Eye Care Blog

The colour of your eyes is inherited. The colour and pattern of the iris are as individuated as fingerprints. It is coated on its back with brown-black pigment. This prevents light penetrating to the back of the eye except through its centre, or pupil. This hole can change its size, dilating or contracting as the level of light requires. In bright light, or when we wish to scrutinize a near object, it contracts. In dull light, or when we wish to relax our eyes and stare in the distance, it dilates. It also becomes bigger if we are frightened or excited, and it dilates in death. The general behaviour of the iris/pupil system may be compared to the shutters of an automatic light-sensitive camera. That is to say we cannot control the dilations and contractions of the iris consciously, although various drugs can be used to induce dilation, while morphine, for example, causes contraction. These operations are controlled by tiny muscles, and it is these which react to any chemical changes in the system.

If you look very closely at an eye you will notice that the pupil is not in fact dead-centre, but is generally inclined to one side of the iris. The word ‘pupil‘ is derived from the Latin word meaning a ’small person’. If you look at your pupil in the mirror you will indeed see a small picture of yourself, provided you can focus at such a short distance. And the same happens when you look at someone else’s eye. When you see your own image, either in your own eye (using a mirror), or in someone else’s eye, what you are actually looking at is an image on the curved surface of the cornea. It is in fact the cornea, not the iris/pupil, that acts like a mirror-reflector. This function of the front lens, however, is incidental and is not used in the formation of an image at the back of the eye.

The cornea, or front lens, is like a small contact lens. It is only half a millimetre thick at its centre, and one millimetre at its edge where it meets the white of the eye. It keeps its shape as a lens because of the pressure of the liquids behind it. In this respect the eyes are rather like small balloons filled with water (and made airtight). If the eye is severely ruptured or cut the cornea will collapse.

The iris-and-pupil is suspended in a fluid that forms the second of the four ‘lenses‘. Behind this fluid is the small crystal lens, usually called the ‘inner lens‘ of the eye. This is shaped like a lentil, and is usually about 10 mm in diameter and 5 mm thick. It is suspended by its equator to the inside coatings of the eye. In front of it the fluid is watery, behind it a transparent jelly. At this stage of description we may compare the eye to a white ball which is hollow. Into the front surface is a more steeply curved circular lens like a watch glass set in a watch face (the cornea), while about 4 mm behind this, towards the centre of the ball, is a globular lens (the crystalline) suspended by fine ligaments that run from all round its perimeter to the circular muscle which is itself attached to the wall of the ball.

As the eye develops, the crystal lens becomes very specialized. Although it is not as powerful as the cornea, what power it does have it can change. Its front part can be made to bulge forward at the centre, thus forming a thicker lens with greater refractive capacity. This change of power is necessary whenever the eye wants to look at a close object, and is called accommodation. At birth this lens is too far forward to function properly. But, and coinciding with the greater activity of the infant’s brain, it gradually moves back so that both good close vision and good distance vision can be obtained.

The average eye is only some 2.4 cm long, and yet rays of light entering through the cornea have to focus accurately on the retina at the back of the eye. The retina is a photo-sensitive tissue that, at this stage, may be compared to the film in a camera. The power that an optical system needs to achieve this focusing ability is +60 dioptres, supposing that such power were concentrated in a single lens at the front of the eye (which it is not). In fact as we have seen there are four lenses. There is no air inside the eye, and so all the optical parts of the eye are separated by fluids. There is no advantage in this, but, except for the lungs, the body has no air spaces, and so the eye has had to evolve as a complete transparent material-and-fluid system. The required refractive power is equivalent to three times that of an ordinary magnifying glass such as is used for reading minuscule print, and the image formed on the retina is very small. An object that is two metres high and which is placed three metres away from the eye will form an image of between one and two millimetres at the back of the eye. But this is only the beginning of the description. Not only do humans require to see detail at all distances: we need to know what is happening around us just to be able to walk around and maintain our balance, and we need to see both when the light is very dim and when it is very bright. More than that, we need also to be able to cope with sudden changes in light intensity, from one extreme to the other.

The retina has evolved to meet these demands, or at least most of them. If the light suddenly intensifies, the retina adapts quickly enough, although it takes longer to adapt to a sudden onset of darkness. The centre of the retina (plural retinae) is reserved for distinct and accurate vision in normal daylight conditions. But it is also sensitive to the red end of the light spectrum so that, with the use of infra-red light, the eye can see in detail at night. (This device has been used both by the police and the armed forces and by criminals to assist them in their night work.) Towards its edges, on the other hand, the retina takes in less detail, preferring instead to give a ‘general description’ of the broader visual field. In this respect the analogy with photographic film fails, except where a special lens has been fitted on the camera.

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