Berlin, 12 February 1915

'How perfidious nature is': reporting on two important proofs, the first of the redshift of spectral lines, the second on Ampère's molecular current hypothesis. Einstein also mentions his separation from Mileva, and the beginning of his relationship with Elsa Einstein.

Einstein has 'two fine things to report', of which the first is 'Gravitation. Redshift of spectral lines. Spectroscopic double stars have the same mean velocity in the radius of vision. The masses of the stars are revealed in the Doppler periodic line fluctuation. The components of greater mass should exhibit a mean redshift of spectral lines against the components of smaller mass. This is confirmed. In that the radii of the stars can be estimated (apparently from the spectral type), thus an approximate quantitative test of the theory is revealed, which is satisfactorily accurate'. The second of his discoveries is the 'experimental confirmation of Ampère's molecular current hypothesis' – Einstein sets out the formulae, and goes on, 'If the magnetisation of a small hanging rod is reversed, it experiences an axial rotation moment, whose existence I together with Mr De Haas have demonstrated through experiments at the Reich Institute ... With it the existence of a "zero point energy" is proven in a single case. A wonderful experiment ... And how perfidious nature is, when one wants to approach it experimentally! I am developing in my old age a passion for experimenting...'

At the outset of the letter, Einstein also reports more personal news: he has recovered from a bout of influenza; and he provides Mileva Einstein's new address in Zurich: 'I am very satisfied with the separation, although I hear very seldom from my boys. The peace and the tranquillity do me a lot of good, and no less the very beneficial, really sweet relationship with my cousin, the durable nature of which is guaranteed by the renunciation of any marriage'.

Elsa Einstein, who was to become Albert's second wife in 1919 after his divorce from Mileva, was also his cousin twice over (their mothers were sisters, whilst their fathers were first cousins). Their relationship had begun nearly three years previously, in April 1912.