2 6 0 D O C U M E N T 2 6 0 A U G U S T 1 9 2 8 to slowly moving masses. is thus easy to calculate. For the spacelike indices , one can simply replace by . The second of the equations I yields , since it can be thought of as necessary only for the external field alone. If we want to take into account only terms up to the 2nd order, then, due to the above equation for , we must set i.e., the terms of order must be considered. Then our equation for spacelike becomes or, leaving off the terms that are quadratic in the , These are the equations of motion, where of course the must still be calcu- lated.— Mathematically, all this is still rather ugly. But when it has been made more or- derly, then we can calculate analogously how the singularities behave in our new theory. For this method is evidently transferable. At present, I request that you read this and try to clean up the matter mathematically, to smooth it out and to work it out fully before we continue to work on the new problem. We will thereby create an instrument that can be utilized for the interpretation of every relativistic field theory. Please send this letter back to me the next time that you write, so that I can keep everything together. With warm greetings, yours truly, A.E. i x ----------- - x ---------- - + 0 = 4 = i 0 0 0 j1 r - - - . - - - -. j1 · r . . r – = 3 2 d dt ---- - -- r A r ----- - + 0 = · ·· A + 0 = A