D O C U M E N T 9 0 O C T O B E R 1 9 2 5 1 0 7 I believe I may conclude from this that your set of variations (insofar as one iden- tifies with ) for charge distribu- tions as we sometimes imagine them (and can realize macroscopically) conflicts with the Maxwell equations. If I am right with this, then I cannot see what else one could do with this variation principle. I still have to tell you that during the assembly of the Union of Physics[7] (chair- man: W. H. Bragg,[8] London secretary: H. Abraham,[9] Paris), held at the begin- ning of July in Brussels, on the day before the International Research Council[10] met, it was decided to convoke an international physics congress only at a time when the Germans can also participate. If I remember correctly, I wrote this to Planck[11] as well. As regards the Research Council, a way out of the problem that arose this sum- mer is still being sought. Maybe the executive committee, which is holding a meet- ing that had been arranged specifically to address this issue, will find a solution. I feel optimistic Locarno will surely also have a positive influence on it.[12] Last week we had the first meeting of the management committee of the Institute of Intellectual Cooperation in Paris. It will interest you that Mr. Eisler is among the eight nominated “adjutants.”[13] A Dutchman is also among them, namely, the as- tronomer de Vos van Steenwyk,[14] a teacher at the Lyceum in The Hague and for- mer observer at the Leyden Observatory. With kind regards, yours truly, H. A. Lorentz Postscript Determination of the quantities . We start from your equation (10a), which we can write with the previous assumptions in the following form (22) where … introduces a known differential expression. We will consider the ’s and their derivatives as known quantities in which we want to express and . Because the course of the calculation is what is involved here, we do not need to explicate the known quantities, which we write on the right-hand side of the equa- tions. Exchange ν and α in (22) and subtract the new equation from (22) (23) Exchange ν and μ and add the new equation to (23) (24) ψ23, ψ31, ψ12, ψ41, ψ42, ψ43 Ex, Ey, Ez, Hx, Hy, Hz Γαν μ gννΓμα ν gμμΓαν μ gμνϕα gμαϕν + + + … = Γμν α Γμν α ϕα gννΓμα ν gμμΓαν μ gααΓμν α gννΓνα μ ––+ … = gμμΓαν μ gααΓμν α – gννΓαμ ν gααΓνμ α –+ … =