D O C . 2 6 4 J A N U A R Y 1 9 2 0 2 1 9 264. From Hendrik A. Lorentz Haarlem, 16 January 1920. Dear Colleague, I wrote you a while ago about an “Extraordinary” (or “Special”) Professorship at Leyden and take the liberty of reiterating this briefly, as I fear from your failure to respond that my letter did not reach you.[1] There is a vibrant desire among Leyden physicists to have you in Leyden from time to time with some regularity, a desire that also immediately found favor with the faculty. Opportunity to satisfy it is given by the Leyden University Council, a governing body that has the authority to name “Extraordinary” Professors (i.e., chairs independent of the state).[2] The directorate of the council[3] was immediate- ly ready to provide for an appointment for a term of three years, with an annual sal- ary of 2000 guilders, and has now commissioned me with asking you whether you would consent to such a post. If this be the case, the call can then be issued on Feb- ruary 8th, or 9th, because of the Sunday,[4] at the council’s annual meeting. Until then, I must ask you to consider this letter confidential. I need add little, since Ehrenfest has already written you.[5] Allow me only to emphasize, first, that by no means is it our intention that you relax your commit- ments to Berlin in any way [6] we would be very satisfied if you came to Leyden once or twice a year for a few weeks. Second, that it is left completely up to you to arrange your work in Leyden as you like. If you can deliver some lectures, that will please us very much, but if you prefer other forms of instruction (colloquium, discussions), that is also agreeable to us.[7] It is our concern again to reap the stimulus you gave us during previous visits. The university council would certainly set great store by an inaugural speech, but this is no condition. Third, that if you say “yes,” you will please me, and equally Onnes and Kuenen[8] (I need not even mention Ehrenfest) exceedingly. We wholeheartedly hope that you do not have any reservations. It would be of great value to Onnes to discuss the investigations conducted in the Cryogenic Laboratory. ————— Even though we are still a few weeks away from February 8, the secretary of the university council[9] would prefer to know your decision now. When he wrote this