Møller and Kristensen thank Pauli for his instructive letter. They answer the question he asked and give some details of their form factor theory.

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.

Källén writes about his work on quantum electrodynamics and especially about renormalization.

Källén writes on an interim report on his work on quantum electrodynamics.

Källén criticizes Thirring's remarks about his theory communicated by Pauli.

Källén comments on the paper by Heisenberg and Haag. He writes on Wightman's axioms.

Källén has difficulties in understanding Pauli's last letter. He talks about rumours regarding the discovery of spin in $\Pi^{+}$-mesons.

Källén talks about Heisenberg's claim that the states in the Hilbert-space I possess a positive norm.

Källén writes on the results of his and Wightman's calculations on the properties of certain analytical functions.