Heisenberg is preparing the manuscript on He. He writes about the problem of the molecule and makes critical remarks about the physical part of Schrödinger's theory. The connection with De Broglie is not yet discovered.

Pauli comments Wentzel's paper.

Pauli expresses doubts about Wentzel's results concerning higher relativity corrections for the H-atom. He makes remarks about the mathematical method used by Wentzel.

Pauli discusses the question of a degenerated gas and compares statistics of Bose-Einstein with that of Fermi-Dirac. Another part of the letter deals with scattering which Pauli does not yet understand.

Pauli writes about the relativistic equations and the expressions for charge- and current-density.

Heisenberg objects to Pauli's analogy between quantum mechanics on the one hand and quantum electrodynamics and classical Maxwell theory on the other hand. There is not only Maxwell's equations must be quantized, but also the De Broglie Waves. He explains his ideas on the intuitive meaning of quantum mechanics: 1) position of the electron 2) "orbit" of the electron. He agrees with Dirac that their experiments introduce statistics 3) on the "velocity" of the electron. 4)the transition from micro- to macro-mechanics. He writes that every new observation defines a new state. There are a few examples as illustrations.

Pauli discusses the properties of spin matrices and efforts toward a quantum electrodynamics.

Pauli suggests postponing his visit to Copenhagen. He talks about his work on quantum electrodynamics and makes a criticism of Klein's paper in the Zs. f. Phys. Pauli thinks that Klein is not ready to see the use of multidimensional spaces as an inherent part of the approach of quantum phenomena.

Heisenberg writes about Pauli's and Jordan's preliminary considerations about quantum electrodynamics.