Ever since Pauli and his wife gave the Bohrs the great pleasure of coming to Bohr's birthday, it has been on Bohr's mind to write to Pauli to say how much their friendship has meant to him during the many years, and how thankful he was again to feel how well they understand each other. Not least when one has come so far in years and has to seek the greatest joy in following the younger generations' progress, all the kindness shown to him was a very great encouragement. It was sad to hear about the death of Pauli's elderly father, and they are all sorry for the loss of Placzek and Weyl. Under these conditions Pauli's and his wife's visit to Princeton will be a great encouragement to all of them, not least for Oppenheimer, of whom they are often thinking. Bohr mentions some questions about the invitation to Lindau, he says that he and his wife plan to go there in the summer. It should be a special joy if Pauli and his wife also came.

Bohr does not agree with Pauli's and Heisenberg's criticism of Thomas' work in: Nature : 117 (1926), pp. 514. He discusses the problem in detail and asks for further comments from Pauli.

Bohr will pass through Hamburg together with Klein on their way to Holland. He hopes that Pauli will join him on his way back to Copenhagen.

Bohr has been thinking of writing to Pauli since the appearance of the Sommerfeld Festschrift, but the letter grew in bulk into a paper, which is not yet finished. Klein is working on relativity problems and is very hopeful about his work for: Z. Phys. : 53 (1928), pp.157".

Bohr thanks Pauli for his letter which gave him a welcome occasion for a small heart-outpouring. He has recently returned to his old primitive view on the problems of the electron theory after having been alarmed by Pauli's difficulty with the infinite self-energy of the electrons.

Bohr has sent a manuscript with the preparation of his discussion remarks from Brussels to Langevin. He is sending now another copy to Pauli and he would be very glad to hear his opinion before he gets the proofs. Bohr and Rosenfeld are eagerly awaiting the promised article with Weisskopf. He need not say how pleased he was to hear that there is a possibility of a visit by Pauli to Copenhagen in Spring. Perhaps Heisenberg and Dirac will come too. Bohr's own journey to Russia should take place in the beginning of May, and Margrethe and Rosenfeld will accompany him on the journey.

Bohr is sorry not to have written for so long, but his time has been very occupied since his return from America. He says that it was nice to be at the Princeton Institute again, and it was a special pleasure to see in how quiet and dignified a manner Oppenheimer took the decision of the Atomic Energy Commission. Both he and his wife were more glad than they have been for a long time, and it has been a great encouragement to them to feel what good friends they have not only amongst the physicists but also in wide circles of the American community, for whom this unreasonable matter has been a great shock. Now he hopes that Robert will have more time for his scientific work, and he is - like the whole circle in Princeton - deeply interested in the many problems which the experiments with the elementary particles always make more contemporary. When Bohr was in Princeton they had also many discussions about the general biological and psychological questions in which also Delbrück took part. Bohr hopes that Pauli will soon come to Copenhagen and speak to the CERN group, and that his wife will come too.

Bohr asks for Pauli's comments on his paper with Kramer and Slater and for suggestions on how to improve this text.

Bohr has heard that Pauli developed new thoughts on the mystic of relativity in the atom theory and is asking for some developments.

Bohr discusses Pauli's paper "Zs. Phys. : 31 (1925), pp.765" of which Pauli has sent a manuscript. The letters contains a long discussion of the electron groups and the periodic table. He also writes on fluorescence radiation.