It is examined which meson field theories are in accordance with the requirements of projective relativity; if the interaction with nucleons is taken into account, there appear to be 5 basic types, namely: (~) a mixed vector-pseudoscalar, ([3) a vector, (y) a pseudoscalar, (8) a scalar and (a) apseudovector theory. Case (cยข) represents the Moller-Rosenfeld theory of nuclear forces. The two main features of such a treatment are: 1. A reduction of the number of universal constants in the cases (cยข) and (ยข); 2. The automatical introduction of the interaction between mesons and the electromagnetic field. *) Another possibility has been proposed by B e t 11 e *).
--267 ---*) It is true that this connection might just as well be inferred from a pure pseudoscalar theory 8); however, the situation with regard to the nuclear forces seems to impose the adoption of the mixed theory. *) We prefer this name to ,,covariant gauge-invariant derivative" as the latter might suggest the invariance of meson theory with respect to the group of gauge transformations which in fact is not the ease. **) a[i.L by] = al~ by --a v bl.,. ***) All = 52. AIBI; g = [detgtzv]. i *) I heartily thank dr. J. P o d o 1 a n s k i for instructive discussions on this point.