1. Kay, L., J. Brit. Inst. Rad. Eng., 24, 309 (1962).
2. Delany, M. E., and Rennie, A. J., National Physical Laboratory Report No. AP 20 (1965).
3. Editorial in New Beacon, No. 613, 113 (1968).
4. Leonard, J. A., and Carpenter, A., American Foundation for the Blind, No. 4, 70 (1904).
5. Riley, L. H., Proc. Intern. Cong. on Sensory Devices for the Blind (edit. by Dufton, R.) (St Dunstan's, London, 1967).