The big facts page

84 size illusion facts, all checked

Every fact on this site in one place, each linked to Michael Bach’s Optical Illusions & Visual Phenomena, Encyclopaedia Britannica or the Exploratorium. Our build checks each one against the source text; see the sources page.

The Müller-Lyer illusion

1 The Müller-Lyer illusion dates from 1889. Michael Bach: Müller-Lyer illusion
2 Franz Müller-Lyer’s original figure was two double-ended arrows; the version with a single line split in the middle is Franz Brentano’s variant. Michael Bach: Müller-Lyer illusion
3 Asked to find the middle of the line in the Brentano version, big errors are completely normal: Michael Bach, whose illusion collection this comes from, says he is out by about 10%. Michael Bach: Müller-Lyer illusion
4 Britannica explains the Müller-Lyer illusion with the Gestalt ideas of convergence and divergence: the fins at the ends seem to lead the eye inwards or outwards, giving a false impression of length. Britannica: Müller-Lyer illusion
5 The perspective explanation (Richard Gregory, 1968): fins pointing in look like the near corner of an object, fins pointing out like the far corner of a room. Michael Bach: Müller-Lyer illusion
6 With no other clues, the brain takes the “angles in” line to be nearer and, because both lines throw the same-sized image, decides the nearer one must be shorter. Michael Bach: Müller-Lyer illusion
7 The perspective explanation has had critics (Segall and colleagues in the 1960s), and a 2025 paper by Amir and Firestone pushed back on the idea that the illusion is a product of culture. Michael Bach: Müller-Lyer illusion
8 Around 1970 the German satirical magazine Pardon ran a piece on the illusion, showing it works on whole pictures too: the elephant drawn in the bigger frame looked larger (and happier). Michael Bach: Müller-Lyer illusion
9 Michael Bach files the Müller-Lyer illusion under “size constancy”, because he finds the perspective explanation very convincing. Michael Bach: Müller-Lyer illusion

The Ebbinghaus illusion

10 In the Ebbinghaus illusion, two equal circles look different: the one ringed by small circles looks bigger than the one ringed by big circles. Michael Bach: Ebbinghaus illusion
11 It is named after Hermann Ebbinghaus, a pioneer of memory research, who probably found it in the 1890s but never published it. Michael Bach: Ebbinghaus illusion
12 Edward Titchener printed it in a 1901 textbook, without claiming it as his, so it is also called the Titchener illusion. Michael Bach: Ebbinghaus illusion
13 It is one of the few illusions with a big age effect: for children under 10 it is weaker. Michael Bach: Ebbinghaus illusion
14 Nobody fully understands why it works; it is probably a close relative of the Delboeuf illusion. Michael Bach: Ebbinghaus illusion
15 A 1995 study suggested the illusion fools the eye but not the grasping hand. After years of repeat experiments, Michael Bach now sees agreement that it affects grasping and seeing alike. Michael Bach: Ebbinghaus illusion
16 A moving version, made by Christopher Blair, Gideon Caplovitz and Ryan Mruczek, makes the effect even stronger and won the 2014 Visual Illusion Contest. Michael Bach: Dynamic Ebbinghaus illusion
17 In that moving version, two perfectly parallel guide lines can even seem to bulge. Michael Bach: Dynamic Ebbinghaus illusion

The Delboeuf illusion

18 The Delboeuf illusion was published by Franz Joseph Delboeuf in 1865: a disc inside a ring only a little bigger looks larger than the same disc inside a much bigger ring. Michael Bach: Delboeuf illusion
19 Hermann von Helmholtz called this kind of effect “size contrast”. Michael Bach: Delboeuf illusion
20 Michael Bach finds the Delboeuf illusion fleeting: some days it hardly seems to be there at all. Michael Bach: Delboeuf illusion
21 Delboeuf and Ebbinghaus are “size contrast” illusions; illusions that rely on depth and perspective belong to a different family. Michael Bach: Delboeuf illusion

Ponzo and size constancy

22 In the Ponzo illusion, a shape touching two converging lines looks bigger than an identical shape where the lines are further apart. Britannica: Illusion
23 Linear perspective also makes parallel lines, such as railway tracks, seem to meet in the distance. Britannica: Illusion
24 Assuming size constancy, the visual system works out how big something is as its size on the retina multiplied by how far away it assumes it is. Michael Bach: Shepard’s “Subterranean Terror”
25 Roger Shepard’s drawing of two identical monsters in a tunnel is a classic example of the Ponzo illusion: the two look very different in size. Michael Bach: Shepard’s “Subterranean Terror”
26 Roger Shepard drew his own illusions, and also invented the “ever rising scale”, a sound that seems to climb for ever. Michael Bach: Shepard’s “Turning the Tables”
27 Shepard’s “Turning the Tables” shows two table tops that look completely different but are the same shape and size on the page. Michael Bach: Shepard’s “Turning the Tables”
28 Size constancy is always at work. It is why someone walking away from you doesn’t seem to shrink into a dwarf. Michael Bach: Size constancy
29 Paste three identical cars along a bridge drawn in perspective, and they seem to grow the further away they are. Michael Bach: Size constancy
30 Richard Gregory called this “inappropriate constancy”: the cars are pasted in flat, but the brain assumes they follow the perspective like everything else, and corrects for distance. Michael Bach: Size constancy
31 Mario Ponzo’s 1911 paper turns out not to contain the famous Ponzo picture at all; it is mainly about an illusion of touch. Michael Bach: Size constancy
32 A 2021 review of Ponzo-like illusions found that a modernised version of Gregory’s explanation fits the experiments best. Michael Bach: Size constancy
33 Britannica: the size of the image on the retina is one of the clues to distance; the larger the image, the closer we judge the object to be. Britannica: Space perception

The moon illusion

34 The moon illusion has two parts: the moon looks bigger than it really is, and much bigger near the horizon than high in the sky. Michael Bach: Moon illusion
35 It isn’t just the moon: any object in the sky looks bigger near the horizon. Michael Bach: Moon illusion
36 Test it: at arm’s length, about two-thirds of the width of your little finger covers the moon, wherever it is in the sky. Michael Bach: Moon illusion
37 If anything, the low moon is a tiny bit squashed from top to bottom, because its light passes through more air near the horizon. Michael Bach: Moon illusion
38 Aristotle wrote that the sun and stars seem bigger when rising and setting; he blamed the air. Michael Bach: Moon illusion
39 Hermann von Helmholtz credited the size-constancy explanation to Ptolemy. Michael Bach: Moon illusion
40 Bend over and look at the moon between your legs, and the illusion gets much weaker. Michael Bach: Moon illusion
41 Photos of a giant moon behind a building are taken with a telephoto lens; that is a camera effect, not the moon illusion. Michael Bach: Moon illusion
42 In 1962 Irvin Rock and Lloyd Kaufman tested the illusion with artificial moons and found the moon’s apparent size depends on whether you can see terrain. Michael Bach: Moon illusion
43 The moon’s image on a photograph is the same size whether it is low or high: the change happens in your brain. Exploratorium: Looking at the moon
44 Birds, clouds and planes are closer when overhead and further away on the horizon, so we expect things on the horizon to look small. The moon doesn’t shrink there, so it seems huge. Exploratorium: Looking at the moon
45 The Exploratorium’s test: trace a rising full moon onto clear plastic held at arm’s length, then check again hours later. The high moon still fits the outline. Exploratorium: Looking at the moon
46 The moon is on average about 384,000 km (238,600 miles) from Earth. Britannica: Moon (natural satellite)
47 The moon’s real size in the sky does change a little: its oval orbit brings it closer to and further from Earth. Britannica: Moon (natural satellite)
48 The sun is far bigger than the moon but also far further away, so the two look almost the same size in the sky. That is why total solar eclipses happen. Exploratorium: Size and Distance (Science Snack)

The Ames room, window and chair

49 An Ames room looks like an ordinary square room from one peephole, but one side’s ceiling is about twice as high as the other. Exploratorium: Distorted Room
50 Because the brain decides the room is square, people walking across it seem to grow and shrink. Exploratorium: Distorted Room
51 Film-makers have used Ames rooms for special effects: in The Lord of the Rings, some Shire scenes used them to make hobbits look tiny next to Gandalf. Exploratorium: Distorted Room
52 Adelbert Ames Jr was born in 1880 and worked on visual sensation and perception; he is best known for the Ames room. Exploratorium: Ames Chair (Science Snack)
53 The Exploratorium’s Ames Chair is a jumble of bent stems on angled lines in a box. Through a peephole they line up into a chair. Exploratorium: Ames Chair (Science Snack)
54 Ames and Egon Brunswik argued that we perceive under the strong influence of learned assumptions. Britannica: Adelbert Ames, Jr.
55 The Ames window is a slowly turning trapezoid, the shape a rectangle would be if seen at an angle. As it turns, it seems to reverse direction partway round. Michael Bach: Ames window
56 To see a real Ames window properly, close one eye so your two eyes’ depth clues don’t spoil the trick. Michael Bach: Ames window
57 The Ames demonstrations were written up by William Ittelson in 1952. Michael Bach: Ames window

The T illusion

58 In the T illusion (also called the vertical-horizontal illusion), the upright stroke of an upside-down T looks longer than the crossbar, even when they match. Michael Bach: T-illusion
59 Size constancy isn’t as good for up-and-down distances: from the top of a tower, people below look tiny, like toys. Michael Bach: T-illusion
60 The top hat illusion is a version of it: a hat exactly as wide as it is tall looks taller than it is wide. Michael Bach: T-illusion
61 Johann Oppel wrote about geometric-optical illusions in 1855. Michael Bach: T-illusion
62 Scientists planned to test the T illusion on a zero-gravity parabolic flight. Michael Bach: T-illusion

The Jastrow illusion

63 In the Jastrow illusion, two identical curved shapes stacked one above the other look different: the lower one looks bigger. Swap them and the size seems to swap too. Michael Bach: Jastrow illusion
64 Joseph Jastrow described it in 1891, and it still isn’t well understood. Michael Bach: Jastrow illusion
65 Jastrow’s own explanation: the long edge of the lower shape is compared with the short edge of the upper one. “We judge relatively, even when we most desire to judge absolutely.” Michael Bach: Jastrow illusion
66 Jastrow also described the famous duck-rabbit picture. Michael Bach: Jastrow illusion
67 Müller-Lyer actually published the same illusion three years before Jastrow. Michael Bach: Jastrow illusion

The sine illusion

68 In the sine illusion, upright bars of equal height riding along a wavy line look like they change height. Michael Bach: Sine illusion
69 Robert Day and Erica Stecher described the sine illusion in 1991 and linked it to the Müller-Lyer illusion. Michael Bach: Sine illusion
70 The same trap catches people reading graphs: the brain judges the shortest distance between two curves, not the vertical distance (Cleveland and McGill, 1984). Michael Bach: Sine illusion

Experiments

71 Close one eye and hold a small coin and a large coin at different distances: you can make them look exactly the same size. Exploratorium: Size and Distance (Science Snack)
72 A bright, plain background works best, so the coins show up as silhouettes. Exploratorium: Size and Distance (Science Snack)
73 Large, distant objects can look the same size as small, nearby ones. Exploratorium: Size and Distance (Science Snack)
74 With one eye closed, your depth perception is much weaker, so you can’t tell how far away each coin is. Exploratorium: Size and Distance (Science Snack)
75 With both eyes open you get more points of view and judge size and distance much better. Exploratorium: Size and Distance (Science Snack)
76 The Exploratorium’s shoebox version mounts a quarter-sized and a dime-sized paper circle on straws, viewed through a hole with one eye. Exploratorium: Size and Distance (Science Snack)
77 In the Exploratorium’s museum version you look at two discs through a peephole; the smaller disc is always closer to you. Exploratorium: Size and Distance exhibit
78 A butterfly pupa has a pattern like a monkey’s face. It is only half as wide as a human fingernail, but birds seem to mistake it for a more distant, and so larger, monkey. Exploratorium: Size and Distance (Science Snack)

People and the big picture

79 Britannica lists the Müller-Lyer illusion (lines that look different lengths) and the moon illusion among the classic visual illusions. Britannica: Illusion
80 Britannica’s own panel of classic illusions includes “lines are equal in length”, “circles are equal in size” and “black dots are equal in size”. Britannica: Illusion
81 Hermann Ebbinghaus was born on 24 January 1850 in Barmen, Germany, and died on 26 February 1909 in Halle. Britannica: Hermann Ebbinghaus
82 Ebbinghaus’s famous “forgetting curve” appeared in his book on memory in 1885. Britannica: Hermann Ebbinghaus
83 After memory, Ebbinghaus turned to colour vision, and in 1890 co-founded a psychology journal with the physicist Arthur König. Britannica: Hermann Ebbinghaus
84 Michael Bach sees the Ames window as a good example of the brain rebuilding the world from incomplete information, using experience: usually right, but easily misled by unusual shapes. Michael Bach: Ames window