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Frenkel's derivation of the electromagnetic mass and the equations of motions of the electron are very suggestive and simple. But we cannot admit that our method of reasoning is unjustified. There are two points: (1) We have proved exactly that the two variation principles using the Lagrangian and the Hamiltonian are entirely equivalent, both leading to the same results (p. 436). The reason why we have to use the Hamiltonian, not the Lagrangian, for the derivation of the equations of motion is the additivity of thepkl (D, H) and not of thefkl (E, B) in accordance with Frenkel's statement; we regret that we have stated this important point only in a footnote (p. 449).

Born's theory of the electron-Remarks on the paper by Frenkel on born's theory of the electron