1. M. Cini andB. Touschek:Nuovo Cimento,7, 422 (1958).
2. C. G. Bollini andJ. J. Giambiagi:Nuovo Cimento,21, 107 (1961).
3. Such an analogy was noticed also byM. E. Rose andR. H. Good jr.:Nuovo Cimento,22, 565 (1961).
4. L. L. Foldy andS. A. Wouthuysen:Phys. Rev.,78, 29 (1950).
5. In a recent communication, to be published,Strocchi has obtained an E-representation for the Sakata-Taketani form (Sci. Pap. I.P.C.R.,38, 1 (1940)) of the Kemmer equation, and he has shown that in the limit of vanishing mass, this representation goes over directly into Maxwell’s equations, despite the fact that the Sakata-Taketani equation itself does not exist in this limit.