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Everyone knows cats can see in the dark. Not everyone realises what this must be like for the cat. Imagine being able to creep up on your prey in nothing more than starlight - to see the hapless mouse (zebra/elephant/drunk human in the bush) at the crucial moment before it becomes . . . dinner! If police and Securitas officers had a cat's eyes burglars and late-night pyromaniacs wouldn't stand a chance, and just imagine the electricity we'd save at night by not having to reach for the light every time we enter a room.

Actually, cats cannot see in the total dark any more than we can. What they can do is see adequately in about 1/6th the light minimal for us. For a cat, an average moonlit night is as bright as one on a cloudy day for us and a starlight night is like the full moon - but there still has to be a little bit of light. A cat's eyes are light-intensifiers, not infra-red cameras. Yet they are a real miracle of natural engineering, and it's worthwhile going into the mechanics (and optics) behind these amazingly sensitive eyes.

You know how the eye works, of course. Light reflected off your friend over there, or emitted by your computer screen, radiates into your eye through the pupil (the dark hole in the centre) and is focussed by a little flexible lens inside before painting a coherent image on the light-sensitive retina at the back of the eyeball. The pupil gets larger for dim light and smaller for bright light, and the lens focusses for far or near objects; the nerve signals generated by cells in the retina are coded and "cleaned up" and then sent on to the thalamus and primary visual centres of the brain, via the optic nerve.

The eye of a cat is just the same - but it's evolved some pretty amazing modifications for seeing at night. For a kickoff, its pupil is larger than a human's. Even a domestic cat can open his pupils wider than we can, and a lion or tiger - with an eyeball twice the size of ours . . . well, you can imagine how much light that  can collect at night! Then there's the lens, which has less laminations (layers) than ours - it's not as flexible as ours, and cats cannot focus on nearby objects (your computer screen will appear to a cat as a fuzzy white rectangle) - but with less laminations for the light to get through, the image is brighter.
And finally there's the retina. Mother Nature made a goof when evolving animal eyes: the light-sensitive cells are actually behind the network of bipolar, ganglion, horizontal, and amacrine nerve cells that do all the image pre-processing. This means all the light we see by has to filter through a dense tangle of nerve cells and fibres to get to the retina itself. In humans, nothing was done about it and we put up with a dimmer image. But cats - and many other night animals - evolved a compensation - the tapetum lucidum. It's a mirror, sited behind the light-sensing cells of the retina. Light passing through the retina bounces off and comes back onto the sensing cells, this time without the messy tangle of nerve fibres - thus more than doubling the image strength. The tapetum is what makes a cat's eyes shine in torchlight in the dark - note how the shine comes from within the pupil, not the iris. This mirror called the tapetum also has a phosphorescent coating, which actually lights up when struck with ultraviolet light - so cats may be able to see UV frequencies we cannot.

The other modification to the retina is the sensory cells themselves. Animals have two kinds of light-sensing cells in their retinae: the rods, which pick up light in general, and the cones, which pick up light of one of the three primary colours - red, green, and blue. The rods, being sensitive to all colours simultaneously, are much more sensitive to light that the cones which only "see" light of one colour. Humans (and birds) have a full complement of colour-sensing cones in their eyes, lots of red, green, and blue sensors, and relatively few high-sensitivity rods. Thus we can see in gorgeous full colour, but never in dim light - look at someone else's car at night with no streetlights and try to tell what colour it is. Cats have sacrificed one type of their colour-sensing cones - the red ones - and many of the cones overall, in favour of the much more sensitive rods. To them, colours in broad daylight are blue-green in tint and rather pastel - but when the sun goes down, boy, can they see things!

The rods in a cat's retina are often wired together in groups, increasing their collective sensitivity even further - but this, and the fact the mirror tends to blur the image a little, means cats cannot see fine detail - a cat could never read a book, unless it was one of those large-print children's books. But that doesn't matter. Cats don't need to appreciate the grain of the fur of a mouse (zebra/etc) - all they need to do is see the body shape in the dark, identify it, and pounce.
The rest is left to their acture hearing and sense of smell, which I'll talk about in my next section.

Why are a cat's pupils slit-like? In fact, only the small cats (the Felis genus) have vertical-slit pupils that widen laterally to admit more light. The big cats, the lions and tigers, have oval or round pupils, rather like ours. I think it's because small cats tend to work in more varied lighting: they hunt by day as well as night, whereas many of the big cats prefer night or crepuscular (dawn/dusk) hunting hours. The vertical slit of small cats lets them fine-tune the amount of light entering their eyes by consciously opening or lowering their eyelids over the slit, during daylight hunting, so they get enough for a clear image but not so much it hurts their eyes.

Why do cats not have a "white" to their eyes? In fact, I'd put it the other way around. Why do humans have a white to their eyes? It's because our eyes are more mobile. The six ocular muscles that move the eyeball left, right, up, and down are move powerful in human eyes than those of most other mammals - mainly because our upright stace emans we've lost the catlike ability to turn our heads to look directly behind.

What does a cat see? Cats have proven to be great subjects for experiments in vision, because they're eager for rewards as part of the testing. The testing does not hurt them in any way. So scientists know a lot about feline vision as a result. A typical look around your living room would appear to a cat to be bluish green, with any pure red lights such as digital LED displays, totally invisible. The TV screen would have a pronounced flicker, and any image on it would be more or less visible only at a distance of, say, across the room. All nearby objects would be fuzzy and out of focus. Movements, especially small, darting ones as made by mice, would be very strong and attractive - but when the moving object stopped, it would become much less seen. And when the humans had given up watching TV and gone to bed, and turned off all the lights, the room and its contents would still be perfectly visible - in a flat, shadowless, colourless scene, quite grainy and coarse like a low-res B/W JPEG.

And if you were a lion, you would also see the entire breadth of the horizon of the Serengeti Plains without having to move your head.
 
 

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