Even behind a fixed gaze the eyes are in a frenzy of activity, as densely packed nerve cells respond to light in a neverending series of high-speed chemical reactions. The two main type of visual receptor cell are called rods and cones, and there are 125 million of them at the back of each eye, in the retina.
Rods are light-sensitive cells that help us to see in dusk and dark conditions. So sensitive are they that it has been estimated that, in total darkness, they would enable the eye to see a lighted candle 8 km (5 miles) away.
Cones are less sensitive than rods and help us to see in daylight or bright artificial light. They also filter colour.
Both types of cell contain light-sensitive substances called pigments. The pigments change rapidly when light hits them and this acts as a stimulus to send signals to the brain, where they decoded and perceived as pictures. Normally the rods and cones adapt automatically and instantaneously to changes in light and darkness. However, if the shift is dramatic, such as moving from, say, an indoor theatre to bright outdoor sunshine, there will be an adjustment that we may notice - the images we see may momentarily appear indistinct or bleached.
Exactly how cones contribute to colour vision is not fully understood. Rods have only one type of pigment and cannot discern colour, but cones have three pigments and each cone responds to a single primary colour - red, green, or blue. The effect might be compared to what you see when examining a colour television image with a magnifying glass: the screen becomes a mass of tiny red, green and blue dots. Our cones too break down various hues into these three basic colours, and the brain interprets these sensations as the full palette of the world around us.
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