1. Andrew, A. M. andRoberts, T. M. D. (1954)Electron. Engng.,26, 469–474.
2. Cornwall, J. B. andMelville, A. W. (1962)Electron. Engng.,34, 537–539.
3. Mackay, R. S. (1963)Med. Electron. Biol. Engng.,1, 109–111.
4. MacNichol, E. F. andJacobs, J. A. H. Rev. Sci. Instrum.,26, 1176–1180.
5. Manzotti, M. (1956)Electron. Engng.,28, 446–448.