Phenakistoscope

Cut a disc of stiff card 250 mm in diameter. Stiff matters — a floppy disc wobbles and the figure jumps out of position.

Mark the centre and divide the circumference into 12 equal sectors of 30°. Use a protractor or step the radius round the circle six times and halve each arc.

Cut a radial slit 2 mm wide and 25 mm long near the rim on each dividing line — 12 slits for 12 drawings. Equal numbers is what makes the image hold still instead of creeping round.

Draw one phase of the motion in each sector, all radiating from the centre with their feet toward the rim. A bouncing ball or a walking figure works; twelve frames is one full cycle.

Make frame 12 flow back into frame 1. A disc has no start, so any sequence that does not close shows a visible jolt once per revolution.

Push a pin or a dowel through the exact centre into a handle so the disc spins freely. Off-centre mounting makes the whole image swim.

Hold the disc with the drawings facing the mirror and your eye at the slits. You are looking at the reflection through the slits — this is the one genuinely awkward thing about the device, and there is no way round it.

Spin the disc and adjust the rate until the motion looks smooth. Time ten revolutions and multiply by 12 to get images per second.

Let it slow until the motion breaks into separate flashes and record that rate too. The gap between those two numbers is the working range of every projected moving image since.

Cut a second disc with one slit fewer than the number of drawings. The figure now creeps slowly around the disc as it animates, because each glimpse arrives slightly out of register. Deliberate stroboscopic drift.

Cut a third disc with 5 mm slits and compare. Wider slits give a brighter, blurrier image because more movement happens while the slit is open — the same exposure trade every shutter makes.

If you have a zoetrope, run both. The zoetrope needs no mirror and lets several people watch at once; the phenakistoscope is sharper and brighter but strictly one viewer. That single difference decided which one the Victorian toy trade sold.

1832, invented twice. Joseph Plateau, a Belgian physicist, produced the phenakistoscope in 1832. In late 1832, independently, the Austrian mathematician Simon von Stampfer arrived at essentially the same device, which he called the stroboscopic disc. Simultaneous independent invention is common in this period of optical toys, and neither man was copying the other.

Plateau's two discs. His arrangement used two discs on one spindle — one carrying equidistant radial windows to look through, one carrying the sequence of images. Single-disc versions viewed in a mirror became the standard form because they are far cheaper to make.

The explanation of the day. It was described in terms of persistence of vision, with an impression said to remain on the retina for roughly a fourteenth of a second. As with the thaumatrope and the zoetrope, that account is now considered insufficient: flicker fusion and apparent motion are handled by mechanisms beyond the retina. The device works exactly as it always did; the textbook explanation attached to it has been revised.

Why it lost. The mirror is the problem. One person at a time, at arm's length, facing a mirror. Horner's slitted drum of 1833 and especially Lincoln's 1867 zoetrope with interchangeable strips let several people watch and let one machine show many sequences. The phenakistoscope survived as a printed object rather than a device — the lithographed discs of the 1830s are collected as some of the finest colour printing of their decade.

Where the line goes. Thaumatrope, phenakistoscope, zoetrope, praxinoscope, projector. Each one solves a specific defect of the one before: two images become a sequence, the mirror becomes a drum, the slit becomes a mirror ring, and finally a lamp is put behind the strip. The whole of early cinema is in that list.