Bhabha's and Heitler's assumptions confirm Heisenberg's idea of the necessity for introducing a universal length. He writes about difficulties in the interpretation of cosmic radiation experiments with the Yukawa theory.

Heisenberg thinks, after receipt of Yukawa's paper, that his Hamiltonian is better than that of Kemmer or Bhabha. He writes on the Beta-instability of the yukon. He sends Pauli a short paper on the limitations of quantum mechanics and asks for his comments.

Heisenberg writes on the impossibility of measuring the power law exp (-k.r)/r at large distances and about Stark's article in "Nature".

Heisenberg explains certain formulae in the manuscript on quantum electrodynamics. He replies to questions by Pauli on the symmetry of conservation laws, on a stronger approximation to Pauli's and Jordan's formalism and on the meaning of certain values for the self-energy of particles.

Heisenberg writes about the "Vertauschungsrelation" in the theory of holes.

Heisenberg makes some remarks on Fermi's paper. He writes on his matrix elements for pair creation and gives some calculations about the exchange energy of proton and neutron.

Heisenberg expresses doubts about the theory of holes; Pauli's schema is logical, but too complicated and a bit arbitrary. The questions are: "Are the expectation values for charge-and energy-density finite?" "Are the latter not occasionally negative?"He writes about the latest results of de Broglie and Joliot-Curie.

Heisenberg uses Dirac's method for the calculation of certain quantities. The concept of polarization of vacuum is not defined according to this theory.

Heisenberg replies to Pauli's questions on the possibility of calculating the total energy of matter created by the introduction of fields. He gives suggestions for the improvement of the paper and talks about Pauli's theory regarding the Klein-Gordon equation and the possibility of finding by this a method of field quantization, provided that modifications are introduced in the quantization rules of field, leading to spinors and Fermi-statistics. He writes about the scattering of light by light.

Heisenberg replies to Pauli's and Weisskopf's letters about the self-energy of the photon.