2 6 6 D O C U M E N T S 2 5 2 , 2 5 3 A P R I L 1 9 2 6 252. From Emil Rupp [Heidelberg,] 11 April 1926 Dear Privy Councillor, Delayed by the holidays, I gained possession of your valued letter of 31 March[1] yesterday and thank you very much for the communicated trains of thought. As nothing more is lacking for me to perform the experiments, I shall begin right away with the execution of the proposed experiments in the coming week. I believe that I can carry out all the trials with the required precision. The only reservation I have is whether I can measure precisely enough the slight effect of the 2nd-order Dop- pler effect, because the velocity distribution & the low ray intensity here stand in the way. I shall keep you informed about the progress made in the experiments. As pertains to the publication, I very gladly agree to your offer of a joint publi- cation. I do not need to particularly point out to you the attitude of my boss toward a research collaboration or even toward an exchange of ideas.[2] However, that is practically insignificant for the execution of the experiments themselves, because I have all the required tools at my disposal. Respectfully and sincerely yours, E. Rupp 253. To Paul Ehrenfest [Berlin,] 12 April 1926 Dear Ehrenfest, I. From the point of view of the momentum theorem, the two cases you sketched[1] would not necessitate the introduction of angular momentum to a quantum. The principle of the conservation of angular momentum can be satisfied (seen from the direction of the magnetic field) in your examples like this: [2] Quantum approaching off-center at absorption [3] quantum emitted off-center [4] recoil onto the molecule [2] [3] [4]