formula of Lorentz–Einstein, and had therefore also disproven the principle of relativity. In his view, the only remaining options were between the theories of Abraham and Bucherer. Lorentz was perplexed and wrote that he was "at the end of his Latin". However, criticism of Kaufmann's experiment arose. Shortly after Kaufmann published his results and the conclusions of his analysis, Max Planck decided to re-analyze the data obtained by the experiment. In 1906 and 1907, Planck published his own conclusion on the behavior of the inertial mass of electrons with high speeds. Using just nine data points from Kaufmann's publication in 1905, he recalculated the exact setup of the fields for each point, and compared the measurements against the predictions of the two competing theories. He showed that Kaufmann's results are not fully decisive and would lead to superluminal velocities. Einstein remarked in 1907 that although Kaufmann's results were better in agreement with Abraham's and Bucherer's theories than with his own, the foundations of the other theories were not plausible and therefore had only little probability of being correct. Subsequent experiments Bucherer The main problem of Kaufmann's experiments was his use of parallel magnetic and electric fields, as pointed out by Adolf Bestelmeyer (1907). Using a method based on perpendicular magnetic and electric fields (introduced by J. J. Thomson and further developed to a velocity filter by Wilhelm Wien), Bestelmeyer obtained considerably different values for the charge-to-mass ratio for cathode rays up to 0.3c. However, Bestelmeyer added that his experiment was not precise enough to provide a definite decision between the theories. Therefore, Alfred Bucherer (1908) conducted a precise measurement using a velocity filter similar to Bestelmeyer's. See Figs. 4 & 5. A radium beta source was placed at the center of a circular condenser consisting of two silvered glass plates spaced 0.25 mm apart and charged to about 500 volts, set in a homogeneous 140 Gauss magnetic field. The radium emitted beta rays in all directions, but in any particular direction α, only those beta rays exited the velocity filter whose speed was such that the electric and magnetic fields exactly compensated each other. After leaving the condenser, the rays were deflected by the magnetic field, and exposed a photographic plate set parallel to the condenser rim and perpendicular to the non-deflected rays. For his final analysis, Bucherer recalculated the measured values of five runs with Lorentz's and Abraham's formulas