1. P.A. Sturrock, Proc. R. Soc. A (London). 210, 269 (1951)
2. V.K. Zworykin, G.A. Morton, E.G. Ramberg, J. Hillier, A.W. Vance, Electron Optics and the Electron Microscope (Wiley, New York, 1945)
3. O. Scherzer, Optik 2, 114 (1947)
4. D.F. Hardy, Dissertation, University of Cambridge (1967)
5. H. Rose, Optik 85, 19 (1990)