The cat's eye is constructed in much the same way as that of a human being, but there are important modifications that enable the animal to do things we cannot.
It is often said that "cats can see in the dark". Not so. In a totally blacked-out room, a cat can see no better than you or me. What it can do is gather the faintest quantities of light in its surroundings. Even on a moonless night the sky is never completely empty of light. Faint starlight or the the pale reflections of high cloud are always present, and the cat's eye is designed to gather and use such minute scraps of luminosity.
It uses an ingenious though logical method in the form of a "mirror" placed behind the light-sensitive retina. This "mirror" is composed of up to fifteen layers of glittering cells and is called the tapetum lucidum. Faint light-beams enter the eye, and pass through to hit and stimulate the light receptor cells of the retina (rods and cones). They then carry on past to be reflected by the "mirror" so that they contact the rods and cones for a second time. This "double dose" multiplies the effect of the light and increases the feline night vision immensely.
We know that domestic cats can make clear visual discrimination at one-sixth of the light levels required by human beings. But obviously, as I have said, the "mirror" cannot work where there is zero light.
The shining of the the mirror is what produces the characteristic golden or green gleam of a cat's eye in the dark. ("Tiger! Tiger! burning bright, in the forests of the night". William Blake's famous lines, were perhaps inspired by this phenomenon.) Human eyes do not gleam in the dark: the red glow of our pupils that is occasionally seen in flash photographs is produced by blood vessels behind the human retina.
Another advantage for cats is that they have a wider angle of view than we possess. We have a visual field of about 210 degrees of which 120 degrees is binocular. Cats have a total visual field 285 degrees, 130 degrees of which is binocular.
The 130 degree binocular vision of the cat is another hunting adaptation that allows the animal to judge depth and distance with accuracy. In practice, there is more to judging distance than merely having binocular vision, and cats can be shown to be not quite as good as humans at estimating range. Humans make up for a somewhat narrower field by far more extensive eye movements, permitted by the larger area of white that surrounds the cornea and iris.
How The Eye Works
The pupil of a cat's eye, like that of other mammals, constricts in bright light and dilates in dim conditions, but the actual shape of the pupil varies among different feline species. Bigger wild cats possess broadly oval pupils, the puma has a round pupil, and only members of the genus Felis (including the domestic cat) have a vertical slit pupil. The virtue of having a slit pupil lies in its ability to close more efficiently and more completely than a circular pupil. This serves to protect the ultra-sensitive retina. Total closure never in fact occurs - a minute pin-hole remains open at each end of the slit.
The rods on the cat's retina give good night vision and are sensitive to lo light levels. The cones provide resolving power. The feline eye contains relatively more rods and fewer cones than a human's. It can therefore see better in dim light but isn't able to discern fine detail quite as well as we can.
Cats focus like we do, by changing the shape of the lens through the involuntary control of tiny muscles. This process, which is known as "accommodation". can either bow the lens to bring close object into focus, or flatten it to concentrate on objects further away. Man and cat share equally good powers of focusing.
Does your cat quietly admire the lavender-shaded new curtains or grit its teeth at the first showing if your youngster's psychedelic T-shirt? In short, do cats see in colour? They do possess cones of at least two and possibly three kinds, and in human beings, cones undoubtedly play a major part in colour vision.
Scientists believe that although cats can see colour it means absolutely nothing to them! The eyes distinguish colours but the brain does not interpret them. This almost philosophical distinction between seeing and perceiving is important, for it has been demonstrated that cats can, with difficulty, be trained to understand colour. In general, however, cats do not use colour perception - it is not an essential part of their normal life and plays no part in hunting a mouse or approving a bowl of favorite food.