1. IEEE Thermionic Conversion Specialist Conference, 162, IEEE, New York, 1967.
2. A. A. Lebedev:Elektronnaia Mikroskopiia, Moscow, USSR, State Publishing House of Theoretical and Technical Literature, 1954.
3. D. Schenk:Ann. Phys., 1935, vol. 23, p. 240.
4. B. N. Popov and A. V. Druzhinin,Radiotek. Elektron., 1958, vol. 3, No. 8, p. 1084.
5. E. F. Burton and W. H. Kohl:The Electron Microscope, Rheinholt, 1942.