"Unlike human eyes, which work best in day light, the cat’s eyes must function well in extremely low light condition and as such are well suited to an animal that is predominantly nocturnal and crepuscular in activity. In darkness, cats eyes are able to function in approximately one-sixth of the light needed for human vision. However they must also be able to function well in daylight – so just how is this achieved.
In low light levels the cats pupil must be able to open as wide as possible, but also be able to contract to very small size to protect the sensitive retina in bright sunlight. In human eyes, this size variation of the pupil is controlled by a circular ciliary muscle, but this limits the amount of size variation. In cats however, the same process is controlled by two, shutter-like ciliary muscles, which gives the cat it’s characteristic slit-like pupil in bright light conditions. "