[playful page] A toy, not a test. It runs in your browser; nothing you do here is sent anywhere, and the only thing kept is a "played" tick in your own browser so the toy shelf can show it.

TOY #2 OF 6 works with a mouse, a finger or a keyboard

wo lines. The top one has fins pointing outward and stays put. Use the buttons to stretch or shrink the bottom one, the one with arrowheads, until the two shafts look the same length. Then press "they look equal".

This toy needs JavaScript. The notes below still say what it would have shown.

What it shows. A line between outward fins looks longer than the same line between arrowheads. Franz Müller-Lyer published the figure in 1889 (Müller-Lyer, 1889). Most people set the arrowhead line too long to compensate. With the fins on again after the reveal, the top line probably still looks longer: knowing does not switch it off. Segall, Campbell and Herskovits tested people in many societies and found that how strongly the illusion worked differed between groups (Segall, Campbell & Herskovits, 1963) [paraphrase, not a quotation]; they suggested that growing up among rectangular buildings and straight edges plays a part, an idea still argued over. What it cannot show. One setting, on one screen, in whole pixels, by someone who may already know the trick. A lab would use many trials, randomise which line is fixed, and vary the fin angle. Why the illusion happens has several competing explanations; this toy does not choose between them.

Sources

1. Müller-Lyer (1889). Optische Urteilstäuschungen. Archiv für Anatomie und Physiologie, Physiologische Abteilung, Supplement, 263–270. The paper that introduced the figure; no DOI. Scan of the 1889 supplement at the Biodiversity Heritage Library. www.biodiversitylibrary.org/page/35372625
2. Segall, Campbell & Herskovits (1963). Cultural Differences in the Perception of Geometric Illusions. Science. doi:10.1126/science.139.3556.769

see also

	replication — which findings hold up, and in whom
	Müller-Lyer illusion — in the glossary
	test patterns — more things to look at hard