Dirac & Oppenheimer in Leiden

(© AIP Visual Archives) Photograph taken in front of the physics laboratory during Dirac's visit to Leiden in July 1927. I am indebted to Alexei Kojevnikov for the identification of Lev Solomonovich Polak, and to Jos Engelen & Louk Lapikás for correcting me on Woltjer (Jan, not his brother Herman). Three other persons remain unidentified. Does anyone know who they are?
Update (October 2023): Further detective work by Engelen and Lapikás, aided by the Casimir and Kramers families, has identified the three unknowns: one student of Ehrenfest (Roelf Krans), one student of Kramers (Henk Govers), and a student from Amsterdam (Josephus Spier). Govers recalls the visit of Oppenheimer in a 2001 interview.

Paul Dirac visited Paul Ehrenfest and his group in Leiden in July 1927, traveling to The Netherlands from Göttingen together with Robert Oppenheimer. Both would give a colloquium and sign the wall (search for their signatures). Ehrenfest had recognized the originality of Dirac's physics, which he admired but found hard to understand. After Dirac had agreed to visit Leiden, Ehrenfest wrote to him that the Dutch physicists had discussed "the last three Diracian crossword puzzles: Physical interpretation, emission and absorption, dispersion. We spent many, many hours going over a few pages of your work before we understood them! And many points are still as dark to us as the most moonless night!"
Oppenheimer would return to Leiden in the Fall of 1928 on a US Fellowship. At Ehrenfest's recommendation he did not go to Niels Bohr's institute in Copenhagen, as he had originally planned, but instead joined Wolfgang Pauli's group in Zürich. In an interview, Oppenheimer recalled that what influenced him was "Ehrenfest's certainty that Bohr with his largeness and vagueness was not the medicine I needed but that I needed someone who was a professional calculating physicist, and that Pauli would be right for me.... He thought in other words that I needed more discipline and more schooling."
H. Kragh, Dirac: A scientific biography
D.C. Cassidy, J. Robert Oppenheimer and the American Century

More on Oppenheimer's Leiden connection (in Dutch).
Did Oppenheimer speak Dutch?

Paul Dirac, Olga Trapeznikova, Igor Tamm, Ivan Obriemov.
Leiden, April 1928. (© AIP Visual Archives.)
Dirac made a second visit to Leiden in April 1928, coinciding with the stay of Igor Tamm (January-June 1928). Olga Trapeznikova, Lev Shubnikov's wife, describes in her memoires an excursion with Tamm and Dirac:
Tamm and Dirac spent a lot of time together. Several times we joined them for walks, and once we all had a boat trip. The boat we got was huge and unruly. We didn't sail too far from the shore. That's when Dirac opened up. He has a nerdy look but was a source of much trouble. Tamm was rowing and could not make the boat go straight, no matter how hard he tried. Dirac was teasing him relentlessly. Eventually Tamm got angry and pretended to go to sleep. Then the two of them (I was not participating, obviously) started pouring water on poor Tamm and soaked him completely. Things escalated to a wrestling match, and so it went.
Tamm's correspondence with Dirac can be found in this article by Alexei Kojevnikov. It was not only physics that Dirac picked up in Leiden:
I have not forgotten the cycling that I learnt in Leiden, and have already cycled about 2000 km. in the neighbourhood of Cambridge.
_{(Dirac to Tamm, January 3, 1929).}