Sight

VHS cover

*Sight* is the third episode of the third season of Eyewitness. The topic of the episode is about the sense of vision in both humans and animals, the abilities and physics of vision and how optics was discovered by scientists. The episode aired on 29th September 1997.

Sight is not based on a book title, and no Eyewitness book about the subject has been published since the episode aired.

## Episode

### Introductory sequence

Fragments of white light appear in a black void, that is then gradually surrounded by a blue border. A barn owl flies out of the black void. The camera zooms out to show the black and blue are parts of a human eye, which blinks closed before fading to white.

### Episode outline

Click here for the full transcript for Sight.

Sight is the sense we rely on more than any other. Two-thirds of information in the human brain comes through the eyes. Vision relies on three elements – eyes to gather images, a brain to process them and light energy to make it all possible. The returning sunlight of each dawn powers the entire food chain and enables animals to see what they’re doing and where they’re going.

Light travels faster than anything else in the known universe. Even the earliest human cultures understood that life depends on light. One ancient terror is that night may never end, so cultures carried out precise rituals to ensure that the sun would rise every morning. When morning comes, the night creatures rest while others race into action. The eyes of the hunters and the hunted reveal the roles they play, with cheetahs having forward-facing eyes and their prey, gazelles, having eyes on the sides of their head to see attackers from any direction. Depending on an animal’s role, there are striking differences in eye position, but all vertebrate eyes share a strikingly similar structure.

The next section looks at the studies of optics being made around 1,000 years ago by an Arabic scientist called Ibn Al-Hazan. He described how the eye worked, identifying the toles of the cornea, the lens and the retina within the eyeball. TUntil the time of Al-Hazan, it was thought light came from the eyes, but the astute scientist correctly suggested the reverse – the eye takes light in.

When light enters the eye, the lens focuses the image onto the retina, a living screen about the size and thickness of a postage stamp. The image shows upside-down in the retina but when the impulse reaches the brain it is interpreted right-side up. Many animals form sharp images only at the centre of the retina. A cheetah sees clearly in a thin horizontal line, ideal for scanning the horizon for prey.

Basic eyes can only tell light from dark, but can still help trigger a getaway. As the animals with the best sight thrived, evolution brought about yet more eyes ranging from the simple eye of the planarian to the huge eyeballs of squid. The ways that several animals see the world is explored, including frogs that only see contours and moving objects, horses which cannot see directly in front of them and birds of prey with their incredibly sharp eyesight.

Midday light can be blinding but they eye has a ring of muscle that can open or close depending on the brightness. This is called the iris, and it gives the eye its colour. Both cheetahs and the ancient Egyptians use black markings around the eyes to minimise glare from the sun. The iris surrounds the pupil, from the word for doll. This may be because, in another’s eyes, you can see a small doll-like reflection.

The next section looks at the many superstitions surrounding the evil eye, a belief that a person could harm others by the power of their gaze. Protective amulets and ceremonies were used to defeat the evil eye, and blindfolding a condemned prisoner may have its foots in the belief. Other frightening eyes are also considered here - Argus had hundreds of eyes until they were put on the peacock's tail. Real animals use false eyes to confuse or frighten predators. The cyclops was a creature with just one eye, possibly inspired by discoveries of the skulls of pygmy elephants. A tiny crustacean called Cyclops is the only living creature with a single eye. A solitary eye also appear on the US dollar bill.

The eyes of invertebrates are particularly fascinating to scientists. Dragonflies have compound eyes with hundreds of facets that provide a single image as a mosaic. They offer a wide-angled view without the need of eye sockets or muscles, but can see nowhere near as clearly as human eyes can. The compound eyes of a fly can detect movement ten times faster than we can.

We blink to keep our eyes moist and shut them in emergencies. Some animals blink with their lower eyelids, while bird eyelids pass sideways over the surface of the eye. Polar bears have extra eyelids to protect against bright arctic light. Humans once had this feature, but evolution has mainly done away with them.

Nature has created variations of the roving eye, with the most extreme case being the chameleon. Many birds can't move their eyes at all, so have to move their heads instead to see. Birds are champions of sight, with some even able to see underwater.

At midday, light penetrates to its greatest depth in water. Some aquatic animals with particularly impressive eyesight at the arowana, which catches insects above the water, the four-eyed fish and the mantis shrimp, which can see more colours than a human can.

Many animals can't see colour that well, instead detecting motion. Cats and dogs both detect movement much more easily than colour, and the colour of a rag may be irrelevant to a bull. Humans have phenomenal colour vision, and can distinguish ten million different shades in good light. However some degree of colour blindness is common, particularly among men.

The real job of seeing is done by the brain. For centuries, the connection between the eye and the brain was a mystery, but we now know that a tangle of nerves makes the connection. Humans acquired their extraordinary colour vision from their primate ancestors, that were looking for ripe fruit in green foliage.

Where does colour come from? Sunlight looks white. Isaac Newton showed how a beam of light passed through a glass prism split into the colours of the spectrum. The colour of an object is determined by the colour of light it reflects, with every other colour absorbed. For example, leaves absorb all light except green, which is useless in the process of photosynthesis. Some of light's colourful performances fascinate us, such as the aurora, rainbow and mirage. Flowers, and some insects, have more colours than we can see, as they are painted with ultraviolet hues.

The next section looks at how the brain-eye combination can be tricked through optical illusions. The brain can make startling adjustments, including being able to navigate in a world turned upside-down. Equally, the limitations of our eyesight can be important - our brains can't keep up with a film at its projection speed, but if it were slowed down they pictures would not flow together.

Many of us rely on glasses with tailor-made lenses to see clearly. The development of glasses, including early water-filled lenses and the eyeglasses worn by monks, is outlined. The myth of eyesight being improved by eating carrots is also addressed here, as it was propaganda released by the British during World War II to cover up the invention of radar. New worlds of sight continue to open, including the discovery of very small worlds visible through microscopes and distant worlds visible through telescopes.

As day ends, the sunlight fades and nocturnal animals emerge. They have challenges, as the light is so much more sparse. The night sight of hunters like the owl and jaguar are explored, as well as the folklore inspired by the glowing eyes of cats. The heat-sensitive vision of rattlesnakes is also included here. Dawn lifts the danger for the prey of these night hunters. The energy source behind every glimpse returns. Of all there is to see on Earth, the return of daylight is surely a sight to behold.

## Animal species filmed for the programme

### Studio footage

* Barn owl, *Tyto alba*
* European brown common frog, *Rana temporaria*
* House mouse, *Mus musculus*
* Giant owl butterfly, *Caligo memnon*

### Stock footage

* Red-eyed tree frog, *Agalychnis callidryas*
* Atlantic spotted dolphin, *Stenella frontalis*
* Bald eagle, *Haliaeetus leucocephalus*
* Winter moth, *Operophtera brumata*
* Thomson’s gazelle, *Eudorcas thomsonii*
* Cheetah, *Acinonyx jubatus*
* Hoverfly, *Syrphidae*
* Horn shark, *Heterodontus francisci*
* Pacific angelshark, *Squatina californica*
* European brown hare, *Lepus europaeus*
* Domestic dog, *Canis familiaris*
* Swift fox, *Vulpes velox*
* Blue wildebeest, *Connochaetes taurinus*
* Lion, *Panthera leo*
* Ruppell’s griffon vulture, *Gyps rueppelli*
* Plains zebra, *Equus quagga*
* Egyptian goose, *Alopochen aegyptiaca*
* Lesser flamingo, *Phoeniconaias minor*
* Red fox, *Vulpes vulpes*
* Mountain hare, *Lepus timidus*
* Horned puffin, *Fratercula corniculata*
* Eastern chipmunk, *Tamias striatus*
* White-faced scops owl, *Ptilopsis granti*
* Verreaux’s sifaka, *Propithecus verreauxi*
* Whitetip reef shark, *Triaenodon obesus*
* Gecko, *Gekkota*
* Squid, *Decapodiformes*
* Great grey owl, *Strix nebulosa*
* Waxy monkey leaf frog, *Phyllomedusa sauvagii*
* Common green grasshopper, *Omocestus viridulus*
* European brown common frog, *Rana temporaria*
* Domestic horse, *Equus ferus caballus*
* African white-backed vulture, *Gyps africanus*
* Common kestrel, *Falco tinnunculus*
* Short-tailed field vole, *Microtus agrestis*
* Indian peafowl, *Pavo cristatus*
* Zebra longwing butterfly, *Heliconius charithonia*
* Great eggfly butterfly, *Hypolimnas bolina*
* Io moth, *Automeris io*
* Cyclops, *Cyclops sp*.
* Common darter dragonfly, *Sympetrum striolatum*
* Emperor dragonfly, *Anax imperator*
* Common prawn, *Palaemon serratus*
* Camel shrimp, *Rhynchocinetes durbanensis*
* Common housefly, *Musca domestica*
* Polar bear, *Ursus maritimus*
* Mediterranean chameleon, *Chamaeleo chamaeleon*
* Painted lady butterfly, *Vanessa cardui*
* Eurasian woodcock, *Scolopax rusticola*
* European robin, *Erithacus rubecula*
* Brown pelican, *Pelecanus occidentalis*
* Silver arowana, *Osteoglossum bicirrhosum*
* Largescale four-eyed fish, *Anableps anableps*
* Peacock mantis shrimp, *Odontodactylus scyllarus*
* Reef lobster, *Enoplometopus sp*.
* Domestic cat, *Felis catus*
* Domestic sheep, *Ovis aries*
* Domestic cattle, *Bos taurus*
* Yellow-handed titi monkey, *Callicebus torquatus*
* Dromedary camel, *Camelus dromedarius*
* Blue butterfly, *Polyommatinae*
* Small tortoiseshell butterfly, *Aglais urticae*
* White tern, *Gygis alba*
* Red-tailed tropicbird, *Phaethon rubricauda*
* Common rook, *Corvus frugilegus*
* Greater galago, *Otolemur crassicaudatus*
* Jaguar, *Panthera onca*
* Desert woodrat, *Neotoma lepida*
* Western diamondback rattlesnake, *Crotalus atrox*
* House mouse, *Mus musculus*
* Barn owl, *Tyto alba*

## Stock footage sources

Wildlife on One: Shadow of the Hare (1993) – Greyhounds chasing hare

Natural World:

* Amazon – The Flooded Forest (1989) – Silver arowana leaping for prey
* Through Animal Eyes (1985) – Frog vision, chameleon hunting butterfly, rattlesnake heat vision, ultraviolet flowers

## Credits

* Director - Jack Weber
* Writer - Anne MacLeod
* Music - Guy Dagul
* Graphic Director - Morgan Almeida
* Camera - Nick Squires and Martin Bobbin
* Set Design and Visual Effects - Chris Reynolds and Andy David
* Research - Inge Samuels
* Production Co-ordinator - Maryrose Upjohn
* Production Team - Louisa Bowman, Philip Whelman and Jackie Byrne
* Unit Manager - Jackie Williamson
* 3-D Animation - Michael Lawson
* Videotape Editors - John O'Connor and Andy Attwell
* Dubbing Mixer - Peter Collins and Kian Wong
* With thanks to:
	+ Imperial Cancer Research Fund
	+ All M C Esher works © 1996, Cordon Art, Baarn, Holland
	+ Ministero per i Beni Culturali e Ambientali

## Making this programme

Sight is partly covered in The Making of Eyewitness 3: Worlds. It shows the filming of the optical illusion with the barn owl, which combined the challenges of filming with animals, models and graphics. The set took three days to construct, with every detail having to be measured and lined up precisely before the hand-painted backdrop was brought in. Getting the owl to line up with the shot took more time, as the bird had to familiarise itself with its surroundings. But a carefully calculated camera move across the set and a well-behaved owl produced the remarkable final shot.

## Home media releases

Sight has only ever been released on VHS. None of the three different DVD releases included this episode.

**Writing staff**: Anne MacLeod • Ben Southwell • Ben Steiner • Brian Meehl • David Hanson • David Stafford • David Helton • George Bloom • Lynette Singer • Mike Tomlinson • Paul Thomas • Stan Cullimore
**Directors**: Gavin Maxwell • Justine Kershaw • David Hutt • Andrew Jackson • Justin Hardy • Ludo Graham • Jerry Foulkes • Derek Hall • Sanjida O'Connell • Caius Julyan • Bonni Cohen • Leanne Pooley • Edwina Vardey • Alexandra Beazley • Sam Hodgkin • Karen Walsh • Jack Weber • Richard Simkin • Alex Hearle • Brian Cummins • Ben Southwell
**Composers**: Guy Michelmore (season one) • Guy Dagul (seasons two and three)

**Season One**: Cat • Fish • Horse • Dog • Jungle • Bird • Amphibian • Insect • Shark • Elephant • Reptile • Dinosaur • Skeleton
**Season Two**: Desert • Tree • Ape • Mammal • Butterfly and Moth • Volcano • Rock and Mineral • Seashore • Pond and River • Weather • Arctic and Antarctic • Prehistoric Life • Shell
**Season Three**: Flight • Bear • **Sight** • Island • Survival • Mountain • Monster • Human Machine - Plant • Ocean • Planets • Life • Natural Disasters