1. For a detailed discussion of this model and the calculational techniques employed, see Chapter 3 ofThe Analytic S-Matrix,R. J. Eden, P. V. Landshoff, D. I. Olive andJ. C. Polkinghorne (Cambridge, 1966).
2. B. Hamprecht:Nuovo Cimento,42 A, 493 (1966).
3. The case of particles with spin will be discussed byD. Branson: Cambridge preprint (in preparation).
4. The argument here follows a related calculation given in detail in ref. (2).
5. V. N. Gribov andI. Ya. Pomeranchuk:Phys. Lett.,2, 239 (1962).