Since 1934, Egon Orowan, Michael Polányi, and G. I. Taylor had independently proposed the existence of dislocations — linear defects in the crystal lattice of metals — as a theoretical mechanism to explain why real metals deform plastically at stresses far lower than predicted for a perfect crystal. For more than twenty years, dislocations remained a mathematical construct: no one had ever observed them. An intermediate technical step proved decisive in making direct observation possible: around 1949, Robert D. Heidenreich, at Bell Labs, developed an electrolytic thinning method capable of producing metal sheets thin enough to be penetrated by an electron beam, and applied Hans Bethe's dynamical diffraction theory — rather than kinematic theory, which proved insufficient — to interpreting the resulting images. Heidenreich did not manage to resolve individual dislocations with the 50 kV instrument he had available, but his work on sample preparation and diffraction-contrast theory was, according to later accounts by the protagonists themselves, the route by which the Cambridge group — via a visit by Michael Whelan to Bell Labs — learned the techniques they would apply shortly afterward with the higher-voltage Siemens Elmiskop I microscope. In 1956, at the Cavendish Laboratory in Cambridge, Peter B. Hirsch, Robert W. Horne, and Michael J. Whelan, working with that 100 kV transmission electron microscope on thin aluminum foils, directly observe for the first time the arrangement and individual motion of real dislocations inside a metal. The finding occurs partly by chance: upon removing the condenser aperture, a sudden flood of electrons locally heats the aluminum foil, setting into visible motion several dislocations pinned at a subgrain boundary. The team succeeds in resolving dislocation networks, nodes, tilt boundaries, and the cross-slip phenomenon of screw dislocations, demonstrating that the transmission electron microscope can reveal crystal defects at a scale unreachable by the optical microscope. The work provides the first irrefutable visual proof of an entity that until then had existed only as an abstract model, and shifts the study of materials from a mostly empirical discipline toward a science based on direct observation of physical mechanisms. The diffraction-contrast technique developed from this work becomes a standard tool of modern materials science, with applications later extending to semiconductors and aerospace alloys.