Each receptor - rod or cone - on the retina is only briefly stimulated by incoming light. So if the eye was held in a fixed position, an image focused on the retina would fade within seconds. So how can you gaze longingly at a piece of cake without it disappearing? Well, no matter how hard you stare, your eye doesn't stay still. There are constant movements of various kinds - frequent little jerks called "saccades" (from an old French word meaning the snap of a sail in the wind), as the eye scans what you are looking at, constantly bringing different sets of receptors into play. (The minute size and close packing of the receptors means that only small movements are necessary.) Look in a mirror and you'll never see your own saccadic movement - or any other kind of eye movement for that matter - because there's a clever neural trick that shuts down the visual system while the eyes are actually on the move and protects you from the visual shakes. Try shutting one eye and gently pushing the other with your finger if you want to know what life might be like without that protection.