The ghost circle
This optical illusion, created by Daniel White, is an example of how our retina perceives colors. Fix your sight on the white dot located in the center of the red circle for about three or four minutes (you can blink, but do not look away). Little by little you will see that a luminous halo forms on the edges of the red circle, but keep your eyes fixed on the central white point. After a few minutes, put your head back and you will see a bright ring around the red circle. The most curious thing is that this ring is a blue-green color that is perceptually different from any other color that your computer screen can display. The range of cyan colors that your PC can show is that shown in the square. 
Another great optical vision is this picture that is black and white and you will see it in color:
Stare at the black dot until the time bar runs out. You will see the colors of an image that is in black and white. 
