Zoetrope
A spinning cylinder with vertical slits — the 19th-century device that proved motion is an illusion the brain constructs from discrete still images. Look through the slits as the drum rotates and watch static frames fuse into fluid animation. The same persistence of vision that makes cinema, television, and every screen you own possible.
Persistence of vision
The zoetrope works because your visual system retains an image for roughly 1/25th of a second after it disappears. When the slits pass your eye in rapid succession, each brief glimpse shows the next frame in the sequence. Your brain fills the gaps, merging the discrete snapshots into continuous motion. This is persistence of vision — the same mechanism that lets you see a continuous image on any screen refreshing at 24 fps or higher.
From toy to cinema
William George Horner described the zoetrope in 1834, though similar devices date back to 180 AD China. The slits act as a shutter — without them, the images blur together into a smear. Each slit freezes one frame for a fraction of a second, just long enough for your retina to register it before the next slit reveals the next frame. The same principle drives the rotating shutter in a film projector and the sample-and-hold behavior of LCD pixels.
Frame rate and flicker fusion
Increase the number of frames and the motion becomes smoother — but only up to a point. Beyond the flicker fusion threshold (roughly 50–90 Hz depending on brightness and peripheral vision), individual flashes merge into steady light. Below it, you perceive flicker. Early cinema ran at 16–24 fps, relying on a two-bladed shutter to double the flash rate and push it past the threshold. The zoetrope is the ancestor of every trick used to turn samples into streams.