Anderson positron
Historical photograph of one of the first positron trajectories observed by Anderson in 1933 in a cloud chamber. A magnetic field deflect the particle trajectory and the insertion of a lead plate allow to determine the direction of its movement (arrow). The deflection indicates a positive particle (an electron would have followed the trajectory in red) which could not be a proton whose path would be much shorter.
C.D. Anderson, Physical Review 43, 491 (1933).