During this time, Heisenberg came under vicious attack by the supporters of "Deutsche Physik." One such attack was published in "Das Schwarze Korps", the newspaper of the "Schutzstaffel", or SS, headed by Heinrich Himmler. Heisenberg had been lecturing to his students about the theory of relativity, proposed by the Jewish scientist Albert Einstein. In the editorial, Himmler called Heisenberg a "White Jew" who should be made to "disappear." These verbal attacks were taken seriously, as Jews were subject to physical violence and incarceration at the time. Heisenberg fought back with an editorial and a letter to Himmler, in an attempt to get a resolution to this matter and regain his honour. At one point, Heisenberg's mother visited Himmler's mother to help bring a resolution to the affair. The two women knew each other as a result of Heisenberg's maternal grandfather and Himmler's father being rectors and members of a Bavarian hiking club. Eventually, Himmler settled the Heisenberg affair by sending two letters, one to "SS-Gruppenführer" Reinhard Heydrich and one to Heisenberg, both on 21 July 1938. In the letter to Heydrich, Himmler said Germany could not afford to lose or silence Heisenberg as he would be useful for teaching a generation of scientists. To Heisenberg, Himmler said the letter came on recommendation of his family and he cautioned Heisenberg to make a distinction between professional physics research results and the personal and political attitudes of the involved scientists. The letter to Heisenberg was signed under the closing ""Mit freundlichem Gruss und, Heil Hitler!"" ("With friendly greetings and, Heil Hitler!")