Our next illusion is a ghost image by Tom Remy. This one takes a little work, and doesn't always work right the first time, so you may have to try again. Stare steadily at the image for about thirty seconds, focusing on the center of the face. Then quickly turn and look at a blank white paper. You should see a classic yellow smiley for a few moments. This is an example of a negative afterimage, which is a physical illusion based on how the light receptors in the eye work. Your eye has three types of cone receptors, that perceive blue, green and red light. The relative stimulation of these receptors is interpreted by the brain to give the perception of the whole spectrum of colors. When you stare at the figure, the blue receptors are overstimulated, and adapt by losing sensitivity. When you shift your vision to the white paper, the blue receptors are slower to respond than the red and green receptors, and so your brain receives stronger signals from those two types, which is interpreted as the negative of blue, or yellow. If the illusion didn't work for you it's probably because you didn't hold your eyes steady enough, as your body normally compensates for this receptor overload by flitting the eyes back and forth.