Figure 2. The famous observation of the τ meson in nuclear emulsion exposed to cosmic rays. This was the first clear indication of the existence of the many heavy mesons which have now been identified. The experiment used electron sensitive emulsion, which had just been produced, and was carried out by Brown, Camerini, Fowler, Muirhead, Powell and Ritson in 1948. The τ enters the emulsion from the top of the photograph; b and c are π+ mesons; a is a π– meson which gives rise to a star at B.