How does the Optical Illusion Test work?
You see three classic size illusions, each twice. In each, one figure is yours, drawn in color: drag the slider until it looks exactly the same size as the other, then press Done. The background then fades away and the two figures show at their true sizes, with how far apart they were. The illusion is how far from truly equal you set yours.
What are the Müller-Lyer, Ebbinghaus and Ponzo illusions?
- Müller-Lyer (Franz Carl Müller-Lyer, 1889): two equal lines, one ending in arrowheads (<—>), the other in fins turned outwards (>—<). The one with fins out looks longer.
- Ebbinghaus (Hermann Ebbinghaus, made widely known by Edward Titchener's 1901 textbook): two equal circles, one ringed by large circles and one by small ones. The one among small circles looks bigger.
- Ponzo (Mario Ponzo, 1911): two equal bars between lines that converge like railway tracks. The bar nearer the point where the lines meet looks longer.
In every layout here, yours is the figure the illusion makes look bigger, so being fooled always means setting it too small.
How is your Optical Illusion Test result calculated?
For each setting, the strength of the illusion is how much smaller than the other figure you made yours, as a percentage of its size: set a line 12% too short and the illusion fooled you by 12%. A negative number means you made yours too big, overcorrecting.
Each illusion is measured twice, once with your figure starting 30% too big and once 30% too small, and the two are averaged. People tend to stop a little early, on the side they started from; starting from both sides cancels that out. This is the method of adjustment, one of the oldest ways of measuring what people perceive. Your result is the average of the three illusions, each listed beside it.
Why do these illusions fool us?
No single answer is agreed. Richard Gregory proposed in 1963 that the Müller-Lyer and Ponzo illusions come from the brain correcting for distance: fins and converging lines look like the corners and edges of rooms and roads, so one figure is read as farther away and scaled up. In 1966, Marshall Segall, Donald Campbell and Melville Herskovits reported that people from places with fewer straight-walled buildings were less fooled by the Müller-Lyer lines, which fits that idea. The Ebbinghaus illusion is usually explained by comparison: a circle among large ones is judged against them, and looks smaller.
Knowing about an illusion doesn't switch it off: the lines still look different after you have measured them. For judging sizes without an illusion, try the Line Length Test; for angles, Guess the Angle.