You shouldn't always believe your eyes

Isaac Newton struggled to conceptualise the nature of light; Albert Einstein chased a theory of everything. All intellectuals seem to grapple with subjects that threaten to overwhelm them - and Charles Darwin's nemesis was the eye. In his 1859 book, The Origin of Species, Darwin set aside a special section solely to criticise his own theory of evolution, and the eye took centre stage. While he could conceive that a thin-beaked finch could have evolved from a fat-beaked relative, an organ as sophisticated and perfect as an eye lay poles apart. Surely, he thought, such a faultless organ couldn't possibly result from bumbling natural selection?

Isaac Newton struggled to conceptualise the nature of light; Albert Einstein chased a theory of everything. All intellectuals seem to grapple with subjects that threaten to overwhelm them - and Charles Darwin's nemesis was the eye. In his 1859 book, The Origin of Species, Darwin set aside a special section solely to criticise his own theory of evolution, and the eye took centre stage. While he could conceive that a thin-beaked finch could have evolved from a fat-beaked relative, an organ as sophisticated and perfect as an eye lay poles apart. Surely, he thought, such a faultless organ couldn't possibly result from bumbling natural selection?

Darwin was less complimentary of what the eye can see - colour. He considered that evolution simply "does its best" to adapt the colour of animal skins, scales, furs and shells to shield their owners from the all-conquering retinas of their enemies. When he thought of colour, he made analogies with an artist's palette. Yes, all the colours of the rainbow may well be represented in animals, he thought, but they all result from pigments - uncomplicated, rudimentary pigments. Due to the technological gap, evolutionary attempts to deceive the eye with colour would surely have been futile.

In my book, Seven Deadly Colours, I demonstrate the opposite. With the aid of NanoCam, the world's most minuscule camera (one that can weave between, and even penetrate, the hairs on a flea's leg), I find flaws in the eye. In an evolutionary arms race spanning 500 million years, colour has often outwitted the eye. The "blue frog" story provides one example.

On the painter's palette lie violet pigments, blue pigments, green pigments, yellow pigments, orange pigments. On nature's palette are violet nano-optics, blue bioluminescence, green pixel permutations, yellow fluorescence, orange optical illusions... There are even colours we cannot see. NanoCam reveals the vivacity of the microscopic factories of skins that relentlessly churn out animal hues, including electrons bustling with life and chemicals reacting to produce blinding flashes.

There are not only seven colours in the rainbow, but seven different ways in which animals produce their colour. Although pigments are involved in the blue frog story, there is an additional element not on an artist's palette. The virtuosity and sophistication of colour in nature, it will emerge, can counter that of the eye.

While Darwin turned his attention to barnacles, other Victorian naturalists applied the new science of taxonomy to the remaining fauna. In the back-room laboratories of stately homes, corks were popped on alcohol-filled bottles that had for years been preserving all manner of exotic creatures - the riches of global expeditions from the age of sail. Of these, one notable expedition was Cook's voyage to the South Pacific and "Terra Australis Incognita", on the Endeavour, and the subsequent voyage of the First Fleet in 1788. The 11 ships of the First Fleet were filled not only with convicts and officers but also with scientists, including John White, a surgeon fascinated by natural history.

White made collections of the Sydney fauna, including a frog - a rather placid and completely harmless frog. With time at a premium, the animals accumulated were abruptly preserved and casually shelved away. "Alcohol is alcohol," thought White. The frog, no longer alive, was duly squeezed into a bottle of rum, and labelled with its New South Wales address - right down to the tree on which it was caught.

Back in England, some years later, White blew the dust off his rum bottle, cracked the glass and lifted the Australian frog from the alcohol. Inspiration for its Latin name was blatant - the frog was uniquely blue. So Litoria caerulea became the species name, from the Latin meaning "pale blue". Joseph Banks, the Endeavour's scientific officer, had previously described the tree on which this frog had been sitting (according to the specimen label) as having green leaves. So why should the blue frog - a defenceless chunk of protein - choose to sit among green leaves?

Generally, when we view a field packed with animals, from beetles and spiders to mice and wrens, almost none of that fauna is observed. The reason for this is that eyes exist in and around that field. Accordingly, everything exposed in the field will leave impressions in them. To avoid becoming easy prey, any animal in that field must adapt the impression it leaves on a retina. The commonest adaptation is camouflage.

True to form, the Australian "blue frog" turned out to be green. Another of these "blue frogs" was later encountered in the wild, this time by a more progressive, Victorian scientist. At first, he must have assumed that it was a new species - the frog peeled from its leaf was not blue but green. But all the other features of the original "blue frog" were there, including the shape of the sucker-like feet, the size of the body and the webbing of the hind feet only. This was undoubtedly White's "blue frog".

So why did White's green tree frog appear blue when it was lifted from its rum bottle? Enter NanoCam. When NanoCam's microscopic fibreoptics make contact with the green skin of a living example of this frog, it observes cells in two distinct layers. It punctures the membrane of a cell in the first layer. Light rays are observed bouncing around within the cell, heading in all directions, which is unusual since sunlight enters from only one direction. The fluid cytoplasm fills the cell and holds the cell membrane rigid. Within this watery matrix, debris lies in NanoCam's path. The cytoplasm is littered with tiny particles, arranged completely at random, all smaller than the wavelength of light. NanoCam's film rolls as sunlight strikes the frog and hurtles towards these cells.

Sunlight consists of waves of different sizes, representing different colours. The red rays are longest and straddle the tiny particles. They pass straight through the cell. The blue rays are smaller, and collide with the particles in the cell. They bounce from particle to particle, pinball fashion, and are reflected back into the environment. In fact, blue rays are the only type to be significantly reflected, and that's the cause of the frog's blue. As Leonardo da Vinci first postulated, the sky appears blue for the same reason (blue rays reflect off water molecules in the atmosphere). To explain the frog's green colour, NanoCam ventures to the second layer of cells.

The second type of cell contains no particles but molecules on an even smaller scale. NanoCam observes a single molecule as it is struck by the remaining sunlight. Crash! Light rays hit the molecule head-on, and trigger unnatural movement of its electrons. Driven by the energy contained in the sunlight, they begin to leap about within the molecule, eating up most of the sun's rays. NanoCam warms up as the energy from the eaten rays is dumped by the molecule as heat. But not all of the sun's rays are consumed by the molecule. Some - the yellow ones - are instead reflected back out into the environment.

These yellow rays mix with the blue in the frog's upper layer of skin, and we see green light. The frog appears green! So, then, why did that specimen in the rum bottle look blue? Alcohol destroys pigments - which is why you can clean paintbrushes in turpentine. The rum in the bottle destroyed the frog's yellow pigments, so yellow light was no longer reflected. The skin cells, nonetheless, did not dry out. Alcohol preserved each cell, fixing any particles in their natural positions. Hence the blue reflections persisted. The pickled frog became blue.

So Darwin's apparently perfect eye is observing a blue and yellow frog, but thinks it's seeing a green one. The eye does not see the true colours represented by the light emanating from the frog's skin. The eye is fooled; it is not perfect after all. This has consequences back in the wild. A predator that strives to catch a green tree frog has great difficulty in finding it. The frog is actually incredibly obvious, shining out in blue and yellow amid a canopy of pure green. But the retina cannot distinguish between blue-plus-yellow=green and pure green. Due to the design of the eye, a bird or reptile predator cannot see its obvious food.

The truth is that a variety of eyes exist on Earth, and all have their pros and cons. Each has its own, unique innovations that are particularly good at certain tasks, such as seeing in the dark or detecting movement. But as a result, each eye type falls short in other areas - humans cannot see ultraviolet light, for example. Ultraviolet is a part of the spectrum where other animals flaunt themselves in front of our very eyes yet we have no idea. While birds and insects communicate in their ultraviolet language, we are left looking foolish. Our own eyes are not perfect, either.

Dr Andrew Parker is research leader at the Natural History Museum, London SW7 ( animaloptics@aol.com). His book, 'Seven Deadly Colours' is published by Free Press (£16.99)