How Do Still Pictures Start Moving? Look Through a Slot
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A spinning drum with slits cut in its side turns a strip of drawings into a moving image. The device explains what cinema does and the usual explanation for why it works is wrong.
How the device works
A drum open at the top spins on a spindle, with a strip of sequential drawings around the inside and a row of narrow vertical slits cut in the wall above them. A viewer looks through the slits at the drawings on the far side of the drum. Each slit passing the eye gives a brief glimpse of one drawing before the wall cuts it off, so the eye receives a rapid series of separate still images rather than a continuous blur. Without the slits the whole thing smears into nothing, which is easily demonstrated by looking over the rim instead of through them.
What the slits are doing
The narrow openings solve the problem that makes the whole thing possible:
- •A moving image on the retina blurs, so movement must be interrupted
- •Each slit exposes one drawing for a very short time
- •The wall between slits blanks the view while the next drawing arrives
- •So the eye sees a sequence of still pictures, not a moving strip
- •Film projectors do the same job with a rotating shutter
- •The blanking is as important as the images
Why persistence of vision is the wrong explanation
The standard account says that an image persists briefly on the retina, so successive images overlap and are perceived as continuous, and that explanation has been in textbooks for a century and is not adequate. Persistence explains why the gaps between images are not seen as darkness, which is real, and it does not explain why anything appears to move, since overlapping still images would produce a superimposed smear rather than motion. Motion perception is an active process in the visual system, which infers movement from changes between successive views, and the effects responsible are studied under the headings of apparent motion and the phi phenomenon. Textbooks have been slow to change.
How to make the strips work
Building a working strip involves constraints that are easy to get wrong and that explain why early commercial strips look as they do. The number of drawings must match the number of slits, or the sequence drifts round the drum and the image creeps sideways as it plays, which is either a fault or a deliberate effect. The action must loop, since the last drawing is followed immediately by the first, so anything that does not return to its starting position produces a jump. Backgrounds are usually plain, because detail flickers badly through narrow slits. And the drawings must be spaced evenly or the motion stutters.
The family of devices
The drum belongs to a sequence of nineteenth century inventions that led directly to cinema. A disc with slits around the edge, viewed in a mirror, came first in 1832. The drum followed in 1834 and was commercially successful from the 1860s, since several people could watch at once. A mirror drum in 1877 replaced the slits with a ring of mirrors at the centre, which is brighter. Flip books appeared in the 1860s. Each solved the same problem of presenting separated still images in rapid sequence, and projection onto a screen rather than viewing through an aperture was the remaining step.
The takeaway
Slits in a spinning drum expose each drawing briefly and blank the view in between, so the eye receives separated still images rather than a smear, and a film projector's shutter does exactly the same job. Persistence of vision explains why the gaps are not seen as darkness and does not explain the motion, which the visual system infers from changes between successive views.