Optical illusion is taken great advantage of in most M. C. Escher's artistic works, such as Belvedere, Waterfall, Ascending or Descending, etc. His work in "symmetry" also ignites the ispiration of a Stanford profeesor, named Sam Savage(who was at one time teaching economics at the University of Chicago) to invent a jigsaw puzzle game - "shmuzzle". In the game, you can arbitrarily put each block in any three directions (angles between each direction is identical, i.e., 120 degrees), and each one can be combined into another piece either in head, or in leg or tail. Quite funny, 'cause among trillions of combinations to pursue the final outcome, only one combination is right. You can try one simple online version, it's a interesting game but a little bit time consuming.Another contribution of this Dutch artist is his man-made "Droste Effect" in some of his drawings. Part of the picture is exactly the same as the picture itself, only in a smaller scale. This technique can make the picture look like as an infinite self repetition. Hendrik Lenstra, a Maths professor from the University of Leiden held a lecture "Escher and the Droste Effect" at Princeton last month, and he illustrated Escher's genius on an academic basis. You can watch this online. By the way, professor Lenstra was introduced by Dr. Andrew Wiles.
