The footnote to the German original says that the derivation is described in mor...

		schoen on April 18, 2015 \| parent \| context \| favorite \| on: Einstein's Letter to Marie Curie (1911) The footnote to the German original says that the derivation is described in more detail in this letter to Lorentz: http://einsteinpapers.press.princeton.edu/vol5-trans/249 (It's the section beginning "I have little that is new to report in physics".)