1. R. Delbourgo, A. Salam andJ. Strathdee:Phys. Lett.,22, 680 (1960).
2. We are grateful to Drs.R. Omnès andH. Jones for discussions on this point.
3. H. Pagels:Phys. Rev.,144, B 1250 (1965).
4. These operators will contain spin matrices and will also include possible Schwinger terms. The precise form of ΓR depends on the interaction Lagrangian and is exhibited in eq. (15) for a quark model. Schwinger terms have been shown to be present in certain [θ, θ] commutators byG. Boulware andS. Deser (Journ. Math. Phys., to be published).
5. The Green's function method in the Appendix gives additional nucleon pole contribution on the right side of (11), which come about because all particles are treated off the mass shell. This ambiguity is discussed further in the Appendix.