Prague, 26 December 1911

A long analysis of a faulty experiment on residual rays, with comments on electron theory, the failings of the Solvay Congress, an important proof about spectral lines – and news of an old girlfriend.

'I recently wrote a short paper about residual rays...'. Einstein goes on to discuss at length some experimental findings by Heinrich Rubens, which he considers wrong, providing a series of diagrams showing Rubens's measurement of the residual rays of sodium chloride, the expected finding (as an even series of waves) and the actual result ('a sort of beats'): 'Rubens interpreted this as a sort of beat, produced by the fact that there are two almost monochromatic residual rays in a bundle, and NaCl has two proper frequencies'. Three further diagrams show the results of the analysis of a single proper frequency using dispersion theory, and showing the reflectivity, and finally showing the true curve.

The letter opens with a reference to the failed attempts at the Solvay Congress to base radiation theory on electron theory: 'I made no further progress with electron theory. In Brussels too they acknowledged the failure of the theory with lamentations, without finding a cure. The congress there generally very much resembled a lamentation over the ruins of Jerusalem. Nothing positive came from it...'. There is positive news of Habicht's 'Maschinchen' (for measuring small quantities of electricity), which has had a successful demonstration in Berlin; and there are many other interesting scientific matters to talk about – too many to commit to paper: 'As the most important result to have been confirmed by physicists seems to me the proof that spectral lines can be emitted and absorbed by neutral metal vapours'. Einstein concludes with a brief mention of his impending return to a post at the Zurich Polytechnic, and a reference to his former romantic attachment to Besso's sister-in-law Marie Winteler, with a hint of residual guilt: 'I am very pleased to hear of Marie's marriage. Thus dwindles a dark point in my life...'. The accompanying letter by Mileva Einstein expresses her relief that they will be returning to Switzerland soon from Prague, and sends news of their recent activities ('We go often to the theatre and concerts'), and of the first schooldays of their eldest son, Hans Albert, in particular regarding his religious education.