1. Zworykin et al.Electron Optics and Electron Microscope (John Wiley and Sons, Inc., New York 1945);K. R. Spangenberg,Vacuum Tubes (McGraw-Hill Book Company, Inc., New York, 1948.
2. G. Henneberke, Nucl. Instr. & Meth.7, 89 (1960).
3. M. Morikawa,Electron Optics in Multi-Stage Lens System (I), ZAMP13, 167 (1962).
4. Zworykin et al., see [1] p. 665;E. B. Bas et al., ZAMP10, 533 (1959).
5. K. R. Spangenberg, see [1]; p. 344;E. Koltay, Nucl. Instr. & Meth.6, 45 (1960).