Heisenberg comments on Neumann's manuscript. He is convinced that the correct quantum wave theory will only be found after the introduction of a universal length. He corrects his statement in the last letter about the state described by the Thomas-Fermi method.

Pauli thinks that Heisenberg's letter of the 19th November contains terrible nonsense.

Pauli makes a recapitulation of the properties of Pauli's and Heisenberg's model and gives a critical consideration of Fermi's theory of Beta-radioactivity. He makes remarks on recent publications on nuclear forces.

Heisenberg writes about the consequences of a Delta-interaction in the case of a finite number of particles. This interaction exists only for point-energy show that the "shape" of the electron in quantum electrodynamics is represented by the properties of the density matrix. He writes on a possible semi-classical interpretation of Fermi's theory. He writes about the idea of the Americans that forces between all heavy particles are equal, and some consequences of it. On Euler's application of the Hertree-method to heavy nuclei and the resulting improvement of results.

Heisenberg talks of results of calculations about their model: lack of relativistic invariance. He gives suggestion for a new model, defined by another Lagrange-function and some comments on the recent ideas about nuclear forces.

Pauli talks about further discussion of the Bethe-Heitler formula. He comments on the hard component of the primary cosmic radiation.

Pauli writes on the problem of wave-quantization as the fundamental problem of physics. He is inclined to drop the idea of the existence of a universal length, because such an assumption cannot lead to finite energy values in a quantized wave theory. He talks about the interpretation of recent results of shower-measurements.

Heisenberg comments on Bloch-Nordsieck's paper. It would be worthwhile to calculate the end of the spectrum. From papers by Anderson, Neddermayer and Blackett it seems to follow that particles exist with masses intermediate between electron and proton, and this is a plausible result. On Cosmic radiation, he says that very great showerscannot be explained by the Bhabha-Heitler formula alone.

Pauli replies to Kemmer's letter (14 February) to Fierz. He talks about his forthcoming lectures on field quantization and gives some remarks on Dirac's equations for particles with higher spin.

Pauli asks a question about a possible magnetic spin moment of yukons, and calculations in connection with this.