from the early 20$^{\mbox{th}}$ century to study radioactive radiation, but it had not been sensitive and reliable enough until the improvements that Powell and his collaborators introduced.[^34] They improved the treatment of the material, the research technique and the optical equipment for analysing the tracks. In the meantime, new emulsions, more concentrated and sensitive, were produced by Ilford, so that in those years the photographic method was to become one of the most precious tools of particle physics.

In particular, a few months after Pancini, Piccioni, and Conversi experiment, in May 1947, Powell, Occhialini and their collaborators in Bristol presented two events, found on photographic plates exposed at the Pic du Midi (2800 meters altitude), both showing a meson coming to the end of its range in the emulsionwhich required hours of patient observation, was of course precious and could significantly improve the scientific results of a laboratory. In Padua, there were in those years about 12 to 15 “osservatori”; many of them later obtained a diploma and became specialized technicians within the Institute of Physics. The knowledge acquired at that time on photographic plates and the contacts set up with Bristol and Bruxelles were to bring Padua, as we will see, to play a leading role in the European cosmic-ray collaborations of the early ‘50s. In those years, collaborations were also set up with other Italian Universities, in particular, as for photographic plates researches, with Bologna, Genoa (with Alberto Bonetti and his group), Milan and Pavia. Riccardo Levi Setti, for instance, who was working in