You won't believe your eyes: The mysteries of sight revealed

HOW MANY COLOURS ARE IN A RAINBOW?

Human colour vision is a relatively recent acquisition. It is, at most, 63 million years old, and it may be a lot younger. On a genetic level, it is a mess: misalignments and redundancies in the genes that code for our "red" and "green" colour perceptions account for 95 per cent of all variations in human colour vision, and it is quite usual for up to nine genes to cluster together in an attempt to code for these colours. This is why the perception of colours - especially blues and greens - varies so much between individuals.

Humans perceive colour through three types of colour-sensitive cell, called cones, but some have four types. Equipped with four receptors instead of three, Mrs M - an English social worker, and the first known human "tetrachromat" - sees rare subtleties of colour. Looking at a rainbow, she can see 10 distinct colours. Most of us only see five. She was the first to be discovered as having this ability, in 1993, and a study in 2004 found that two out of 80 subjects were tetrachromats.

WHY YOUR EYES NEVER STAY STILL

If our eyes did not move - if they simply "drank in" the view before them - we would go blind. Our retinas can only process contrast, and soon become exhausted looking at the same thing for too long. They must tremble constantly in order to bring still objects into view.

THE SIGHTS WE ALL MISS

Human vision captures only two degrees of the world with any clarity, so we tend to miss things that happen outside our focus of attention - and the more we concentrate, the more extreme our "attention blindness" becomes. This makes us easy prey for psychologists such as Daniel Simons and Christopher Chabris, whose notorious experiment of 1999 asked its viewers to score a three-a-side, 90-second basketball game. Afterwards, the viewers were told to relax, put down their score cards and watch the video again. Only then did the game's most remarkable feature come to light: the invasion of the court, a few seconds in, by a 7ft-tall pantomime gorilla.

A VISION OF THE FUTURE

Our eyes stay several steps ahead of us, whatever we happen to be doing. When negotiating a turn in the road, for example, a driver's eye will provide motor information to his or her arms almost a second before he or she makes any movement. By then, the eyes will already be looking elsewhere. Visually at least, we operate in the world not as it is, but as it existed half a second ago. This raises a not insignificant question: how does the eye know where to direct its gaze next?

THE CURE FOR BLINDNESS

The concept of a bionic eye is nothing new. In the 1970s, bio-engineer Paul Bach-y-Rita, now at the University of

Wisconsin-Madison, was turning different parts of the body into eyes. His prototypes were vests containing hundreds of mechanical vibrators. Pixelated images from a low-resolution video camera, worn on a pair of glasses, were translated into mechanical vibrations against the skin of the chest or back. Bach-y-Rita's volunteers were able to recognise faces using the system. Proof that they could see came when Paul threw balled-up papers at them: they ducked.

SEEING BENEATH THE SEA

Because light behaves differently in water and air, land-adapted human vision is lousy in water. Someone, however, forgot to tell the Moken - gypsies who ply the Burmese archipelago and Thailand's western coast. Moken children, who spend days diving for clams and sea cucumbers, can see twice as much fine detail underwater as European children. While the pupils of the latter expand underwater, in response to the dimness of the light, Moken pupils shrink to their smallest possible diameter, improving acuity underwater. Mokens also use the lenses of their eyes more, squishing them to the limit of human performance.

HOW RETINAS WORK

A rod cell is the commonest form of light-sensitive cell in the human eye. When it is exposed to light, it expands like a Slinky toy to twice its length. In the dark, it contracts again. Rods behave like muscle cells, and muscle fibres expand and contract in response to electrical stimulation. The retinal rod, too, is responding to an electrical signal - one that comes from a biochemical reaction to light.

The working retina is a glorified Pin Art machine. On 16 November 1880, in the German town of Bruchsal, a young felon was beheaded by guillotine. A short while later, in a gloomy room, its windows screened with red and yellow glass, Wilhelm Kühne, professor of physiology at Heidelberg, dissected the dead boy's eyes. Ten minutes later, he showed colleagues a sharp pattern on the surface of the left retina. This, Kühne said, was an optogram: a dying vision, preserved as a chemical pattern on the retina.

WHY ANIMALS HAVE NO WHITES IN THEIR EYES

Few animals risk making a feature of their eyes. The "whites" of most vertebrates' eyes are dark, concealing the direction of their gaze. Only a social animal - a parrot, say, or a human - would make its eyes noticeable. Our bright whites enable us to use gaze-direction to convey emotion. A downward gaze indicates sadness; looking down and away suggests shame; looking away is a sign of frustration or disgust. The lateral rectus eye muscle is labelled "amatoris" in early anatomies because lovers use it to flirt.

HOW MOLES CAN 'SEE' UNDERGROUND

The nose of the star-nosed mole, Condylura cristata, has evolved into a mobile fleshy organ only about one centimetre across. Its nerves - five times as many as run through the human hand - are arranged across the nose's 22 "fingers", so that the mole's nose is most sensitive at its centre. The whole distribution of nerve endings bears a more than passing resemblance to the retina of a mammalian eye.

WHEN EYES SWITCH OFF

Although our eyes can move smoothly when tracking a target, they more usually snap from position to position to capture a selection of "stills". To avoid disorientation, our optic nerves fall silent while our eyes are moving between stills. This leaves us blind for about 10 per cent of our waking lives.

THE CRYING GAME

Crying is difficult to fake. Even actors have to generate some feeling before they cry. The Israeli evolutionary biologist Amotz Zahavi proposed that you can infer the honesty of a social signal by measuring the cost of the expression. Harvard's Marc Hauser, applying this principle to the eye, regards tears as the human equivalent of a dog rolling belly-up to show submission. "Unlike all of the other emotional expressions, tearing is the only one that leaves a physical trace," he says. "It blurs one's vision, therefore it's costly."

WORLD'S BIGGEST EYE

Arguably, the largest eye in nature is currently lying on a slab at the Te Papa museum in Auckland, New Zealand. It belongs to a colossal squid, Mesonychoteuthis hamiltoni. Identified in 1925, the colossal squid evaded capture for years until, this February, the trawler San Aspiring snagged an adult male on a fishing line. It is about 39 feet long. To be the biggest ever measured, the squid's eye will have to beat the previous record, set by a giant squid, found alive and stranded in Newfoundland in 1878. It had eyes 40cm across - wider than my computer screen.

MORE THAN A MYTH

About a week into a baby's development in the womb, a single eye socket appears in the middle of its forehead. Soon, it splits into two. The Cyclops - the one-eyed giant of mythology - has inspired many explanations, but the most likely is also the saddest. Very rarely, a developing child's brain fails to divide into two, and the central eye socket doesn't divide. This happens a handful of times each year.

WE'RE ALL LOSING OUR FOCUS

In 1996, about 60 per cent of American 23- to 34-year-olds were short-sighted, compared with only about 20 per cent of people over the age of 65. In Asia, things are worse. The Singapore National Eye Centre estimates that more than 80 per cent of the country's 18-year-old men are myopic. In the developed world, severe myopia is the leading cause of blindness. Evidence suggests that children grow more short-sighted in term-time than during the holidays.

The Eye - a Natural History by Simon Ings is published by Bloomsbury on 19 March, priced £14.99. To order a copy for £12.99 (including p&p), call Independent Books Direct on 08700 798897, or go to www.independentbooksdirect.co.uk