From 1932 to mid-1933, Casimir worked as an assistant to Wolfgang Pauli at ETH Zurich. During this period, he worked on the relativistic theory of the electron, in particular, evaluating deviations of the Klein-Nishina equation in the case of bound electrons. To attack the problem, he invented a number of mathematical tools. One in particular is still referred to as the “Casimir Trick": in particle interaction calculations, it is a familiar procedure of trace formation and projections using products of Dirac matrices.