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 belong to vector quantities. If several such variables come into account, such as in the electro-magnetic theory of light: the dielectric polarization, the electric force, the magnetic force, or even the earlier vectors $$\mathfrak {D}'$$ and $$\mathfrak{H}'$$, then we have to imagine that for a given body (at a given beam direction, relative oscillation period and translation) these vectors were all determined by one of them. Thus it will be sufficient, to choose one of the vectors for consideration. This we call the light vector and introduce the following presuppositions, which partly includes a hypothesis about the nature of bodies and light, and partly a limitation in the choice of the light vector.

1°. If a state of motion exists in a system of bodies, in which the components of the light vector are certain functions of relative coordinates and time t, thus also the functions that arise when we replace t by -t, represents the values of the components that correspond to a possible motion. But, in the course of this reversal, we also have to reverse the motion and the velocity $$\mathfrak{p}$$.

2°. We also arrive at a possible motion, when we take the mirror image of an arbitrary, given motion in relation to a stationary plane, namely in such a way that both the translational velocity, as well as all light-vectors are replaced by the mirror images.

If we are dealing with the pure aether, then we satisfy these conditions if we choose the dielectric displacement as the light vector.

§ 98. In a  polarized light beam, the light vector is parallel at all points to a certain line; it can be decomposed into three mutually perpendicular components, the first having the direction of the beam, while the second lies in the plane of incidence and the third is perpendicular to it. Now, since the properties of a polarized beam, except the intensity and period of oscillation, only depends on one magnitude — such as the azimuths of the polarizer —, then the ratios between the mentioned components must have specific values, as soon as the ratio between the second and third is given; yet this single ratio