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		"The Blue Jay, as we clearly see, Is so much like the green Bay tree That one might say the only clue Lies in their dif-fer-ence of hue, And if you have a color sense, You'll see at once this difference." ---- R. W. Wood 1917
Image and text taken from S. Coren, C. Porac & L. M. Ward, Sensation and Perception.

Many of us have at least once come across optical illusions in our lifetime. Beside being fun to solve, optical illusions have the power to puzzle and bewilder because we have an implicit trust that what our senses tell us is physical reality.

Illusions are a nice window into how the brain works. "The mind sees and the mind hears. The rest is blind and deaf." said Epicharmus 450 years before the birth of Christ. It is surprising to find out that the eye cannot see, it merely perceives information which is passed down to the brain where innumerable processes of classification, comparison, and decision-making are initiated.

In this section we have gathered many instances where the senses (the brain?) tell lies. You can find them grouped for your easier access in the menu on the left side. Probably you have seen some of the listed optical illusions before, such as the "classical" - the geometrical-optical - illusions, since they have been known a fairly long time.

Beside the collection of optical illusions you can find a brief introduction to perception containing some explanations for these puzzling effects.

Compiled by Ramona Winkler, edited and expanded by Gerd Waloszek (SAP User Experience)

Note on the Source of Images

The images in this online book come from different sources:

• Some of them can be found on many Websites and seem to be treated as if they were in the public domain; typically, we added "Source: unknown" to these images
• Other images are taken from books, such as Roger Shepard's book Mind Sights: Original Visual Illusions, Ambiguities, and Other Anomalies. Of course, the copyright belongs to the respective authors.
• The remainder of the images has been redrawn by us according to originals found in journals or textbooks; these images are not in the public domain but you may ask us for permission to use them.

In all cases, where the images were not redrawn by us, we added the respective source information.

Status

• Version 1.3.3 (May 2007): Note on the source of images added
• Version 1.3.2 (May 2006): High-resolution versions of the Liar and Triangle illusions added
• Version 1.3.1 (April 2006): Minor technical and typographical updates
• Version 1.3: New induced motion images added
• Version 1.2: Comments from Walter H. Ehrenstein jr, University of Dortmund, added; a couple of minor changes made

This paper can be found in Resources on the SAP Design Guild Website (www.sapdesignguild.org).

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Source:  Optical Illusions