1. Glossary of terms used in radiology, p. 19 (British Standard 2597; 1956).
2. Vries, L. de, German English Science Dictionary, p. 558 (N. York: McGraw-Hill, 2nd edn, 1946).
3. Kapp, G., J. Soc. Tele. Engrs. and Elect. (GB), 15, 518 (1886).
4. Meggers, W. F., J. Opt. Soc. Amer., 41, 1064 (1951); 43, 411 (1953).
5. Bladergroen, W., Nature (GB), 167, 1075 (1951).