In the fall of 1939 Pais dedicated himself to preparing for his master's degree. Utrecht experimental physicist Leonard Salomon Ornstein provided him guidance in his independent physics studies. Uhlenbeck, in anticipation of his departure, introduced Pais to physicist Hendrik Anthony Kramers at Leiden University, who became his mentor and friend. He was also influenced then by discussions with Léon Rosenfeld of the University of Liège, who was invited to Utrecht to give a colloquium in an effort to find a successor for Uhlenbeck and reported on the work he was then doing with Christian Møller on the meson theory of nuclear forces.