Consider the two images above. They show color photos of the same object, with a filter overlaid. In the left image the filter is yellow, while in the right image the filter is pale blue. Note the object is constructed of plastic pipe sections of various colors. Focus your attention on the very center of the X-shape. In the yellow-filtered image the center appears grey, while in the blue filtered image the center appears yellow. However, here is the incredible fact: in both images, the center is precisely the same color! To test this you can just cut two holes in a sheet of paper to block off everything except the two centers, or you can go to the site below which allows you to choose to block off more and more of the images until you are left with just the centers. Colors such as grey, green and yellow are extremely difficult for the human eye-brain system to judge. Generally the cue as to what color is perceived is taken from the surrounding context. When the context is removed, the perception can change drastically.
[By the way, there is another illusion built into these two images. The perspective of the photos of the two X-shaped constructions implies that (in the photos) the two arms of the X do not meet at right angles. But in fact, they do! To see this place a corner of a sheet of paper against any one of the intersections in either image.]
Probably the best interactive version of this illusion will be found here. [Illusion 1.]
This illusion was created by Dale Purves and Beau Lotto. To see interactive versions of some of their other illusions, click on their names or here and look for the interactive demonstrations.
This page is the first of more than 100 pages currently posted here, collecting some of the better and most surprising perceptual illusions around today, plus some distressing cultural delusions.
Here's a Lotto-Purves bonus. Note the two cushions above. Note the one square on each with a dot in its center. Despite appearances these two squares are precisely the same in color, as you can verify by cutting appropriately positioned holes in a sheet of 8-1/2 by 11 copier paper. Or just click here, clicking on the appropriate thumbnail under "color."
Another version of this illusion is here.
Jump to Illusion 25, 50, 75, 100, 110, 120, 130, 140, 150, 160, 170.
An extremely nice collection of very colorful illusions, many of which were new to me.
Individual Project LITE pages are linked elsewhere on these illusion pages, but here's the full directory.
A large collection of illusions; it helps if you can read Japanese.
Individual ZONEFLASH animated illusion pages are linked elsewhere on these illusion pages, but here's the full directory.
Although Akioshi Kitaoka's illusion pages are randomly distributed, like the ones you are looking at now, there actually is an index, here.
A long article on illusions, with dozens of animated illustrations.
In closing, here are some wise words from Prof. Michael Bach, regarding illusions in general, and all the 150 or more illusion pages included here: “Don’t distress yourself if you don’t see the effect described, even if trying carefully. For many illusions, there is a small percentage of people with perfectly normal vision who just don’t see it, for reasons unknown.” Your eye-brain image processing system is working normally if you can see the page at all; don't worry if you don't see a particular illusion; just go happily on to the next page.