FASCINATING FACTS

THE EYE

INTRO

Forget CCDs and microchips, you already own the most powerful and sophisticated image sensor and processing system there is,  and  you're using it right now, to read this...

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1.

There are a number of structural similarities between the eye and a video camera with a CCD image sensor. In both cases light passes through a lens, and then through a opening called an iris, this changes in size according to the intensity of the light. The light rays then form an image on a surface made up of photosensitive elements, which converts the information into electrical impulses.

2.

Over the years camcorder manufacturers have turned to the eye for inspiration and there's plenty of  examples of science imitating nature. For example, some camcorders have sliding covers to protect the surface of the lens,  very similar to our own eyelids in fact. Several manufacturers have dabbled with 3D camcorders, which use two image sensors instead of one to produce a stereoscopic image. Sony developed the variangle prism for their Sureshot image stabiliser system; it's a transparent fluid-filled sac which changes shape, altering the path of light rays passing through it. The lenses in our own eyes do a similar sort of thing, though for a rather  different reason.

3.

Our eyeballs are seated in fat-lined sockets in the skull. The tough outer layer of the eyeball is called the sclera and it forms the whites of the eyes and the transparent cornea at the front which is responsible for primary focusing. Behind the cornea is a flexible sac full of aqueous humour (watery fluid), and behind that is the iris, which controls the amount of light entering the eye, and gives it its characteristic colour. The central hole or pupil appears black, and its size is varied by a set of tiny muscles. The main lens in the eye lies behind the iris, it's made up of a crystalline material and is held in place by delicate fibres attached to a ring of muscle called the cilary body. When this contracts it alters the shape of the lens and consequently its focal length. The main cavity of the eyeball is filled with a transparent gel called the vitreous humour. The retina is on the inside of the back of the eye, this is covered with a complex network of nerve tissues terminating in light-sensitive structures called rods and cones. There are between four and seven million colour-sensitive cones. Science still has quite a way to go to catch up with nature, the latest CCD image sensors, still in the development stage still only have one million light sensitive picture elements or pixels...

4.

The inverted image formed on the retina creates a pattern of impulses which are transmitted along the optic nerves which emerge from the middle of the retina. The two optic nerves pass though holes at the back of the eye sockets into the skull, and pass into the brain where they partially cross, and then part, as they pass along the underside of the brain before entering the visual cortex,  an area on the back part of the brain. so in a sense you do have eyes in the back of your head... Movement of the eyeball is controlled by a set of six muscles attached to the sclera. The actions of the muscles of both eyes are co-ordinated by a network of nerves in the brainstem.

5.

Contrary to popular belief and what you may have read in the more lurid horror/comic books there is no slack in the optic nerve, so the eyeball cannot be removed without damage, allowing it to dangle on the chin...Contact lens wearers will also be relieved to learn that their lenses cannot drift onto the back of the eye,  a flexible membrane called the conjunctiva seals the eyeball off from the outside world.

6.

'Eye strain' is another well-established myth . According to the experts eyes cannot be damaged by over-use, nor will incorrect glasses, or no glasses at all, (when they should be worn)  damage the eye. The pain that is often associated with 'eye strain' is normally a type of headache, caused by tiredness of the muscles around the eye, or an inflammation of the eyelid.

7

Eyelids contain thirty tiny glands, situated just behind the eyelashes,  which secrete an oil to prevent the lid sticking to the eye during sleep. The oil also slows down evaporation of the film of tear fluid. Blinking is a protective reflex and helps spread the tear film evenly across the eye. Underneath each eyelid there's a thin but strong muscle; this 'screws up' the eye in response to danger, pushing the eyeball into its socket,  protecting it with a mass of tightly bunched tissue.

8.

Our ability to perceive colour evolved as an aid to finding food. The mechanics of colour vision are still not fully understood but the main colour receptors are the cone cells, rod cells do not vary significantly in their response to light of differing wavelengths.  There are three classes of cone cell,  most highly sensitive to wavelengths of light of  445, 535 and 570 nanometres respectively . This corresponds to violet/blue, green and green/yellow light, though all cone cells are sensitive to the whole visible spectrum. The retina also contains a number of other cell types, called bipolars and ganglions,  which research suggests carry out a form  of preliminary processing, before the information from the retina is sent to the brain by the optic nerve.

 9.

Colour blindness, where the sufferer sees only in black and white, is extremely rare. ereditary aHHnnnn Hereditary abnormalities in colour vision are quite common, though, especially amongst white European males where the incidence in some countries is as high as 8 percent. The commonest forms are an inability to discriminate between reds and greens. Those with green deficiency (deuteranopia) have trouble distinguishing between brown, green orange and pale red ; reds appear dull and uniform to those with red deficiency (protanopia). There is a form of blue deficiency (tritanopia) but this is also very rare.

10.

Most people with colour vision problems are rarely aware of their affliction until they take a colour test, simply because they have no way of comparing  their colour  perception with others. The condition is genetic and cannot be cured, Those with colour discrimination problems are usually barred from certain occupations where their condition could be dangerous, these include pilots, train drivers, electricians and magazine designers....                 

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(c) R Maybury 1993 0803