This reply to Pauli's letter, concerns the representation of the wave equation with spin.

Kita talks about the behaviour of Heisenberg's S-function. He asks a question about the translation into Japanese of Pauli's handbook-article of 1933.

The letter contains a critical discussion of Heisenberg's quantization of non-linear wave equations.

Historical remarks from Klein about Poincaré's note in "Comptes Rendus 146" and in "the origins of relativity".

Klein's reply to Pauli's letter of 4 May, and detailed comments on several passages in the encyclopedia.

Klein's further remarks and suggestions about the encyclopedia article.

Klein's comments on the proofs of Pauli's encyclopedia article.

A few more suggestions from Klein for changes to Pauli's encyclopedia.

Klein sends a reprint with remarks. Klein reports on his activities in Ann Arbor.

Klein's writes about his and Hilbert's view that the specific significance of Einstein's theory is to be found in its physical conception, but that its precise formulation is a purely mathematical problem. He makes further comments about relativity.