U.S.A. Physical Review, 1947. Royal8vo. Without wrappers (as issued). Offprint from ""The Physical Review"", Vol. 71, No. 10, May 15, 1947. Fine and clean. Pp. 661-666 + 1 blank.
Offprint of Broström, Huus and Tangen's important paper which was reprinted in several monographs.