Just once in its long history has a Royal Medal been awarded but not presented. John Tyndall FRS (1820-93) was the chosen recipient in 1853 for his early work on diamagnetism but declined to accept it. The story of why Tyndall felt compelled to turn down this considerable honour sheds light on the scientific politics and personal relationships of the time, on the importance given to the study of magnetism, and on Tyndall's own character and career.