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phenomena. Fresnel based his explanation of Arago's prism experiment upon the value $1-{\frac {1}{N^{2}}}$ of the dragging coefficient. Subsequent scientists have applied this equations to many other cases, and have derived from it, that the motion of earth, as regards most of experiments with terrestrial light sources, is without influence, and that experiments with the light of a celestial body must give a result, as if the direction altered by aberration would be the real one. How easy the theoretical considerations are formed, when we look, not upon the direction of the waves, but on the path of light rays, I have demonstrated (following the example of Veltmann^[1]) in my treatise of the year 1887.^[2] At that time, I restricted myself to isotropic bodies, since it wasn't known to me yet, how to extend Fresnel's law for crystals. Now, since it was demonstrated, that the propagation velocity of light rays obeys in these bodies the simple law expressed in formula (87), it is easy to show, that also the birefringence of rays is independent of Earth's motion.^[3] For this purpose we can start with a simple theorem that follows from the principle of Huygens, and I allow myself to shortly state it at this place.

Let A and B two arbitrary, which may lie within different mutually adjacent media. In general, only a restricted amount of light rays can travel from one to another. If we now form (for one such ray, as well as for other ways between A and B with only small deviations) the integral

$\int {\frac {ds}{U}}$,

in which U means the velocity for a light rays that follows the line element ds,

1. Veltmann. Pogg. Ann., Bd. 150, p. 497, 1873.
2. Lorentz. Arch. néerl., T. 21.
3. A derivation of this theorem from formula (87) was published by me in Zittingsverslagen of the Akad. T. Wet. te Amsterdam, 1892—93, p. 149,