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The optical illusion is credited to Professor Joseph Jastrow, a pioneering psychologist, who used it to illustrate optional illusions in 1891.
It's sold as part of many magic kits to children.
- Presented as an optical illusion in Simple Conjuring Tricks by Will Goldston (1913)
- A kind of bet in 200 More Tricks You Can Do by Howard Thurston as "Which Is Larger?" (1927)
Variations of the shape