Then in 1928 he was appointed as extraordinary professor at Cologne, a position he held for two years before returning to the Charles University of Prague in 1939.
Pagkatapos ay sa 1928 siya ay hinirang bilang katangi-tangi propesor sa Cologne, isang posisyon na hawak niya para sa dalawang taon bago bumabalik sa Charles University of Prague in 1939.