Heisenberg talks about some plausibility arguments against Pauli's objections and about a possible solution for the interaction of neutrinos in the case of many particles. He writes about an application of a kind of Thomas-Fermi-method for the state of lowest energy. From this interaction the production of showers can possibly follow.

Pauli writes that his view that Delta-type interaction between more than two neutrinos must lead to infinities was influenced by a paper by Thomas (Phys. Rev. 47,903). This does not affect Pauli's and Heisenberg's case.

The letter asks further question about the same model discussed in his letter of 14 Nov, concerning impact cross-section of suchparticles.

Pauli takes back what he said in the last postcard and confirms Heisenberg's calculations.

Heisenberg gives a description of the present situation of the field-theory. In a more complete future theory there will exist no particles smaller than the universal length, and the divergences of the present theory will be avoided. He writes about more considerations about nuclear theory. The pair production observed up to now can only be interpreted as internal emission. About an experimental paper on shower production in great depth.

Heisenberg talks more about cosmic radiation at great depths and Heisenberg's conjecture that it consists of neutrinos. He continues his calculations with the new formulation of the quantum theory of waves. It leads to relativistic and convergent results, but energy and momentum integrals are not known because of the lack of a Hamiltonian.

Heisenberg writes about his marriage and gives more results about his attempt at convergent wave quantization: appearence of new infinities which cannot be avoid without loss of relativistic invariance. He talks about cosmic radiation: it seems as if his hypothesis of the neutrino origin of showers can be confirmed. He also talks about recent papers about the number of showers as function of layer-thickness.

Pauli comments on a paper by Uhlenck-Konopinski and by Anderson-Neddermeyer on particles with masses between that of electron and proton, which opens completely new vistas.

Heisenberg talks about a letter by Blackett and his experiments about energy loss in Cu plates. He speaks of the possibility of the existence of heavy particles in the showers. He writes on Heisenberg's considerations about explosions in and the existence of a universal length.

Bloch has given Pauli Bohr's letter and the note on the nuclear photo-effect to read. At Bohr's request, Pauli gives his opinion on it.