Bourdon Illusion

from Michael’s   Visual Phenomena & Optical Illusions


What to see

Two black triangles, slanted a little – but are the left sides aligned, forming one straight line? Use the slider “Straight?” until they are aligned, then check “Result”.


This angle illusion was first described by Benjamin Bourdon 1902 in his book “La perception visuelle de l’espace” (siehe Originalabbildung rechts). He writes (my translation) “A direction or an extent tends to be assimilated to a a neighboring direction or range. …”

Verstijnen et al. (1998) found that the illusion is stronger when presented at a steep angle as appearing here by default (change with “Slant”).

There are several explanations put forward for this illusion (see op. cit. for an overview), which all-in-all don’t really convince me. So this one, as many angle illusions, appears ill understood at this time.


Bourdon B (1902) La perception visuelle de l’espace. Paris, Reinwald.

Verstijnen IM, Van Leeuwen C (1998) Anomalous orientation effects in the Bourdon illusion. Psychonomic Bulletin & Review 5:290–294