Part of the signature wall as it looks today. The way it looked in the past is shown here.
An experimental physicist called Van Mierlo was working in the Kamerlingh Onnes Laboratory in the 1920's. I believe he was intelligent but he was somewhat dissipated and had not made much progress in his studies. Finally, he had pulled himself together and felt he could go to Ehrenfest for a tentamen. He was rather nervous though, and although it was only 9 AM, he first fortified himself with a swig of "oude klare" (Dutch gin). That was almost worse than perfume, and after a few minutes Ehrenfest kicked him out and told him that he could not come back until a whole year had past.
The boy was furious; he felt that this was unjust and that his knowledge had not been tested at all. So he took his revenge. He went to the colloquium room and there, on the sacred wall, among the names of Einstein and Bohr and others, he wrote in bold characters his name, underlining it with a deep gash in the plaster. When I discovered it, I decided that Ehrenfest would be enraged by this sacrilege and I did not want to get the poor boy into worse difficulties than he was in anyway. So I carefully erased the name – fortunately it was written in pencil – but I did not repair the scratch in the plaster.
The young man left Leiden and went to Amsterdam. I later heard that during the war he was active in the Resistance and that he had been caught and executed by the Germans. A few years ago the scratch in the plaster could still be seen, a curious monument to a nameless fighter in the Resistance.
H.B.G. Casimir, Haphazard Reality (Harper & Row, New York, 1983)
Jan van Mierlo (bottom right), with comrades in the CS-6 Resistance group. Top row, left-to-right: Gerrit Jan van der Veen, Reina Prinsen Geerligs, Tineke Wibaut-Guilonard; bottom row: Joop de Groot, Jan van Mierlo. The photograph was part of a file that the German police had prepared on this group. Here are more photographs from that file.
The student mentioned by Casimir was Johannes Michael van Mierlo. Born in 1907, he studied physics in Leiden and then became a math and physics teacher at a technical school in Amsterdam. He participated in several acts of resistance against the German occupation in the years 1940-1943, operating under the name "Fons van den Berg", until he was betrayed and caught. Jan van Mierlo was executed on October 1, 1943 in the dunes near Zandvoort, together with 18 of his comrades from the resistance group CS-6.
Here is a link to his memorial grave. The story of the CS-6 resistance group is described (in Dutch) in the book Recht al barste de wereld, reviewed here.