The Eye: A Natural History (2007)

This is a book about the nature of the eye. It is about all the eyes that are, and ever have been, and may yet be. It is about how we see the world, and how other eyes see it. It is about what happens to the world when it is looked at, and about what happens to us when we look at each other. It is about evolution, chemistry, optics, colour, psychology, anthropology, and consciousness. It is about what we know, and it is also about how we came to know it. So this is also a book about personal ambition, folly, failure, confusion, and language.

UK: Bloomsbury. 1st hardback edition, March 2007
UK: Bloomsbury. Paperback, January 2008
Germany: Hoffman und Campe, April 2008
USA: Norton, October 2008
Italy: Einaudi, October 2008
Japan: Hayakawa, 2008
Portugal: Aletheia, 2008

The Eye: what the reviewers said

Ings argues convincingly that the eye has had a profound effect on our language, perception, philosophy and even consciousness… Ings deals with these, as he does all parts of this thoroughly engaging book, with refreshing clarity, enthusiasm and vigour. It’s a real eye-opener, if you’ll pardon the pun.
Doug Johnstone, The Times, March 10, 2007

 

In The Eye: A Natural History, the novelist and science writer Simon Ings explores evolution’s alleged masterpiece from several perspectives, including optics, physiology, history, medicine and biochemistry. It is a rich and eclectic survey, with an intriguing nugget on almost every page. Ings has done his homework and is not afraid to find fault with new ideas that don’t pass muster.
Graham Farmelo, Sunday Telegraph, March 24

 

[Ings] rightly asserts that ‘the story of the eye is epic’, and this is an impressive attempt to summarise its 538-million-year history. There are times when the encyclopaedic scale of the endeavour rather overwhelms the reader, but it’s easy to share his genuine wonder at the sheer oddness of some of the mechanisms of sight.
P D Smith, the Guardian, June 2

 

In this ambitious work, Ings reaches into chemistry, evolutionary biology, anthropology, psychology, aesthetics and his own fertile imagination to produce an agglomeration of ideas and themes, aimed at neither the specialist nor the idiot, but somewhere tantalisingly in between. In many ways it’s the perfectly judged popular-science book: he assumes little or no prior knowledge, but he does take for granted an open mind and a certain curiosity. His book will bring out the intelligent 12-year-old in us all. You may even look on the world with new eyes.
Marcus Berkmann, The Spectator, March 31
(requires subscription; the review has been reprinted here)

 

Ings has succeeded in writing an elegant, entertaining and up-to-date overview of cutting-edge research. He tells the story “episodically”, in a “mix of history, science and anecdote” that is utterly compelling.
Gail Vines, the Independent, April 25

 

Ings has a good eye for memorable anecdotes and striking facts. More importantly, The Eye is always readable, and Ings is a very good explainer of scientific concepts.
Robert Hanks, the Telegraph, March 18

 

The evolution of the human eye sounds a potentially arid subject, but not as treated by Simon Ings, who seamlessly blends natural history with personal observation (the progress of his baby daughter), visual conundrums (illustrations punctuate the text), and a real sense of wonder.
There are fascinating facts galore: our eyes are never still, for example. As well as entertaining, it’s philosophically profound: showing how our eyes, far from simply absorbing the world, are tools with which we construct our own reality.
Katie Owen, the Sunday Telegraph, 27 January 2008

 

Voles communicate by leaving trails of urine that (happily for the hungry kestrel) reflect ultraviolet light. Simon Ings will cheerfully supply you with a whole feast of such tasty morsels in this expansive history of the eye. But while his book may satisfy a nerdy hunger for trivia, it is much more than just a compendium of information, ambitiously blending science with philosophy and drawing on history and anecdotes. The latter, which often focus on his daughter, are impressive for somehow avoiding mawkishness; in one of the book’s most moving sections, Ings considers their respective ageing and sight. It all makes for a surprisingly appealing and readable book, helped along by the odd judicious diagram. Read it and you will never see things in the same way.
Hermione Buckland-Hoby, the Observer, January 27

 

The mirror of the soul? In Homo sapiens, maybe. But in this account of that remarkable organ, the eye, Ings goes beyond the human… The more complex his material, the clearer his prose becomes. He is equally at ease with mathematics, philosophy, palaeontology and history in this cornucopia of facts and folklore about the eye… this far-ranging and wonderfully eclectic work is popular science at its best.
Ross Leckie, The Times January 25

 

Charles Darwin wrote that thinking about the evolution of the eye gave him a “cold shudder”. The organ’s complexity tested his theory to the limits, yet so necessary has eyesight become to species’ survival that some scientists estimate the eye has evolved independently at least 40 times. As Ings puts it, “There are only a handful of really good ideas in nature”, and eyesight is one of them. There is a lot of science in Ings’s account, but it is leavened by engaging forays into history and biography. He relates such fascinating subjects as the ability of the Thai Moken tribe to see underwater, and Woody Allen’s rare skill in raising the inner corners of his eyebrows.
Ian Critchley, The Sunday Times, January 27

 

‘Popular science’ now too often refers to books about penguins’ feet and the like, so it is a relief to find a work that demonstrates genuine learning and intellectual passion. Simon Ings takes us elegantly through the 600-million-year history of the eye, explaining its differing functions in humans and animals, and discussing philosophical thoughts about vision. The result is a narrative as arresting and remarkable as any fiction, accessible but complete, with the reader assumed to be an intelligent adult on the lookout for something substantial. An excellent guide to one of the world’s true wonders.
the Telegraph, February 2

The soul stealers

Guardian, Saturday 19 January 2008

Iris, the Greek goddess of the rainbow, carried the first messages from the gods to man; 3,000 years later, the flow of communication is to be reversed. There are plans afoot, as we learned this week, to harness our irises, those pretty rings of multicoloured muscle in our eyes, to reveal our identities to the Olympians of Homeland Security.

We’ll each need to earn notoriety first: the FBI’s data-sharing proposals, involving an entire suite of biometric data, are directed at catching major criminals and terrorists. The name the Feds gave this project, however, suggests that someone, somewhere, is looking to the future: “server in the sky”. This is either a tip of the hat to 80s rock band Doctor and the Medics’ only hit or, more likely, a grotesque piece of security-state triumphalism.
Mind you, we are all more than likely to offer up our eyes over the next couple of years to any institution that cares to stare into them. Iris scanning is set to replace the passport and credit card as the preferred method of proving identity. Who wouldn’t want to pass through Heathrow in a blink, after all?

But there is something unpleasant about the idea of having one’s eyes scanned, and this is not altogether the fault of the film Minority Report’s stolen eyeballs scene. It is more to do with our intuition that the eyes are windows on the soul. The human eye is built to be noticed. Simply opening the eyes wider can, with other facial movements, express everything from shock to arousal to doubt. Simple gaze direction conveys emotional meaning. The lateral rectus eye muscle is labelled “amatoris” in early anatomies because lovers use it to direct their flirtatious glances.
Eyes reveal our inner state. It is impossible to control our rate of blinking for any length of time, or the way our pupils wax and wane. When aroused, we blink more often, and our irises dilate. Our eyes, with their bright whites, colourful irises, responsive pupils, brows and lashes, have evolved to communicate and carry meaning.

Nonetheless, given the amount of information they carry, eyes are surprisingly hard to read. We don’t count each other’s blinks, and we don’t press our faces up against each other to study the changes in each other’s irises. Of course, we don’t have to: we have language – which lets us lie in a way the eyes don’t. But liars are easy to spot – aren’t they?

Humans have been pack animals for most of their history. When survival depended on cooperation there was little advantage to be had from blatant lying. In a tightknit community, a pathological liar stands to lose too much if they are caught out. Now, things are different. A 65-year-old, Jean Hutchinson, was sent to jail for five years this week. Why? From her secret operations room, accessed through a wardrobe, she had managed to impersonate 76 different people well enough to defraud the British state of £2.4m.

Technology confers anonymity on people far more effectively than it establishes identity. The biometric security market emerged in the US following the passing of two laws. Neither had anything to do with security, the war on terror or other bugaboos. One was the health insurance act of 1996, which made healthcare firms protect their clients’ records more carefully; the other, known as Sarbanes-Oxley, was meant to reduce the fiddling of financial records after the collapse of Enron.

The war on terror is a branding exercise. The war on fraud is real. The technology has a long way to go before machines are invented that can scan our eyes for the secrets of our hearts. Still, this is the path we are on. As our machines learn more about us, we are increasingly learning how to hide behind our machines.

Sensations into symbols

Guardian, Thursday 15 March 2007

Like those jingles you can’t stop humming, some bad ideas stick. This one has maddened me for years: when you and I see a green ball, do we see the same green? When we have toothache, we don’t all have the same toothache. The notion that pain varies between individuals does not disturb us. Why, then, do we resist the idea that different people see different colours?

Just as you and I, each suffering our own very different toothaches, can agree on what a lousy experience toothache is, so we can all roughly agree on what colour is what. We can argue till the cows come home whether this particular shade of turquoise is green or blue, but we both pretty much agree on what green and blue are. There is a lawfulness to colour, and it would help if we knew where this lawfulness resided.
In his 1995 essay The Case of the Colour-blind Painter, the neurologist Oliver Sacks describes the case of an artist who, through subtle but devastating damage to his brain, could not see colour. Though damage of this sort robs an individual of the experience of colour, the mechanisms of colour vision continue to function. Asked to match up coloured counters, people with no experience of colour are still able to match up colours perfectly. They just don’t see them. But if the relationship between wavelength and colour is purely contingent, where the devil do colours come from?

Artists are forever trying to uncover universal meanings behind their colours. It is easy to scorn their efforts, not least because this kind of thinking dates very quickly. Kandinsky’s experiments in colour symbolism may as well have been conducted in the 14th century for all their relevance now. There is, none the less, a growing body of evidence that colours, shapes, sounds and smells do have meanings. Wolfgang Köhler’s delightfully simple 1929 experiment asked volunteers to match a pair of abstract figures to one of two nonsense words, “maluma” and “takete”. Immediately, and virtually without exception, people matched maluma to the soft round figure and takete to the sharply angular one. Some sort of shared symbolism related the sounds to the shapes.

Now Dr Jamie Ward, at University College London, might have uncovered an underlying symbolism to colour. Ward’s interest is synaesthesia – the experience of a handful of individuals who perceive information through an unexpected sense. Some hear colours, others smell shapes. The vast majority see sounds. The experiences of individual synaesthetes are notoriously idiosyncratic. But there are unexpected regularities, and Ward’s bulging address book – he knows 450 synaesthetes by name – allows him to spot trends that were formerly invisible. For example, among synaesthetes who see coloured letters, A is often red, B is often blue, and C is often yellow. “This is likely to hold true for other types of synaesthesia,” Ward says, “assuming that we are able to make a large enough number of observations. For instance, certain musical instruments may tend to produce particular colours, shapes and movements.”

Synaesthesia may simply be an exotic manifestation of something we all enjoy: the ability to turn sensations into symbols, and to think with them. After all, if our thoughts are not made of sensations, what are they made of? And this is why we find it so distressing, you and I, to realise that we don’t see the same colours. Colours – so striking, so beautiful, so manifestly there – are one of the few things we can agree on, more or less. How cast adrift will we feel if colours turn out to be, after all, only our thoughts about light?

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

Independent, Wednesday, 7 March 2007 http://bit.ly/4pzolR

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.

Vision in the womb

Rodsandcones1

By April, Natalie had begun, now and again, to sleep. Every so often she closed her lids and kept them shut, and when she did, her eyes trembled, stirring the oxygen-rich liquid, called aquaeous humour, that lies between the iris and the cornea. (The rapid-eye movement that accompanies dreams has at least one very practical purpose: it feeds the front of the eye.(1)) This trembling woke the light-sensitive cells of Natalie’s retinas. Stimulated, her retinal cells fired at random, preserving and strengthening their connections with each other.(2) Even before she saw, Natalie went through the motions of dreaming, and those motions taught the cells in her retina to hold hands.
Prepared by dreams, Natalie’s retinal cells took their next lessons from light. Even before birth, the body is no stranger to illumination. Flesh itself lights up a little, every time a nerve fires(3). Perhaps this familiarity with light is why any young nerve cell, transported to the retina – the body’s most light-sensitive surface – will learn to see, just as every seeing cell, moved elsewhere, becomes an ordinary nerve(4).
The womb is not dark: it is easily penetrated by light from the outside world. From November to July of 2003, the month of her birth, Natalie’s retinas grew to seize what news they could from the amniotic murk of her home. They adapted to darkness and to blur. One layer of nerves grew into light-sensitive cells called rods, the better to gather the light. At birth, Natalie was well on the way to acquiring good nocturnal vision. Babies see well at night.
In the glare of day, though, they are all but blind. (It is one of the ironies of birth that it fills our world with light – and blinds us in the process.) A sunny day is a million times brighter than a night-time nursery, and a whole other form of vision is needed to handle such a glare – a form of vision Natalie had not yet got.
Exposed to the light, Natalie’s eyes had not so much to ‘adapt’ to the brightness of day, so much as acquire a whole new way of seeing.
In her retinas lay another set of nerve cells, distinct from the rods and only distantly related to them.(5) At birth, they were little more than ordinary nerve cells. Once exposed to the glare of day, however, they began to change. Natalie will be six before these ‘cones’ of hers are fully grown, packed as tight as they can be into the fovea – that tiny circle on the retina where images are focused and light explodes with colour.

(1)Maurice DM. 1998. ‘An ophthalmological explanation of REM sleep.’ Experimental Eye Research 66 pp139-145. This article includes some interesting background on Maurice’s work.
(2)Siegel JM. 2005. ‘Functional Implications of Sleep Development.’ PLoS Biology 3/5 p178
(3)Tarusov, BN., Polivoda, AI., Zhuravlev, AI. 1961. ‘Study of the faint spontaneous luminescence of animal cells.’ Biophysics 6 pp83-85. (This paper is one of the inspirations behind space physiologist Karl Simanonok‘s diverting ‘endogenous light theory of conciousness’.)
(4)Cepko, C. Interview with Norman Swan on The health report: the retina and the brain, ABC Radio National, Australian Broadcasting Commission, Monday 13 December 1999.
http://www.abc.net.au/rn/talks/
8.30/helthrpt/stories/s73272.htm
Livesey FJ, Cepko CL. 2001. ‘Vertebrate neural cell-fate determination: lessons from the retina.’ Nature reviews: neuroscience 2/2 pp109-18. (PDF)
(5) The distinction between rods and cones had already arisen in the eyes of jawless fish, swimming in Devonian seas around 400 million years ago. See Bowmaker, J. K. 1991. ‘Evolution of photoreceptors and visual pigments.’ Pp. 63–81 in J. R. Cronly-Dillon and R. L. Gregory, eds. Evolution of the eye and visual pigments. CRC Press, Boca Raton, Fla.

The true story of the Cyclops

Cyclops1

Natalie, my daughter, began life with one eye placed centrally in her forehead. It appeared early – barely a week after her conception – in October 2002, and had things gone awry, there it would have remained, a single cyclopean orb, glowering in the shadow of a grotesque proboscis that would have grown in place of her nose.(1)

There are many explanations for what inspired the legend of the Cyclopes – the one-eyed monsters of classical mythology. These explanations invoke everything from an ancient find of mysterious dwarf elephant skulls, to the blacksmith’s habit – in the days before protective goggles – of protecting one eye with a patch. But the most likely inspiration for the story is also the saddest one. Occasionally, a human cyclops survives in the womb long enough to emerge visibly disfigured. Once upon a time, someone got a good look at what they had aborted – and wished for ever after that they hadn’t.

It happened again, not so long ago. George Gould and Walter Pyle’s indispensable volume Anomalies and Curiosities of Medicine of 1896(2), includes an account of a woman of thirty-five, the mother of seven children, ‘who while pregnant was feeding some rabbits, when one of the animals jumped at her with its eyes “glaring” upon her, causing a sudden fright.’ The child’s mouth and face were small and rabbit-shaped. Instead of a nose, it had ‘a fleshy growth 3/4 inch long by 1/4 inch broad, directed upward at an angle of 45 degrees.’ Between the nose and the mouth was an organ resembling an adult eye.

Within it lay ‘two small, imperfect eyes which moved freely while life lasted’.

The disfigurement of the face was the mere outward mark of an even more collosal failure – the brain failing to divide into two delicately linked halves. So, mercifully, after about ten minutes, the child died. The mother went on to have two more children, and they were perfectly healthy.

Human beings arise, not from dust, but from stuff hardly more edifying: a gellid spittle. How from this unpromising material anything beautiful emerges – let alone anything that comprehends and communicates something of the world beyond itself – is a mystery too big to be encompassed by just one version of events. How we explain how we grow depends on fashion. It is the fashion now to talk of rules embedded, like a code, in every cell. But this, like most shorthand explanations, fosters misunderstanding. Natalie was not fashioned by rules. Nothing oversaw her growth. Genes set the conditions of the game, but it was the game itself that built her: the touch and slide of surfaces, the little chemical kisses, the partings and the reunions, each part of her tissue communicating chemically with each neighbouring part as she folded herself into being.

It was this dance, known to science as induction, that told her brain to split, that squeezed her single orbit into two, and drew two little strings of brain out to the windows in her skull, there to make her eyes.

Teaching the skin to see

 

Since the early 1970s, Paul Bach-y-Rita has been building prosthetic eyes for the blind: not false eyes, not glass eyes, but fully working organs of vision. With them, Bach-y-Rita – a biomedical engineer at the University of Wisconsin-Madison – has helped the blind to see.

His eyes do not look like eyes. The earliest models look like clothing. Bach-y-Rita’s vests are worn either across the stomach or across the back. Sewn into the material are 256 mechanical vibrators (nicknamed ‘tactors’ because, when they’re activated, the subject can feel their touch). A computer worn at the hip recieves pixellated images from an ultra-low resolution video camera, worn on a pair of eyeglasses, and translates these images into mechanical vibrations, via the tactors. The upshot is a kind of Braille or Pin Art vision.

Bach-y-Rita’s subjects reported that after a couple of hours they were no longer aware of the tingling sensations generated by the vest. They were able to navigate between obstacles, and, eventually, to recognise faces. When the ‘view’ before them changed – because they moved, or because something moved in front of them – they reacted appropriately to the change of view. If you screwed up a piece of paper and threw it at them, they would duck.

Even more suggestive is an experiment reported by Daniel Dennett in which a researcher, without warning, manipulated a zoom button on a volunteer’s camera, making it seem as though he were hurtling forward. The volunteer raised his hands to protect his face. But his vest was strapped to his back.(1)

The artificial sense bestowed upon his blind volunteers by Paul Bach-y-Rita not only works like vision – it feels like vision. It seems that the mind is not overly fussy where it gets its sensory information from. What matters is what ‘shape’ the information takes. If visual information is received through the skin of your back, it only takes your brain a couple of hours to start seeing through your back. If your back starts itching, on the other hand, you won’t mistake the itch for a flash of light. The ‘shape’ of an itch is different to the ‘shape’ of, say, a face, and the brain knows how to deal with each.

 

The senses become specialized over evolutionary time, but they are never entirely compartmentalised. If we look closely at a rod – a photosensitive cell common to almost all vertebrate eyes – we see that it comes in two parts – a fairly normal-looking cell body, and a column made up of thousands of discs containing the pigment rhodopsin. When the rod is exposed to light, the pigment column expands like a slinky to twice its length, with no increase in width. In the dark, it contracts again. Each rod is behaving just like a muscle cell – and for good reason. In many functional respects, it is a muscle cell. Muscle fibres expand and contract in response to electrical stimulation. The retinal rod, too, is responding to an electrical signal – one that comes, not from a nerve, but from a biochemical reaction to light. This is what the working retina looks like on a cellular scale – a vast automated Pin Art machine.

(1) Dennet, D. 1991. Consciousness Explained. New York, Little Brown & Company, pp339-342

For an overview of Paul Bach-y-Rita’s work, see Paul Bach-y-Rita, Mitchell E. Tyler, and Kurt A. Kaczmarek. 2003. ‘Seeing with the brain.’ International journal of human-computer interaction, 15/2:pp285-295. In early 2001, the University of Wisconsin-Madison’s article Tongue seen as portal to the brain first broke the news of Bach-y-Rita’s return to the sense-substitution field. (Since the late 1970s, he had turned his attention more towards to the rehabilitation of victims of brain damage.) The latest applications of Bach-y-Rita’s work are discussed in Blakeslee,S. 2004. ‘New Tools to Help Patients Reclaim Damaged Senses.’ New York Times, November 23.
See also See also Bach-y-Rita’s commercial website Wicab.com.

Matters of light and darkness

How well do you see in the dark? Edward Halsall, a royalist major during the Cromwell era, was imprisoned for twenty months in a windowless room. It took Halsall’s eyes seven months to adjust fully to the dark, but by the end of his imprisonment, he ‘could see the mice that used to feed upon his leavings’; ‘well enough’, indeed, ‘to make a mousetrap with his cup.’ Humans have excellent night vision. (We are, after all, the descendants of nocturnal shrews.) And it’s by juggling two quite distinct forms of vision – one adapted to the dark, the other to the light – that our eyes can cope with virtually any lighting conditions.

This is as well: on a sunny day, our eyes receive a million times as much light as they can gather on a clear, moonless night. How can our eyes cope with such staggeringly different light levels?

In 1867, a young physicist called Ernst Mach pondered this optical illusion. Arrange a series of grey bands, each band slightly lighter than its neighbour, and they look as though they have been lit from the side. The edges lying against darker neighbours appear lighter, while edges lying against lighter neighbours appear darker. The fluting is an illusion, obviously – but why should the eye manufacture dark where there is no dark, and light where there is no light?

Theeye1

Spotting boundaries is essential to vision. Without boundaries, the edges of objects become uncertain, and objects simply bleed away into the background. So the eye manufactures shading to reveal the forms of objects. Mach worked out the mathematics of how the eye could do this. It was a brilliant piece of work, still used today. (Bang & Olufsen’s new televisions handle contrast and picture detail in an intelligent manner by applying algorithms first dreamt up by Mach, nearly a century and a half ago.)

In the 1930s, American physiologist Haldan Keffer Hartline identified the parts of the eye that performed Mach’s mathematical magic tricks; and, in doing so, he discovered something surprising. When the eye studies an evenly illuminated surface, its optic nerve falls silent. The eye can handle a million-fold difference in light level because the eye doesn’t measure the light level at all. All it ever reports are small, local variations in light intensity. Look very closely at portrait of Che Guevara, – a delightful visual puzzle dreamt up five years ago by Dr Steven Dakin of University College, London. You will see, if you look closely enough, that the lit parts of Che’s face are exactly the same shade of grey as his beard and facial shadows. It’s the banded line that tells your eye which side of the line is supposed to be light, and which side is supposed to be dark – and it’s your eyes that then add shading to the picture.

Theeye2

Our perception of colour, too, is a matter of contrast. Vivid as the colours around us seem, their brilliance is manufactured in the eye. Our eyes gauge the brightness, hue and vividness of patches of colour by relating them to the shade, hue and vividness of their surroundings, and we can draw figures, like the ones here, to show how the same colour looks very different when it appears in different surroundings.

Theeye3

Simple figures like these seem to trick the eye into error. But in the rich visual environment of the real world – a world full of multiple light sources, shimmering reflections, dappled shadows, and complex three-dimensional patchworks – our style of vision enables us to identify the colours of things with extraordinary accuracy.

Oddly, this point that was lost on vision science until midway through the last century, and the arrival of Edwin Land. Land was, after Thomas Edison, America’s most prolific inventor. Polaroid photography is just one of his inventions. Land’s startling experiments and demonstrations showed how robust our colour vision is under different lights. He prepared boards of intersecting multicoloured shapes (called ‘mondrians’, after the artist whose work they resembled) and lit them with lamps of different hues. People studying the mondrians described their colours accurately even under the most bizarrely tinted lighting.

But Land’s most famous ‘experiment’ happened by accident. Land and his team were using red, green and blue lights to produce a true-colour image on a screen (cathode-ray televisions work this way). Come evening, Land and his assistants shut off the blue projector and took the green filter out of the green projector. It was then that one of the assistants called Land’s attention to the screen. The red projector was still running, and the unfiltered green projector was projecting its image over the top of the reds in white light. And there, upon the screen, was the original full-colour image. Red and white lights were throwing blues and greens upon the screen! Land realised that the eye was using the little information it had to colour in the image, just as your eye shades in the portrait of Che Guevara.

Our eyes make things up. They snatch trickles of light from a world of blur and shadow, and they manufacture pictures of the world that are both coherent and true. The optical illusions on these pages do not ‘fool’ the eye – rather, they persuade it to reveal its creative power. They show us why, in the real world, we can believe our eyes.