Heisenberg agrees with Pauli's calculations avoiding spin with Bose-statistics. The assumption that a single particle (electron) must be a trivial solution of quantum electrodynamics, should lead to spin with Fermi-statistics. The question is what kind of quantized wave theory will obey the following postulates: the number of particles corresponding to the waves is not necessarily constant, relativistic invariance, a single particle as trivial solution of the equations and energies are always positive.

Heisenberg makes some remarks on Pauli's and Weisskopf's theory. The infinite polarization of the vacuum is the result of certain parts of the quantum-theoretical formalism which need corrections.

Heisenberg cannot discover any mistake in his own calculations of the self-energy of photons.

Heisenberg asks questions in connection with Pauli's and Weisskopf's calculations, in particular how the self-energy of the electron becomes infinite. He writes about the results of calculations and about the scattering of light by light. He makes some remarks about nuclear physics (Fermi's and Bethe's calculations) and the moments of the proton and the neutron.

Heisenberg writes on Euler-Kochel's modifications of Maxwell's theory.

Heisenberg writes on the treatment of an example according to Neumann's method: the case of an electron in a force field with probably 1/2 for right or left spin. He talks of a application of the result to field theory, of the possibility of representing the state of a system by a matrix depending on a finite number of degrees of freedom, and of some of his doubts about this possibility.

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.

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.

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.