Casimir (Hendrik)

Autograph letter signed (‘Henk Casimir’) to [Samuel A. Goudsmit] (‘Beste Sam’), in Dutch and English, discussing spectroscopy, electron spin and matrices, making reference to his work on the Dirac equation and to his life in Copenhagen working alongside Niels Bohr, George Gamow, Lev Landau, and Leon Rosenfeld, saying "It has become clear to me, especially because of some remarks from [Lev Davidovich] Landau, that application of Dirac's or Pauli's theory to a light-emitting electron (although giving the right order of magnitude for the doublet splitting) just isn't very useful. It seems preferable to treat the problem by introducing a current for both orbit and spin. Thus both spin and orbit each give rise to a magnetic field. When writing down the equations one has to take into account that J is a diagonal matrix. But for coupling between s and l no assumption is made. This converges also for s states and then gives the well-known result. One can be sure that one simply has to get the same result as with the Dirac theory, if one neglects relativity effects...", also adding an entertaining account of an automobile trip in the eastern United States, as well as news about his fellow members at the Niels Bohr Institute in Copenhagen "Here we have Gamov[sic], Rosenfelt and Landau. Landau, a young Russian, is astoundingly smart. Grossly unmannered but not un-entertaining. Also Bohr likes him very much [...] Of course we have again some pain-in-the-neck assignments. A Faraday lecture which has to be written is the most urgent. Tomorrow [Niels] Bohr and I will escape and see if we can get it done", eight pages, numbered at top right, staple hole to top left corner of each leaf, 1cm tear at the top right of the final leaf, 4to, n.p. [Copenhagen], 27 September 1930.