Phenakistiscope: A Spinning Disk You Watched in a Mirror

Count the slits on Cooper Hewitt disk 1948-124-4 and you get 10, spaced evenly near the rim. The disk is a phenakistiscope: a card printed with rings of drawings on one face, spun in front of a mirror. You turn the printed side toward the glass and look with one eye through the slits. What you watch is the reflection, and in it the drawings move.

Then count the drawings. Their number decides what they appear to do, and only one set of figures on this disk matches the 10 slits. In an 1833 paper the Belgian physicist Plateau explained the rule: with as many figures as slits, they perform their action on the spot; with slightly more or fewer, they travel in one direction or the other.

How do you look at a phenakistiscope?

Open the diagram full size

Plateau described the setup in French, in the Annales de chimie et de physique; the English renderings of his words on this page are this site’s translations. A cardboard disk is pierced near its edge with small openings, and figures are painted on one face. You turn it about its center “vis-à-vis d’un miroir,” facing a mirror, and look with one eye through the openings. The figures then stop sharing the turn of the disk and make movements of their own.

Lutz’s 1920 handbook, Animated Cartoons, gives the same description in English: in his account the toy is a big cardboard disk with pictures on one face, held in front of a mirror and viewed through its slots. Lutz spells it phenakistoscope; Plateau’s title has phénakisticope. This page follows the Cooper Hewitt object title: phenakistiscope.

The slits do the essential work. Plateau explained that each opening lets the eye see the disk for only a very short time. In that moment the disk turns so little that it looks as if it stood still, so each glimpse is sharp. Lutz compared the plan to a film projector: the slits match the open parts of a turning shutter, and the solid card between them matches the parts that block the light.

Same count stays put, one slot more travels

Plateau’s own examples show the rule at work. For dancers doing pirouettes, he wrote, set “un nombre de figures égal à celui des fentes,” as many figures as slits, each a little further into the turn. The dancers then seem to spin in place. For men walking, use a number of figures “un peu plus grand ou un peu moindre,” a little greater or a little less than the openings, depending on which way they should go.

Lutz put the same idea in terms of slots. With equal numbers, he says, the figures stay where they are drawn. With “one slot more,” and the disk turned in the proper direction, the row of drawings appears to be “going around a circle.”

The loop below models both situations with a single dot. Each frame is one glimpse through a slit. The speed is an assumed 3 turns a second, so 10 slits give 30 glimpses a second. Each drawing shows the dot a little further round a small circle of its own, which stands in for an action like a pirouette.

Ten slits: ten drawings stay put, nine travel round

Every frame, side by side

10 slots, 10 drawings

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10 slots, 9 drawings

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On the left, 10 drawings meet 10 slits, so every glimpse finds a drawing in the same spot and the dot circles in place. On the right, 9 drawings sit 40 degrees apart while the disk turns 36 degrees between glimpses. Each glimpse lands 4 degrees short, so the dot works its way round the disk: 360 degrees in 90 glimpses, or 3 seconds.

Counting the Vienna disk

Back to the disk itself. The printed ring around the center hole reads “Josef Bermann Kunsthändler in Wien Graben 619,” German for Josef Bermann, art dealer in Vienna, Graben 619 (this site’s translation). The museum record dates the disk to about 1840 and gives it as 25 cm across, a hand-colored lithograph on paperboard.

This site’s count, made on the full-size museum photograph, finds 10 slits and four sets of figures. The table sets each one against the slits.

The museum description on the Commons file page agrees with that result. It says a small black circle passes forward over a red-and-white circle, and that black and yellow circles rotate around a star. The disk was not spun for this page, so the last column applies the rule to a count. It is not something seen in motion.

Download the disk count and the slot rule (CSV) · source and license notes

Who made the phenakistiscope?

Plateau’s paper opens with a disclaimer. The phénakisticope, he wrote, had been made from the idea he had given for this new kind of illusion. Yet he was “complètement étranger à l’exécution,” a complete stranger to the making of an instrument that left much to be desired. His own carefully drawn models, he added, were being published in London as the fantascope.

His footnote says he developed the idea in a letter of 20 January that year, printed by the Brussels observatory’s Correspondance mathématique et physique. That letter was not read for this page.

Other published writers gave him the credit. In 1834 W. G. Horner opened a paper in the Philosophical Magazine with “the ingenious and amusing invention of M. Plateau.” In 1920 Lutz wrote that the toy’s invention “has been credited to the Belgian physicist, Plateau (1801–1883).”

So the three sources consulted here all point to Plateau, and Plateau himself separates the original idea from the commercial object. That is a record of who was credited, not a ruling on priority. Any rival claim would need its own source.

One viewer at a time

The mirror had a practical limit. Lutz observed that “one person at a time could view conveniently the reflected pictures,” so people tried to project the disk. One version added a lens, a light and mirrors; another put the pictures on a glass disk turning behind a magic-lantern lens.

Horner’s answer was a drum. Drawings inside it, he wrote, would produce “the same surprising play of relative motions as the common magic disk does when spun before a mirror.” His drum needed no “mirror or any second instrument,” and he claimed it could be shown “to unlimited numbers at once.” He named it the Dædaleum; the same open drum is better known as the zoetrope.

The rotating disk did not disappear from later experiments. Lutz writes that Muybridge built his zoöpraxiscope on the same rotating-disk plan, to project some of his animal photographs, the kind of series shown in The Horse in Motion. Lutz’s shutter comparison points the same way, toward projected film such as The Great Train Robbery.

The disk and the stereograph are both cards that do nothing until they meet an eye. One makes motion from a ring of drawings, the other depth from two photographs.