Pauli writes about his theoretical investigation of the Zeeman-effect. He makes remarks about the validity of Landé's shema for certain multiplets.

Pauli talks about his Zeeman-effect model in the light of Back's experimental results.

Heisenberg writes more about the intensities of spectral lines.

Heisenberg writes about Pauli's coming to Copenhagen. He reports on his recent work.

Heisenberg replies to Pauli's letter. He fully agrees with Pauli's considerations regarding impact processes, which clarify the physical meaning of Born's formalism. He talks about the physical reality of Pauli's p-waves and the indeterminacy relations. Space and time can be applied only to many particles, like temperature and pressure in a gas. He writes about the meaning of the matrix-elements and on the concept of fluctuation of electrical density.

Heisenberg sends Pauli his manuscript, on the whole identical with his long letter. He writes about the postulate character of his quantum mechanical assumption and its similarity with the assumption of the constancy of light velocity.

Heisenberg writes on the difficulties of the phase problem in radiation theory, and on a Hund's paradoxon about optically left- and right-rotating substances and its solution.