Café wall
Richard Gregory & Priscilla Heard; observed by Steve Simpson
Perfectly parallel mortar lines seem to slope.
Size & geometry / 1855 / 1863
Oppel–Kundt Illusion
A subdivided distance can look longer than an empty one.
TRY THIS
Compare the distances between the main boundary marks, counting the intervening lines separately from the overall extent.
THE IMAGE IN THIS ARCHIVE
WA Reiner (talk)
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What is Oppel–Kundt Illusion?
A subdivided distance can look longer than an empty one.
What should I look for in Oppel–Kundt Illusion?
Compare the distances between the main boundary marks, counting the intervening lines separately from the overall extent.
What explains Oppel–Kundt Illusion?
Internal divisions change perceived extent. A span containing several marks can look longer than a neighboring unfilled span.
Another way to see it
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Ebbinghaus circles
Traditionally Hermann Ebbinghaus; popularized by E. B. Titchener
The neighbors change the size of the circle you see.