TWENTY-FIVE years ago Dr. Zeeman, working at Leyden in the laboratory of Prof. Kamerlingh Onnes, achieved the epoch-making discovery which is now so abundantly familiar to physicists. Almost simultaneously he was appointed professor of physics at Amsterdam. To celebrate these e\ents a volume of his collected papers, bearing on this branch of magneto optics, has been published, under distinguished editorship, with a portrait and a few editorial notes and minor corrections. Verhandeligen van Dr. P. Zeeman over Magneto-Optische Verschijnselen. Pp. xv + 341. (Leiden: Eduard Ijdo, 1921.)