Café wall
Richard Gregory & Priscilla Heard; observed by Steve Simpson
Perfectly parallel mortar lines seem to slope.
Size & geometry / 1902
Bourdon Illusion
A straight edge seems slightly bent beside an angular outline.
TRY THIS
Inspect the line from A to E through C. Use a straightedge to compare its actual direction with the neighboring outline.
THE IMAGE IN THIS ARCHIVE
Mabit1
Original file & attribution · CC BY-SA 4.0
This locally stored image is a Wikimedia Commons rendition (resized or rendered from the source file). The full original is linked from the source record. Gallery previews may be resized or converted to WebP; animated previews show the first frame.
Download imageFrequently asked questions
What is Bourdon Illusion?
A straight edge seems slightly bent beside an angular outline.
What should I look for in Bourdon Illusion?
Inspect the line from A to E through C. Use a straightedge to compare its actual direction with the neighboring outline.
What explains Bourdon Illusion?
The direction of the adjacent outline influences the apparent direction of the straight boundary. It can seem to bend at the central junction.
Another way to see it
Explore this category
Ebbinghaus circles
Traditionally Hermann Ebbinghaus; popularized by E. B. Titchener
The neighbors change the size of the circle you see.