9½ pages, 4o, in Dutch, creased where previously folded, minor marginal chipping to one sheet. With English translation. Excellent scientific letters to Goudsmit from the discoverer of the "Casimir effect" (a quantum force that pulls two metal plates, placed a short distance apart, towards one another) and the "Casimir trick" for simplifying calculations with the Dirac equation involving spin value. Both of Casimir's letters discuss spectroscopy, electron spin, matrices, etc.., as the following extract from the Sept. 1930 letter shows:" /> CASIMIR, Hendrik B.G. (1909-2000). 2 Autograph letters to the physicist Samuel Goudsmit, signed ("Henk Casimir"). N.p. [Copenhagen], 27 September 1930 and Zurich, June 22, 1933. <I>9½ pages, 4<V>o, in Dutch, creased where previously folded, minor marginal chipping to one sheet.</I> With English translation. Excellent scientific letters to Goudsmit from the discoverer of the "Casimir effect" (a quantum force that pulls two metal plates, placed a short distance apart, towards one another) and the "Casimir trick" for simplifying calculations with the Dirac equation involving spin value. Both of Casimir's letters discuss spectroscopy, electron spin, matrices, etc.., as the following extract from the Sept. 1930 letter shows: | Christie's