Heisenberg replies to Pauli's letters. He writes about a future formulation of field theory, where matter and radiation fields will appear as different properties of the same field. He talks of Pauli's remarks that the exchange relations of electrical and magnetical field strengths have to be modified.

Heisenberg talks again on the physical meaning of certain terms of self-energy, the problem of the relativistic invariance of self-energy, and Weisskopf's calculations.

Heisenberg writes about Pauli's result that the Balmer-terms must suffer a shift if parameters are smeared out over a domain ~h/mc. He talks on serious deviations of the present theory from reality.

Heisenberg replies to Pauli's letter about two alternations of proceeding with the theory. He writes about a difficulty in the theory of continuous X-ray spectra and its possible solution.

Heisenberg writes about the Uhlenbeck-Konopinski theory of Beta-spectra and a possible relativistic invariant formulation. He also writes about Bothe's refutation of a paper by Shankland, and about a simple model of a non-linear wave theory without universal constant, except c, containing only a pure number to be identified with e$^{2}$/(h.c.).

Heisenberg replies to Pauli's letter, which convinced him that the infinity of the energy values is unavoidable, and that this is connected with shower production. Probably the whole quantum theory of wave fields is wrong. Neumann's suggestion is a possible way out of the dilemma.

Heisenberg thinks that his description of the lowest state (vacuum) is correct, but he does not yet understand how particles as higher states result from his mathematics.

Heisenberg comments on the latest results on showers and talks about possible reasons for the high penetrability of cosmic radiation and calculations on wave-quantization. He writes against the cut-off-method and gives an introduction to the universal length in a new operator. The question remains whether this can lead to a relativistic and convergent theory.

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.