1. H. Becquerel, Compt. rend. 122, 420 (1896).
2. W. Heisenberg, Physik 101, 533 (1936).
3. See, for example, L. B. Okun, “Weak Interactions of Elementary Particles”, (Addison-Wesley Publishing Company, Inc. 1965), Chapter 18.
4. H. Yukawa, Proc. Phys-Math Soc., Japan 17, 48 (1935).
5. See, for example, W. Kummer and G. Segré, Nucl. Phys. 64, 585 (1965) and N. Christ, Phys. Rev. 176, 2086 (1968).