1. Fröhlich, Heitler and Kemmer, Proc. Roy. Soc., A, 166, 154 (1938). Yukawa, Sakata and Taketani, Proc. Math. Phys. Soc. Japan, 20, 319 (1938).
2. Heitler, Proc. Roy. Soc., A, 166, 529 (1938), and Report of the Eighth Solvay Conference, Brussels, in the press, where the reasons for the high cross-section are analysed.
3. Wilson, Proc. Roy. Soc., in the press. I am very much indebted to Dr. Wilson for having sent me his MS. before publication.
4. This possibility was first mentioned to me Bhabha in a private discussion in connexion with his classical theory for neutral mesons. The whole problem was very much clarified in discussions with Bhabha, Fröhlich and Kemmer.