁ and N₂ of the round lens doublet. The coils of the round lenses are connected oppositely in order to avoid an image rotation from the first sextupole, which is independent of the current strength. Therefore, the doublet images the front sextupole with magnification of −1 exactly onto the second sextupole centered about the nodal point N₂ without introducing an off-axial third-order coma. This system can correct the third-order spherical aberrations of electron microscopes [1].

Figure 3610. Schematic illustration of a simplest spherical-aberration corrector. Adapted from [2].

[1] Born M. and Wolf E., 1975, Principles of Optics (Oxford: Pergamon Press).
[2] Rose H. H., Optics of high-performance electron microscopes, Sci. Technol. Adv. Mater. 9 (2008) 014107.