Discovery of the positron. This cloud chamber image was taken during the 1932 work by US physicist Carl David Anderson (1905-1991) that led to the discovery of the positron. This particle is the opposite of the electron and the first antimatter particle to be discovered. The image shows the curved track of a positive particle entering the cloud chamber from below. The particle is known to be positive because of the direction in which it bends in the chamber’s magnetic field. The track is too faint to be caused by a proton, and is more like an electron’s track, hence it had to be the predicted positron. These results were published in 1933

« It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.  »
(Ernst Rutherford , 1911)

Diagram of cyclotron operation from Lawrence’s 1934 patent.