What is a man's eye, but a machine for the little creature that sits behind his brain to look through.
 


SAMUEL BUTLER (1835-1902)

Eyes

Incredible Eyes


The human eye can take in a million simultaneous impressions and can tell the difference between eight million different colours. On a clear, moonless night it can detect a match being struck 50 miles away.
 
Eye colour is determined by the amount of melanin in the anterior border (the outermost part) of the iris. Melanin is the main pigment in the human body and is responsible for the colour of hair and skin. If there is a lot of melanin, brown eyes will result. The amount of melanin in the iris is genetically determined and is related to the amount of melanin elsewhere in the body: non-white people nearly always have brown eyes.

If there is little melanin, it won't show up and the apparent colour of the eye will be blue - because the next layer of the iris (the stroma) is made primarily of collagen, which is blue. If there is a middling amount of melanin, both the melanin and collagen show through, producing green eyes. Red or purple eyes are very rare, and only occur if there is no melanin at all. Such people most often have blue eyes, but the blood vessels of the eye show through - hence the red colour.  Having eyes of different colours is known as heterochromia, and is usually the result of damage to one or other eye.
 
Navahos believe you shouldn’t sleep during an eclipse because your eyes won’t open again, or while herding sheep in case a crow takes your eyes out, and that you will go blind if you open the eyes of a new-born kitten. Amongst the Mayans, cross-eyed people were considered attractive. Mothers trained their children’s eyes by dangling a bead on a string above their noses.
 
On being told by a local clan chieftain that she had nice eyes, Saint Triduana, a Benedictine Abbess, plucked them out and sent them to him impaled on a thorn and presented on a plate.

Animal Eyes


Butterflies have four eyes. Most insects (bees for example) have five eyes. Most spiders have eight eyes. Caterpillars have 12 eyes while houseflies have 4,000 lenses in each eye. A dragonfly’s eye contains about 30,000 lenses. Giant squids have the largest eyes in nature: they can be 16 inches across.

Geckos clean their eyes with their tongues while okapis use theirs to clean both their eyes and the insides of their ears. 

Eye-tracking


Eye tracking determines where a person is looking by recording the ‘point of gaze’. In 1879, Louis Emile Javal (1839-1907) first observed that the eye doesn’t read smoothly, but in a series of jerky movements called 'saccades'. Javal was also the father of orthoptics - the science of correcting squints.
 
Edmund Burke Huey (1870-1913) made the first physical record of eye-movements. He stuck plaster of Paris discs with holes in the middle and aluminium pointers sticking out of them onto people’s corneas. These made marks on the paper as the person read. In 1967, the Russian psychologist Alfred Yarbus wrote an influential book on the subject. He showed that our attention is only held by a few elements of each picture we see, and that these elements vary, depending on our motivation.
 
Modern eye tracking uses video and infrared light not only to record eye movements but to measure changes in pupil diameter. It is widely used in marketing to discover how people look at packaging, at the positioning of products on a shelf or at the information in a brochure or a web page. It is also used in sports coaching, flight simulators and to improve missile defence systems.
 
Eye tracking has shown that men and women respond differently to the same picture. Presented with images of both people and animals of either sex, men’s eyes linger ('fixate') more on the genital areas, whereas women look exclusively at the face.

An ‘eye-baby’ is the tiny image of yourself reflected in another person's eye.

HERACLITUS OF EPHESUS (535-475 bc)

Eyes are more accurate witnesses than ears.

Dolphins sleep with one eye open.

An ostrich’s eye is the same size as its brain.

‘Eyewater’ was 19th century slang for gin.

On a clear, moonless night the human eye can detect a match being struck 50 miles away.