1. D. M. Volkov:Zeits. Phys.,94, 250 (1935);Žurn. Ėksp. Teor. Fiz.,7, 1286 (1937).
2. M. Gell-Mann:Suppl. Nuovo Cimento,4, 848 (1956).
3. E. P. Wigner:Ann. of Math.,40, 149 (1939).
4. See, for example,J. H. Eberly: inProgress in Optics, edited byE. Wolf, Vol.7 (Amsterdam, 1969), p. 361.
5. B. Beers andH. H. Nickle: paper to be presented at theApril 1972 Meeting of the American Physical Society in Washington, D. C.