7 4 D O C U M E N T 5 4 F E B R U A R Y 1 9 2 2 former attempts at interpretation seem to derive from a long-gone era. The theory of Röntgen spectra, of the visible spectra, of the periodical system of elements are all based primarily on Bohr’s thought. What strikes one as so wonderful in Bohr as a researcher is a peculiar combination of audacity and cautious deliberation sel- dom has a researcher possessed to such an extent the ability for both an intuitive comprehension of hidden things and for acute criticism. In spite of all his knowl- edge of the particular, his vision is directed unflinchingly toward matters of princi- ple. He is, without doubt, one of the greatest innovators of our time in the field of science. Niels Bohr is a professor of theoretical physics at the University of Copenhagen. The undersigned recommend him as a corresponding member of our Academy. A. Einstein. 54. From Paul Ehrenfest [Leyden,] 16 February 1922 Dear Einstein, Many thanks for your postcard and printer’s proofs. —As soon as I have read the latter in peace, I’ll probably write you about it—it appears to me to be very nice indeed. The following passage in your postcard caused me much unease: “It would be preferable if we could shift the congress to the end of September. I was with you for so long just recently.”– I begin from the back: a.) If we correspond in January about what you’re supposed to do at the end of April, it is a terrible computational error to confuse the (quite) correct assessment: (A) January minus September = “recently” with the incorrect assessment: (B) end of April minus September = “recently.” b.) There are naturally absolutely no objections to a shifting of the “congress” for me it is even convenient, because I can then prepare everything more calmly. I had just meanwhile, prompted by your penultimate letter about it, written to Lorentz in Pasadena about whether one of the “redshift Americans” might not per- haps be present in Europe around Easter. If Lorentz were to write: yes, then you really can come to Leyden for it for a short time. But presumably Lorentz’s answer will be negative, and then the business can very well wait until September. c.) But it would be very fatal for me if you did not come to Leyden this spring.— You know: Nobody demands that you present lectures not even convening little “conferences” is obligatory. This all can happen one time this way, another time that way, or not at all. But I’ll get into real trouble if you simply don’t come.