as Bucherer's (see Fig. 6). Also in this experiment, the data corresponding to Lorentz's formula are nearly on a horizontal line as required, while the data obtained from Abraham's formula sharply deviate (see Fig. 8). Neumann concluded that his experiments were in agreement with those of Bucherer and Hupka, definitely proving the Lorentz–Einstein formula in the range 0.4–0.7c, and refuted Abraham's formula. Instrumental uncertainties occurred in the range 0.7–0.8c, so the deviation from the Lorentz–Einstein formula in this range wasn't considered as being significant. In 1915, Charles-Eugène Guye and Charles Lavanchy measured the deflection of cathode rays at 0.25c–0.5c. They used a tube with a cathode and anode in order to accelerate the rays. A diaphragm at the anode produced a beam which was deflected. A screen was placed at the end of the apparatus, at which the impacts were photographed by a camera. They subsequently computated the ratio of transverse electromagnetic mass mT and rest mass m0 indicated by the red and blue curve, and obtained good agreement with the Lorentz–Einstein formula (see Fig. 9), supplementing Neumann's result. Neumann's and Guye/Lavanchy's experiments were considered by many as conclusively proving the Lorentz-Einstein formula. Lorentz summarized these efforts in 1915: Later experiments [..] have confirmed the formula [..] for the transverse electromagnetic mass, so that, in all probability, the only objection that could be raised against the hypothesis of the deformable electron and the principle of relativity has now been removed. Further development Zahn & Spees (1938) and Faragó & Lajos Jánossy (1954) argued that many assumptions employed in those early experiments as to the nature and the properties of electrons and the experimental setup, were wrong or imprecise. As with Kaufmann's experiments, the Bucherer–Neumann experiments would only show a qualitative increase of mass, and were incapable of deciding between the competing theories. While the results of those electron deflection experiments were disputed for a long time, the investigations of the fine structure of the hydrogen lines by Karl Glitscher (based on the work of Arnold Sommerfeld) had already in 1917 provided a clear confirmation of the Lorentz–Einstein formula, because the relativistic expressions for momentum and energy were necessary to derive the fine structure, and a constituted a refutation of Abraham's theory. In addition, the first electron deflection experiments with sufficient precision was conducted by Rogers et al. (1940), who developed an improved setup. The radium decay series yields a spectrum