Patrick Blackett, an expert in cloud chambers at the Cavendish Laboratory in Cambridge, partners in 1931 with the young Italian physicist Giuseppe Occhialini, who contributes the Geiger-Müller counter coincidence technique learned under Bruno Rossi in Florence. Together they invent the counter-triggered cloud chamber: two Geiger counters placed above and below the chamber only trigger its expansion when a particle passes through both, so that cosmic rays "take their own photograph". With this device, in autumn 1932 and early 1933 they record hundreds of photographs of particle showers diverging from a common point. After discussing their results with theoretical physicist Paul Dirac, they interpret the phenomenon: cosmic gamma rays produce electron-positron pairs upon interacting with matter, systematically and quantitatively confirming the positron Carl Anderson had detected in isolation at Caltech months earlier, and experimentally verifying the prediction of the Dirac equation. The finding, published in "Some Photographs of the Tracks of Penetrating Radiation" (Proc. Roy. Soc. A 139, 1933), also confirms the reverse process of electron-positron annihilation into gamma radiation. This work is also intertwined with the recent discovery of the neutron by James Chadwick (1932, same Cavendish Laboratory): in his May 1933 Bakerian Lecture, Chadwick notes that beryllium radiation — the same radiation he had identified as neutrons — also appeared to produce positrons, and openly asks whether these come from the neutron itself or from the accompanying gamma radiation. That question motivates, that same year, a second, complementary experiment to the cosmic-ray one: Chadwick, Blackett, and Occhialini place a polonium-beryllium source next to a cloud chamber with a lead target and show that beryllium radiation, upon passing through lead, also produces positrons ("New Evidence for the Positive Electron", Nature, 1933) — confirming, via a nuclear source distinct from cosmic rays, the same pair production. The discovery of the neutron thus acts as a direct conceptual precursor and not merely as shared institutional context: it opens the concrete experimental question the second 1933 experiment comes to resolve. Together this constitutes the first solid, reproducible demonstration of the existence of antimatter as a systematic physical phenomenon, and the counter-triggered cloud chamber becomes a standard tool of cosmic-ray physics for the following two decades.