Müller-Lyer Illusion


Don’t press the button “Ruler” yet!   Rather:
Grab the central arrow with your mouse and drag its tip to the centre between the outer arrowheads.
Now you may press “Show Ruler” and learn how you fared…


Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player


I hope you have enjoyed the “Müller-Lyer Illusion” (1889). When one bisects the horizontal line it is quite normal that large errors are made, so don’t be annoyed or disappointed (I err by at least 1 cm).


This phenomenon is placed in the category “size constancy” because I find the perspective explanation (see bottom) very convincing. Müller-Lyer's original figure showed two double-ended arrows, here I used the Brentano variant.


The German satirical magazine ‘Pardon’ published in ≈1970 an article on this illusion. It was accompanied by the two pictures on the below, demonstrating a Müller-Lyer effect on enclosed objects ;-). [The right elephant not only looks larger, but also happier…]


Müller-Lyer Illusion     Müller-Lyer Illusion


The picture on the below (from Amazingart) demonstrates the perspective explanation (Gregory 1968): When you place the mouse pointer over the picture, the blue lines indicate that the two red arrows are of equal length. The arrow configuration “angles in” (near the ticket counter) is always the front side of an object, the “angles out” configuration occurs at the far end of a room, for instance (here next to the door). So, given no further information (as in the Müller-Lyer arrangement at top), the brain assumes the “angles in” configuration to be closer, computes size constancy on it, and –given identical retinal size of the two angle arrangements– concludes that the “angle in”-line is shorter.


Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player


Some sources


Müller-Lyer FC (1889) Optische Urteilstäuschungen. Archiv für Physiologie Suppl. 263–270

Brentano F (1892) Über ein optisches Paradoxen. Zeitschrift für Psychologie 3:349–358

Müller-Lyer FC (1894) Über Kontrast und Konfluxion. Zeitschrift für Psychologie IX p 1 / X p 421

Lewis EO (1909) Confluxion and contrast effects in the Mueller-Lyer illusion. Brit J Psychol 3:21–41

Rudel R & Teuber HL (1963) Decrement of visual and haptic Müller-Lyer illusion on repeated trials: A study of cross-modal transfer. Quarterly Journal of Experimental Psychology 15:125–131

Segall HH, Campbell DT, Herskovits MJ (1966) The Influence of Culture on Visual Perception. Bobbs-Merrill, Indianapolis

Dewar RE (1967) Stimulus determinants of the magnitude of the Mueller-Lyer illusion. Perceptual and Motor Skills, 24 , 708–710

Gregory RL (1968) Perceptual illusions and brain models. Proc R Soc Lond B Biol Sci 171:24279–296Christie PS (1975) Asymmetry in the Mueller-Lyer illusion: Artifact or genuine effect? Perception 4:453–457

Restle F & Decker J (1977) Size of the Mueller-Lyer illusion as a function of its dimensions: Theory and data. Perception & Psychophysics 21:489–503

Adam J & Bateman L (1983) A correlational analysis of symmetry between the arrowhead and featherhead Mueller-Lyer illusions. Perception 12:119–129

Pressey AW & Martin NS (1990) The effects of varying fins in Mueller-Lyer and Holding illusions. Psychol Res 52:46–53



Created: 2002-08-16

Last update: 2013-10-04