Physics. — ‘Space-time symmetry. I. General Considerations’. By 
N. H. Ko.kmever. Communication N°. 7a from the Laboratory 
of Physics and Physical Chemistry of the Veterinary College 
at Utrecht. (Communicated on behalf of Prof. W.H. Kensom, 
Director of the Laboratory, by Prof. H. KAMERLINGH ONNES). 
(Communicated at the meeting of December 18, 1920). 
§ 1. Introduction. In the literature of later years some propositions 
are found for changes in the original atom-model of RurHERFoRD-Bonr. 
These propositions are founded on different considerations. So Lewis *) 
and Lanemuir*) were led by considerations on the chemical structure. 
SOMMERFELD *) was brought to his combinations of ellipses by investi- 
gations of the cause of the fact that the defect of charge of the 
nucleus is not a whole number for the Z-series. Born, LANpÉ and 
MApDELUNG *) again studied the absolute dimensions of the elementary 
cells of crystals, the transformations of energy, and especially the 
compressibility. By these considerations they were led (partly in 
cooperation) to the invention and nearer inspection of cubical 
models analogous to those of Lewis and LANGMUIR. 
Evidently Born, LANDÉ and Maprerune especially felt the necessity 
of a change in the considerations on symmetry that were valid until 
now, because they considered moving systems. On the same ground 
I felt necessitated to introduce some new symmetry-elements, in 
which time also plays a role *). It now seemed desirable to consider 
the space-time-symmetry more systematically than could be done in 
Communication N°. 4. In this paper the way to attack this problem 
will be indicated. 
$ 2. Restriction to a definite kind of operations. A symmetry 
operation will further on be denoted by A, a complex symmetry 
1) G. N. Lewis, Journ. of the Amer. Chem. Soc. 38 (1916) p. 762. 
2) J. Lanemurr, Journ. of the Amer. Chem. Soc. 41 (1919) p. 868. 
3) A. Sommerretp, Physik. ZS. 19 (1918) p. 297. 
4) M. Born and A. Lanpé, Verh. d. D. Phys. Ges. 20 (1918) p. 210. 
M. Born, Verh. d. D. Phys. Ges. 20 (1918) p. 230. 
A. Lanpé, Sitz.-Ber. d. Berl. Akad. 1919 p. 101. 
Verh. d. D. Phys. Ges. 21 (1919) p. 2, 644, 653. 
ZS. f. Phys. 2 (1920) p. 83. 
E. Mapetune and A. Lanpé, ZS. f. Phys. 2 (1920) p. 230. 
5) N. H. Kotxmener, Comm. N°. 4, These Proceedings 23 (1920) p. 120. 
