sentences:  Until his death in 1921 he was actively engaged in the fields of mathematics, physics and natural philosophy.He corresponded with well-known mathematicians and physicists, such as Hermann von Helmholtz, Heinrich Hertz, Felix Klein and Philipp Lenard.Because of his unusual scientific creativity and versatility the philosophical faculty of the University of Bonn honoured him demonstratively on the occasion of his golden doctoral anniversary with the renewal of his doctor dignity on 31 July 1916.