1. W. Thirring:Ann. of Phys.,16, 96 (1961). See alsoR. U. Sexl:Fortschr. Phys.,15, 269 (1967).
2. S. Deser:Gen. Relativ. Gravit.,1, 9 (1970).
3. G. Cavalleri andG. Spinelli:Phys. Rev. D,12, 2203 (1975).
4. See, for example,G. Cavalleri andG. Spinelli:Phys. Rev. D,12, 2200 (1975), where the electromagnetic field is also included.
5. The integration over a volume containing the test particle is very useful for eliminating the Dirac delta functions which appear in (2) and (4) because of the pointlike character assumed for the particles. In this case the volume integration of components in general co-ordinates does not constitute a problem because of the delta functions. If the delta functions were not present (e.g. a continuum), we would not have to integrate.