The Ebbinghaus and Delboeuf illusions
The same circle, surrounded by different company, looks a different size. Helmholtz had a name for it. Ebbinghaus didn’t bother to publish it.
The Ebbinghaus illusion
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
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 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
Children see it differently
It is one of the few illusions with a big age effect: for children under 10 it is weaker. Michael Bach: Ebbinghaus illusion
Why?
Nobody fully understands why it works; it is probably a close relative of the Delboeuf illusion. Michael Bach: Ebbinghaus illusion
Delboeuf and Ebbinghaus are “size contrast” illusions; illusions that rely on depth and perspective belong to a different family. Michael Bach: Delboeuf illusion
Does it fool your hand?
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
A moving version that wins prizes
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 In that moving version, two perfectly parallel guide lines can even seem to bulge. Michael Bach: Dynamic Ebbinghaus illusion
The Delboeuf illusion
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
Hermann von Helmholtz called this kind of effect “size contrast”. Michael Bach: Delboeuf illusion
Michael Bach finds the Delboeuf illusion fleeting: some days it hardly seems to be there at all. Michael Bach: Delboeuf illusion
The illusion drawings on this site are our own, drawn from scratch from the classic recipes (two lines, some fins, a ring of circles). Where a famous original belongs to its artist, such as Roger Shepard’s monsters, we describe it and link to it rather than copy it.