I N T R O D U C T I O N T O V O L U M E 1 4 l v During Ilse’s illness and convalescence, Betty Neumann replaced her as Ein- stein’s secretary (Doc. 202). Neumann was a first cousin once removed of Hans Mühsam, Einstein’s friend and physician, with whom he was collaborating at the time on a technical paper on the “Experimental Determination of the Pore Size of Filters” (Doc. 72), completed before 2 July 1923 (Doc. 73). Sometime in the summer of 1923 Einstein’s professional relationship with Betty Neumann, who was twenty-one years younger, took on a personal character. But by early August, when it became apparent that Ilse’s return to work would be de- layed, Einstein suggested that Neumann, who hailed from Graz, remain in Berlin and continue to work as his secretary. He wrote to her that he wished to “count her among those close to me” and take care of her, as one would “for a young daugh- ter.” Aware that they “liked each other,” he warned her not to speak to anyone about their relationship, except to Hans Mühsam (Doc. 98). From Leyden, in late Sep- tember, he urged Betty to write to him. He reassured her that he would be “vigi- lant,” so that her letters would not “fall into the wrong hands,” but ultimately would be indifferent if they did, since the main thing was to receive something from her “small paws.” A month later, he complained that he was “not allowed to run after my Betty” (Docs. 126, 135). On the eve of his sudden departure from Berlin to Leyden on 7 November, Ein- stein envisaged taking up a professorship at Columbia University, residing outside New York, and having Betty live with him and the rest of his family (Doc. 140). But a week later, he had reconsidered the move to the United States. The “triangle geometry” of his relationship was becoming too much for him, he avowed. He pit- ied Elsa, and felt “helpless and resigned” to the reality of not being allowed to lead Betty, whom he loved, “through life” (Doc. 151). It appears that, in early December, Betty Neumann became engaged to a man twenty-five years her senior, but by early January 1924, soon after Einstein’s return to Berlin, the engagement seems to have been called off. Einstein advised her to look for someone ten years younger than himself, and confessed feeling “trapped”: “fate is merciless, even in relation to someone as envied” as he was (Docs. 174, 195). However, in another turn of events, by the end of the month he wrote that he had reached a “true peace treaty” with Elsa, according to which he had “the right” to meet Betty two afternoons a week (Doc. 201). But he pleaded with her that she visit him, rather than he visit her, as Elsa was “afraid” that he would make her look “ridiculous” if he visited a stranger’s house “out of love for [her]” (Doc. 203). In June 1924, remarking on their age difference, he again urged Betty to look for some- one “twelve years younger” than he. Yet, in the meantime, they could continue to meet (Doc. 262). While away in Geneva in late July 1924, he confessed that he missed her “in quite an alarming manner in this lively lonesomeness” (Doc. 294). During his vacation with his sons in Bavaria in early August, he foresaw returning