A box of foggy air let physicists photograph a single atom's footsteps
Charles Thomson Rees Wilson built a sealed chamber of supersaturated vapour so sensitive that a single charged particle flying through it leaves a visible trail of droplets, like a jet's contrail. He described the device in a 1911 paper, after years spent trying to recreate cloud formations he'd first observed from a Scottish mountaintop. The cloud chamber let physicists see, and photograph, subatomic particles directly for the first time, and was used to discover the positron in 1932.