Müller-Lyer illusion
≈ 2 MIN · 4 TRIALS · Perception
What this is about
Two lines of equal length, one with outward, one with inward fins — and the second looks shorter. Franz Müller-Lyer described the figure in 1889; it became the most-studied evidence that perception is organisation, not depiction: the eye does not measure, it interprets.
Knowing does not protect: the illusion stays even once you know it. In matching designs like this one, most people misjudge by a few per cent — depending on fin angle considerably more; cross-cultural studies also found its strength varies with the visual environment.
How it runs
- Four trials with random reference length and random starting value.
- Adjust the lower line with the slider (arrow keys work) until both LOOK equally long.
- “Looks equal” confirms; after the fourth trial comes the finding.
Runs entirely in your browser — nothing is transmitted or stored.
You can stop the run at any time.
Your finding
What it means
Limits of this experiment
- Screen size and viewing distance are uncalibrated.
- Four matches, one direction — the original varied angles and methods.
- The slider allows whole pixels only; fine differences vanish.
- The first trial is discarded — it carries getting used to slider and figure; the three after it are scored.
→ Case file 020 · WERTHEIMER→ Topic: Measurement and Method→ Glossary: Gestalt