Positronium reveals its first excited state

The way has been paved to test quantum electrodynamical predictions of the Lamb‐shift‐like transitions in positronium. Karl F. Canter, Allen P. Mills Jr and Stephan Berko of Brandeis University recently observed¹ the long‐sought radiation from the first excited state of positronium with a method that makes it feasible to measure the fine structure of the $n = 2$ state. That measurement is now in progress.