Autograph letter signed (‘P. Zeeman’) to Samuel A. Goudsmit, in Dutch, thanking the recipient for sending a copy of his book on Atomic Energy States and adding ‘The book will be very useful. It is more useful on the issue than the Tables Annuelles. Subsequent editions of your and Bacher’s book will prove to be indispensable […] Did you see Zwikker’s book, the Leerboek der Optiek? It is written in a refreshing and vivid way. It is too bad that the magnetic splitting has been treated completely wrong and even by a student of mine! I am already having some pleasure from a mass spectrograph that I installed following Thomson’s parabola method. By experiment I can indeed prove that […] the percentage H1 H2 had increased by fractionated evaporation and rectification. He found the specific gravity of the gas obtained through his method the specific gravity of the gas obtained through his method to be 1.51-+ 0.005% higher than that of ordinary hydrogen. With the mass spectrogram you see immediately that H2 has to be present. […] Recently we had Francis Perrin to visit. Very interesting. Holweck was going to come too, but he had to leave for Algiers to do gravity measurements for a governmental commission. Now we are waiting for Geiger to visit us on the 11th of May’, four pages, pin holes to top left corner, 4to, Amsterdam, 30 April 1933.