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Rh indeterminate the units of transformation is here illustrated in the theory of Larmor.

In the most recent discussion by Lorentz, the necessity of a general treatment is shown for not only the second but also the higher orders. In a consideration of transparent media, his theory attempts to show that translation would not alter interference, diffraction, or polarization. He would thus, by means of the assumption of so-called "Heaviside ellipsoids" as the shape of electrons, explain the negative results of optical experiments, as well as the observations of Kaufmann on Becquerel rays.

Attention should also be called to the recent theory of Abraham, who gives as the ratio of the axes of the moving electron $$1-\tfrac{4}{5}\left(\tfrac{v}{V}\right)^{3}:1$$, omitting fourth and higher orders. This would give a residual in double refraction of $$\tfrac{1}{5}\left(\tfrac{v}{V}\right)^{2}=2\times10^{-9}$$ for transparent media, which he acknowledges is difficult to reconcile with the experimental results which show no double refraction to the first order beyond this.