In 1942, during World War II, Casimir moved to the Philips Natuurkundig Laboratorium (Philips Physics Laboratory, NatLab) in Eindhoven, the Netherlands. He remained an active scientist and in 1945 wrote a well-known paper on Lars Onsager 's principle of microscopic reversibility. He became a co-director of Philips NatLab in 1946 and a member of the board of directors of the company in 1956. He retired from Philips in 1972.