The Universe of Discourse

Sat, 30 May 2020

Addendum: optical illusion

[ Previously ]

John Gleeson points out that my optical illusion (below left) appears to be a variation on the classic Poggendorff illusion (below right):

A circle, with the
center marked.  A shortest-distance path is drawn in blue between two
blue points on the same radius, and in red between two red points on different
radii.  The blue path goes straight from one blue point to the other.
The red path goes from one point straight to the origin,
then straight to the other point. A narrow gray rectangle,
with a black line proceeding from the upper left to disappear behind
it, and parallel red and blue lines emerging from behind it to
continue to the lower right.  It appears that if the black line were
extended it would coincide with the blue one, but a second version of
the diagram, where the gray rectangle has been rendered
semitransparent, reveals that the blak line actually coincides with
the red one. By Fibonacci. - Own work, CC BY-SA 3.0,

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