|MadSci Network: Science History|
One of the oldest optical illusions created by humans (as opposed to noticing a natural visual phenomenon, such as how a pole half immersed in water will appear to be broken) are the coins of Lesbos, a Greek island. These coins date back about 2500 years ago.
The coin face shows two boar's heads confronting each other, but combined, the two boars heads can be seen as a panther's head, facing outward.
A more familiar optical illusion of a similar variety is the "Old woman/young girl" picture. Depending on how you perceive the picture, you either see an old woman looking to the left with her chin to her chest or a young girl looking away from you over her right shoulder.
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