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the factors given in § 23, then it is clear, that the forces in $S_{1}$ will be mutually canceled, as soon as this happens in $S_{2}$. Consequently, if $S_{2}$ is the state of equilibrium of a stationary, rigid body, then in $S_{1}$ the molecules have exactly those positions, in which they can remain under the influence of translation. The displacement would of course cause this configuration by itself, and thus by (24) it would cause a contraction in the direction of motion in the ratio of 1 to ${\sqrt {1-{\frac {{\mathfrak {p}}^{2}}{V^{2}}}}}$. This leads to the values

$\delta =-{\frac {{\mathfrak {p}}^{2}}{2V^{2}}},\ \epsilon =0$,

which is in agreement with (119).

In reality the molecules of a body are not at rest, but there exists a stationary motion in every "equilibrium state". As to how this condition is of influence as regards the considered phenomenon, may remain undecided; in any case, due to inevitable observational errors, the experiments of Michelson and Morley let remain a considerable wide margin for the values of $\delta$ and $\epsilon$.

The polarization experiments of Fizeau.

§ 93. In the oblique passage of a polarized light beam through a glass plate, the azimuth of the polarization changes in general, namely this phenomenon is depending on the nature of the plate, so that the increase or decrease of its refractive index is followed by a rotation of the polarization plane of the emanating light. This fact was the starting point for the experiments with glass columns, executed by Fizeau^[1], whose results deserve our attention to a high degree. The apparatus employed, consisted of a polarized prism,

1. Fizeau. Ann. de chim. et de phys., 3e sér., T. 58, p. 129. 1860; Pogg. Ann., Vol. 114, p. 554, 1861.