friend and teacher TH. SEXL. But also as assistant and co-worker of E. FUES and R. THIRRING I frequently worked on these problems, constantly using new methods and lines of approach. During the last years of the war and the first ones afterwards Ihad the fortunate opportunity to receive many stimulating ideas and comments on my work from A. SOMMERFELD on the occasion of my frequent visits to Munich. Especially this last period, although partially connected with personal difficulties and troubles of many kinds stemming from the turbulence of lost-war readjustments, I consider to be one of the most valuable times in my life.