Prague, 4 February 1912

On a series of scientific subjects, including the earth's radius, residual rays and photochemical processes – as well as Einstein's dislike of lecturing and the lack of good students in Prague.

Einstein disagrees with a remark of Besso's about 'the influence of refraction on the earth's radius as derived from the size of the horizontal radius', and provides a diagram to demonstrate what he means. Returning to the question of residual rays and the infrared proper frequency of salt (discussed in his letter of 26 December 1911), Einstein admits that he was in error in his critique of Heinrich Rubens's findings: 'However, this does not mean that there are two proper frequencies': a diagram explains the effect of the reflective cap on the theoretical curve, and he has written as much to Rubens. 'In my last paper I proved (in a thermodynamic way) that the radiant energy Nhν must always be absorbed in the photochemical decomposition of a gram-molecule by Wien's light ... So one does not need the quantum hypothesis for this'. Warburg has written to him about a confirmation of this in one specific substance. Meanwhile, Max Abraham has been working on 'the new gravitation theory', although they still have differences of opinion, and '[a] Berlin astronomer is working very hard on the verification of the deviation of light rays around the sun. He has all the solar eclipse photographs sent to him and measures them'.

Besso has been writing about his dislike of lecturing, a feeling Einstein shares ('For this reason I give them all a wide berth'). Einstein wishes he had better students in Prague – 'but the lack of interest for my beautiful field is grievous amongst the students. In my seminar I have one decent male student and otherwise only two halfway useful female students'. In any case, his appointment to the Zurich Polytechnic has just arrived, so they will be moving at the end of July. Habicht has made another important improvement to his 'Maschinchen' (referred to in previous letters). Mileva Einstein's postscript expresses her delight at the prospect of leaving Prague for Zurich, and refers again to their visits to the theatre and the comic opera.