is arising by itself. In relation to this, Poincaré has made the following hypothesis. The electron is a charged and expandable shell, and an inner normal stress of invariable magnitude is resisting to the electric repulsions of the individual points. According to the above, such normal stresses indeed satisfy the relativity principle.
In the same way, all molecular forces acting in the interior of ponderable matter, as well as the quasi-elastic forces and resisting forces acting upon the electron, must satisfy certain conditions in order to be in agreement with the relativity principle. Then, every moving body will be invariable for a co-moving observer, yet it will experience a change of dimensions for a stationary observer, which is just a consequence of the change of molecular forces required by those conditions. From that, the contraction of the body – which was already imagined before to explain the negative outcome of Michelson's interference experiment – follows by itself, and also the negative outcome of all similar experiments which should demonstrate an influence of Earth's motion upon optical phenomena.
Regarding the rigid body (with which Born, Herglotz, F. Noether, Levi-Cività were dealing): the difficulties emerging during the consideration of rotations, will probably be solved by ascribing rigidity to the effectiveness of particularly intensive molecular forces.
Eventually we want to turn our attention to gravitation. The relativity principle requires a modification of Newton's laws, above all it requires the propagation of this effect with the speed of light. The possibility of a finite propagation velocity of gravitation was already discussed by Laplace, who imagined a fluid streaming against the sun as the cause of gravity, which pushes the planets towards the sun. He found, that the speed of this fluid must be assumed to be at least 100 million times greater than the speed of light, so that the calculation remains in agreement with the astronomical observations. The necessity of such a great value of stems from the fact, that the magnitude arises in its end formulas in the first power, where is the planetary velocity. However, if the propagation speed of gravitation shall have the speed of light, as required by the relativity principle, then a contradiction with observations can only then be avoided, when only magnitudes of second (or higher) order in arise in the expression for the modified law of gravitation.
If one confines oneself to magnitudes of second order, then a condition can easily be given on the basis of an obvious electron-theoretical analogy, which defines the modified law in a definite manner.
