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$QQ=l\cdot OD'$. If ${\frac {dx}{dt}}=v$ for the second strip, a simple calculation gives $OD'=OC{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}$, therefore also $PP:QQ=1:{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}$. This, however, is the sense of Lorentz's hypothesis about the contraction of electrons in motion. On the other hand, if we conceive the second electron to be at rest, and therefore adopt the system $x',t'$, then the cross-section $P'P'$ of the strip of the electron parallel to $OC'$ is to be regarded as its length and we shall find the first electron shortened with reference to the second in the same proportion, for it is in the figure

$P'P':Q'Q'=OD:OC'=OD':OC=QQ:PP\,$

Lorentz denoted the combination $t'$ of (t and x) as the local time (Ortszeit) of the uniformly moving electron, and used a physical construction of this idea for a better comprehension of the contraction-hypothesis. But to perceive clearly that the time of an electron is as good as the time of any other electron, i.e., that t and $t'$ are to be treated equivalently, has been the service of A. Einstein.^[1] By that, the concept of time was shown to be unambiguously established by natural phenomena. But the concept of space was not altered, either by Einstein or Lorentz, probably because in the case of the above-mentioned spatial transformations, where the $x',t'$ plane coincides with the x, t plane, the interpretation is possible as if the x-axis of space somehow remains conserved in its position. To step over the concept of space in a corresponding manner, is certainly only to assess as the boldness of mathematical culture. After this inevitable step for the true understanding of the group G^c, however, the word "Relativity-Postulate" for the demands of invariance in the group G^c, seems to be rather weak to me. Because the sense of the postulate is that the four-dimensional world is given in space and time by phenomena only, but the projection in time and space can be handled with a certain freedom, and therefore I would rather like to give to this assertion the name "The Postulate of the Absolute World", (or shortly World-Postulate).

III.

By the world-postulate a similar treatment of the four determining parts x, y, z, t becomes possible. Thereby the forms
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1. A. Einstein, Ann. d. Phys. 17, 1905, p. 891; Jahrb. d. Radioaktivität u. Elektronik 4, 1907, p. 411.