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Cat's Eyes
The cat has rather large eyes in relation to its skull. They have a wider visual, and binocular field than ours:-
Our Visual Field: 210 degrees. A Cats Visual Field: 295 degrees.
Our Binocular Field: 120 degrees. A Cats Binocular Field: 130 degrees.
The cat has the ability to see well in very dim light. This is made possible by the image-intensifying device at the rear of its eyes. The tapidum lucidum ( meaning "bright carpet"), is like a mirror behind the retina. It is this that makes a cat's eyes "glow" in the dark. Using this device it can utilise every tiny scrap of light entering its eyes. It needs only one-sixth of the light that we do to see the same shape.
Despite this remarkable device, it is not true that a cat can see in complete darkness. If there is no light to optimise, then it has to rely solely on its sense of smell and hearing, also the sensitivity of its whiskers.
The cats eyes also have a greater control over the amount of light they let in. Because the eye is very sensitive, and can see in dim light, it is important for it not to be dazzled by sunlight, so the cat can narrow the pupils to tight slits giving it a greater control over the light input. Cats have vertical slits rather then horizontal ones, because by using the vertical slit of the pupil and the horizontal one of the eyelid, it can make very minute adjustments to the light input.
Cats are not colour blind as first thought in the first half of the 20th century. With more sophisticated equipment is has been proven that cats can distinguish between different colours, but not to the degree that we do. But when you have all the acute senses that a cat has perhaps you don't need worry too much about perfectly distinguishing colours.
The Haws ( nictitating membrane)
Cats also have a third eyelid, called the haw. It moves sideways across the eyes to protect them or to lubricate the corneal surface with tears. The haw is not normally visible, unless the cat is in poor health or undernourished. When the haw becomes permanently visible, giving the eye a "half-shuttered look, it is a indication that the cat is in need of veterinary assistance.