Roget's ‘Palisade’ Illusion
What to do & see
1.We certainly all agree that on the right side a wheel is seen rotating.
2.On pressing the “show” button, a brown picket fence
(a palisade) will fade in. When it is fully opaque, it largely occludes the rotating
wheel, but one can still perceive its rotation.
3.On pressing the “move” button, the palisade begins
to, well, move.
4.What do the spokes of wheel look like now? A little distorted, similar to the
icon on the top right?
Yes, it is somewhat difficult to perceive this, due to the timing distortions (“anti-aliasing”)
by computer presentation.
5.When fading out the palisade, the distortion vanishes as soon as a little of the
wheel becomes visible through the semi-transparent palisade.
6.The effect changes drastically when changing the relative speeds. When you
inverse rotation direction or palisade movement (by going to negative numbers),
the “bends” move to the top of the wheel.
7.If you observe the wheel for a while an then stop, you see opposite rotation.
That’s the motion after effect
8.When you inverse rotation direction or palisade movement (by going to negative
numbers), the “bends” move to the top of the wheel.
Comment
Peter Mark Roget (1779–1869), author of the famous Thesaurus, first described
this illusion, hence known as the “Roget” or “Palisade” illusion.
His own explanation is not convincing from our current understanding. Essentially
it is a sampling proplem. Carpenter detailed a geometrical derivation in
1868, which was recently formalized by Jim Hunt (2003). The intersections
of gaps and spokes over time indeed form curves. Due to the
afterimage (“persistance of vision”) our perception connects these and we
perceive the illusionary shape.
Note added 2009-04: Interestingly, this effect can also occur in “finish photos” of bike racing, where the
spatio-temporal intricacies of the camera shutter “bend the spikes” – and quite
strongly too. Furthermore, the Palisade illusion probably also causes an interesting
shape distortion in archery, when capturing
the string release with a photo just at the right time. Beautiful things can be
learned from the web!
References
I am grateful to James L Hunt for pointing me to this illusion
and providing material.
Roget PM (1825) Explanation of an Optical
Deception in the Appearance of the Spokes of a Wheel Seen through Vertical Apertures.
Phil Trans Royal Soc London 115:131–140
Carpenter WB (1868) On the Zoetrope and its antecedents. The Student and Intellectual
Observer 1:427–444
Hunt
JL (2003) The Roget Illusion, the Anorthoscope and the persistence of vision.
Am J Phys 71:774–777
Wade NJ, Heller D (2003) Visual motion
illusions, eye movements, and the search for objectivity. J Hist Neurosci 12:376–395
Created: 2007-01-01
Last update: 2013-10-04