1. F. J. Belinfante, Physica 7 (1940), 449; L. Rosenfeld, Mem. Acad. Roy. Belg. (Cl. Sciences) 18, fasc. 6 (1940), Cf. §5, formula (I) [this volume, p. 711]. This present note supplements these papers.
2. It can be verified (cf. L. Rosenfeld, loc. cit. (1). pp. 24–25) that these two terms have the non-commutative properties required for this interpretation.
3. J. Serpe, Physica 8 (1941), 748.
4. The non-commutative properties of $$ \overline M _{s}^{{ik}} $$ and $$ M_{s}^{{ik}} $$ are obviously identical.
5. H. Bohr, Fastperiodische Funktionen, §46, theorem I. [See English translation, Almost Periodic Functions (Chelsea Publ., New York, 1947) — Ed.].