This is just a quick page to say a little about why I prefer the term "visual illusion" to "optical illusion". The simple reason is that optical illusion implies that the illusion is an illusion of the eye, whereas visual implies that it is an illusion of the brain. The vast majority of the illusions called optical illusions are in fact a result of the working of the visual system as a whole, sometimes even incorporating very high level cognitive recognition tasks. A quick look at the visual illusions page will show that is the case.
There is no doubt though, that some visual illusions are optical illusions in the strictest sense. The cyan illusion of Daniel White repeated below, is an example of a truly optical illusion. The vast majority of the processing which contributes to this illusory effect occurs in the retina itself, and is particularly interesting for that reason.
The following demonstration, by Daniel White, is a great use of the colour aftereffect illusion. The image below (click for a larger, more effective version) will give an example of a colour you will never see on your computer screen. Stare at the dot in the centre of the red circle for three to five minutes. You will see the edges of the circle begin to glow, but do not stop staring at that point - keep on. After a few minutes move your head back and you will see a glowing ring around the red circle. The colour you are seeing is a cyan which will be perceptually totally different from any of the colours you have seen on your computer. The colour gradient to the right shows the shades of 'cyan' which your computer can produce, none of which match the colour you see with this effect.
For more visual illusions, or more detail on this particular illusion, then go to the visual illusions page.