Light gets inside the eye and irritates nerve-endings of the sensitive layer, that is retina and this allows animals to see. Then these irritations pass through the nerve fibers of the eye nerve into the brain and there are decoded into a "picture". If there's no light, then a cat is unable to see anything at all just because light doesn't get inside the eye and irritate the nerve-endings of retina. That's why in absolute darkness cats don't have any advantage over any other animal.
But in the gloom, sometimes so impenetrable, that a human eye may percept it as pitch-darkness, a cat orients itself among different objects much better than we do, especially during movement. There're three main reasons making for such vision.
First reason. There're three types of nerve-endings that according to their form are called sticks and cones. Cones most actively react on bright light; these nerve-endings are responsible for color vision of humans and for perception of small details. Sticks, on the other hand, react on light of faint intensity and are unable to reproduce clear images.
Night vision, or better to say, crepuscular vision of all animals is accounted for by the activity of sticks. The ratio of sticks and cones in the cat eye (25:1 approximately) is much higher than in the human eye (4:1 approximately).
Second reason. A cat along with many other domestic animals, but unlike human has a reflective layer situated directly under the retina. Its' function is to reflect the rays of light that get into eye and reach retina immediately on the selfsame nerve-endings. Consequently, every ray of light has the double effect on a given nerve ending providing a certain type of intensification of image.
The presence of this layer (tapetum) also explains the typical effect of a "cat-eye" that is displayed when the ray of light, for example, headlights is directed out of darkness right into eyes. The light is reflected from a green-yellow tapetum and creates an effect of eyes shining yellow or green.
Third reason. The pupil of all animals dilates in dim light and contracts in bright light trying to keep the amount of incoming light at constant level at which an eye functions best of all. Cat's pupil may dilate rather much. Contracting in bright light the pupil of a cat resembles a vertical slot; when it completely opens in darkness it becomes round and may run almost up to one centimeter across diameter. Cat eyes are big relative to its' body size, and such an ability of pupil to dilate in darkness allows the greater amount of light to get inside an eye and irritate sticks.
The combined effect of these three factors helps cat get much more visual information in comparison with human in the presence of very poor illumination. In fact, a cat may discern objects and other animals when the amount of light is less then 20 per cent of the amount necessary for a human eye.
Translated by Tatiana Karpova (Moscow)
(MSU, Biology faculture, Dep. zoology and ecology).