1. Ruska, E., and Müller, H. O., Z. Phys., 116, 360 (1940).
2. von Borries, B., Z. Phys., 116, 370 (1940).
3. von Borries, B., “Die Uebermikroskopie”, 200 and 312 (Berlin, 1949).
4. Sushkin, N. G., et al., Bull. Acad. Sci. U.S.S.R., 15, 285 (1951). Kushnir, U. M., Biberman, L. M., and Levkin, N. P., ibid., 306 (1951).
5. Cosslett, V. E., and Jones, D., Rep. Int. Cong. Electron Micr., Paris (1950).