In 1947, the first true mesons, the charged pions, were found by the collaboration lead by Cecil Powell at the University of Bristol, in England. The discovery article had four authors: César Lattes, Giuseppe Occhialini, Hughes Muirhead and Powell. Since the advent of particle accelerators had not yet come, high-energy subatomic particles were only obtainable from atmospheric cosmic rays. Photographic emulsions based on the gelatin-silver process were placed for long periods of time in sites located at high-altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains, where the plates were struck by cosmic rays.