Introduction "Whilst part of what we perceive comes through our senses from the object before us, another part (and it may be the larger part) always comes out of our own mind." -- Wllliam James The Great Book of Optical Illusions can be appreciated on many levels. Most books on illusions reprint the same classic illusions over and over again, but this is not the case here. Although you will find the familiar classic illusions, this book contains innumerable illusions, which are not found in any other published source. A number of the illusions come from the results of recent research in vision and perception laboratories. Other examples come from a variety of artists who have deliberately incorporated an "obvious" illusion into a drawing, photograph, or sculpture. There are also quite a number of illusions, which were specially created by us. The illusions are not ordered in any systematic fashion, but are grouped into galleries and at the back of each you will find some notes which give brief scientific explanations of our (and in some cases, my own) best guesses for why these effects occur, and how they are consistent with the processes that mediate normal perception.

Illusions are a very nice window into how the brain perceives, because they can reveal the processes that underlie perception in a way that normal perception does not. At the end of the book is a glossary of technical terms that are used in the textual explanations and suggestions for further reading. Although the book is in a popular format and accessible to all ages, it can serve as a stepping stone to some wonderful projects on perception for students of all ages. Most of us take vision for granted, We seem to do it so effortlessly; however, perceiving images, objects, depth, and motion is a very complicated process. Only in the last one hundred years, and especially in the last twenty years, have scientists started to make some progress in understanding vision and perception. Take a moment to observe the world around you. For example, if you tilt your head, the world doesn''t tilt. Move around an object: the shape you see changes, yet the object remains constant in your perception.

"Sorting it out" is a truly wonderful process. Your visual system, however, usually settles for the correct interpretation. But there are some very powerful constraints on just how your brain does this. For the most part, these constraints work. However, mistakes can happen. Sometimes, an illusion occurs when there is not enough information in the image to resolve the ambiguity. For example, important clues that would normally be present in the real world, and which would have resolved the ambiguity, are missing. It needs to emphasized, though, that our understanding of vision and perception is very far from complete and the explanations in this book are tentative and should be regarded with some degree of skepticism, especially since some of the explanations involve my own speculations! Therefore, I sincerely hope that this collection will inspire some serious thought -- and practical demonstrations -- as to why these effects occur.

It is my hope that this may lead to some new insights about the creative intelligence of vision. Part of the fun will involve being tricked, fooled, and misled. This collection of illusions will definitely do that! It has nothing to do with how smart you are, how cultured you are, how artistic you are, or how old you are. You will be tricked. In fact, many of the illusions are so powerful you will doubt the written description, while others are so convincing that you will perceive nothing wrong. You will know that you are being tricked, but you won''t know how. Vision Science is one of the most exciting areas in current scientific research, and the study of illusions is one that brings great joy. I hope that this book will bring surprise and delight to both young and old alike, as well as stimulating some thought about the most marvelous mystery in the universe, the human brain.

Now go have some fun! Al Seckel California Institute of Technology, 2002.