x x x v i i i I N T R O D U C T I O N T O V O L U M E 1 4 manuscript appears to be nonextant, so that the subsequent correspondence only provides indirect information about Weyl’s initial critique (Docs. 33, 36, 40, 45). In his responses to Weyl, however, Einstein made clear that he no longer had con- fidence in the viability of the approach: “I definitely must publish because Edding- ton’s idea must necessarily be thought through to the end [referring to his manuscript of Doc. 52]. I now also believe that all these attempts on a purely formal basis will not advance our physical knowledge any further. It may be that field the- ory has already produced all that lies within its powers” (Doc. 40). A second letter from Einstein to Weyl reveals even more of Einstein’s attitude: “On the whole, a quite resigned mood has come over me concerning the entire problem. Mathematics is fine and good but Nature is leading us about by the nose. Besides, there are some amusing things about it. I wanted to distance myself from you and […] have arrived at the same equation as you did with your special princi- ple of action. I wanted to discard the potentials , but they come back in through the back door. The entire thought has to be worked through and is strangely beau- tiful but above it stands the marble smile of merciless Nature, who gave us more longing than inspiration” (Doc. 45). In a letter to his close friend Heinrich Zangger in May, Einstein wrote: “All that Weyl, Eddington, and I have done recently is pure mathematical speculation, and maybe completely out of place” (Doc. 49). In spite of the resigned tone of Einstein’s reflections after completion of Einstein 1923n (Doc. 52), he revived the affine approach once more in the first half of July 1923. As first mentioned in a letter to Zangger (Doc. 61), and then in letters to Hen- drik A. Lorentz and to Max Born, Einstein indicated that he had realized that there were empirical consequences of the affine theory that he was hoping to be able to test experimentally. We do not know what those envisaged experiments might have been, but they involved ionized gases (Doc. 87) and space charge (Abs. 150), and carried implications for “an understanding of terrestrial magnetism and the electro- magnetic bookkeeping of the Earth” (Doc. 87). Some documents provide additional hints. On the first page of a draft paper that Einstein and Hermann Mark co-authored some time in late summer or fall of 1923 (Doc. 152) we find a proposed experiment, based on a consequence of the affine theory that was expressed by the equation , where the d’Alembertian operator acts on the electromagnetic four-current , and β is a constant. A page of calculations is extant (Doc. 31) on which just this equa- tion is derived from the affine approach of Einstein 1923n (Doc. 52), and a related equation was scribbled by Einstein on Doc. 89. ϕα fμ βfμ = fμ