The photo-sensitive chemicals in the rods and cones become rapidly exhausted after particles of light have entered the cell. Time is required for the chemicals to re-form. Vitamin A, carried to the retinae from the blood circulation (via the choroid), is vital to the continuing manufacture of these chemicals. There are continuous minute periods when the cells are not functional, but the process is randomized so that not all the cells are `recharging’ at the same moment. Sometimes, however, in sudden very bright light, all the cells are exhausted more or less simultaneously, and there is a noticeable period of recovery. If there is any vitamin deficiency or metabolic malfunction, this period will be yet slower. The cells also have a mechanism for blocking out impulses to avoid complete exhaustion. This time-period between impulses being fired into the brain works out to our advantage. For instance, the cinema presents to the eye different pictures with split-second intervals between them, but the brain does not register the absence of pictures, only their continuity. By this method we appreciate the smooth movement of objects.

Perhaps the most interesting aspect of the whole optical system is the brain’s ability to fuse the two images, one from each eye, into a single ‘picture’ that has the sensation of depth. This happens, or is allowed to happen, because the eyes are set apart, usually by between 50 and 70 mm, but nevertheless can focus on the same spot or object. The horizontal displacement of the eyes means that each eye will see a slightly different object. Only by making allowance for this discrepancy can the brain achieve a fusion. It seems that it does this by operating a size value which it uses as a common reference to both images. The perception of depth is thus caused by the small differentials in relation to the standard exhibited by either one or both pictures. Using one eye on its own, depth can be judged only by memory or knowledge of the shadows formed, and, to a limited extent, by intensity of colour. Foreground tends to have high colour values, and background lower intensities (haze effect). Artists use perspective shadow and changes in colour intensity to mimic depth. Perversion of these techniques, whether or not deliberate, can result in new or original art forms. ‘Realism’ in art often heightens detail and colour intensity to such a degree that every part of the picture appears in focus (which is rather unrealistic compared to natural vision). Complete absence of colour differentiation can cause the viewer confusion, especially if the artist still retains other clues to depth such as perspective. Some forms of abstract and semi-abstract art even separate or `desynthesize’ perspective and colour and use several dimensions or views of one object on a single canvas. (This is particularly the technique of the Cubist painters such as Cézanne, Braque and early Picasso.) Another example of artistic distortion is the work of the Spanish Renaissance painter El Greco, whose elongated figures suggest he may have suffered some kind of astigmatism. The perception we have of objects is stored in our brains as memory pictures, and many people find it difficult to subjugate these so as to accept modern art forms. Their rejection of what they may find in museums of modern art is as often biologically conservative as it is culturally reactionary, and the animosity caused may not be as irrational as the artists themselves would like us to believe.

Biological conservativeness, however, has its own sad limitations. In old age the first tissues to degenerate are those composed of the most highly specialized cells, which are also, generally, the cells that require most energy to function. Live long enough and you will experience a sort of evolutionary build-down. The blood vessels bringing energy-laden molecules to the surfaces of the body begin to block up and contract. In particular the nutrition of the optical nerve system begins to falter. The optic nerve itself, that pipeline of nervous fibres, becomes atrophic in old age. (There are also some diseases that mimic old age in young people, with the same result.) Vision gradually becomes dimmer. It is not uncommon to operate for cataract on very old people, only to discover in the convalescent period that the optic nerve has suffered irreparable deterioration. But even if this does not happen, as age creeps upon us the ability to focus the eyes dwindles. The crystalline lens hardens, and therefore cannot change its shape to form a lens of higher power. The muscles that might do such a job have themselves weakened.

To sum up: at an early stage in life we develop an organ of light reception able to focus at will from distance to very near and see objects in great detail with fine discrimination of colour.

The adult is aware of a field of vision nearly 180 degrees in every direction using both eyes. Only the nose and eyebrows tend to cause obstruction. But then, as middle age approaches (35-45), the ability to change the focus from distance to near begins receding. Thus a young person who could focus on something 10 cm away finds in middle age that he or she can no longer focus on anything closer than 20 cm. By the age of fifty this distance has grown to perhaps 50 cm, and at seventy it is 100 cm. There are exceptions, which will be discussed, but in general everyone must learn to make accommodations as time goes by.

By contracting the pupil the iris assists vision in three ways. First, it stops light from the sides of the cornea forming blurred images on the retina. Secondly, it prevents too much light from entering the eye. Thirdly, it enables depth of focus. As the pupil becomes smaller the need for optical accuracy in the system regresses. In fact, if the pupil shrinks to less than one millimetre in diameter the individual could see clearly without any of the `lenses’. A short-sighted person who looks through a pinhole made in a black card will be able to see things markedly better, even though he or she is not wearing glasses or contact lenses.

The sensory part of the retina is thinner than tissue paper and is almost transparent, and is backed by a layer of pigment. Both are closely applied to a third layer of blood vessels, which ‘feed’ the whole retinal system. This is called the choroid. The retina coats the back half of the eye and consists of light-sensitive cells. It is a little similar to the hemisphere of a radar scanner.

The light-sensitive, or sensory, cells are of two types, called rods and cones. Those cells shaped like rods are arranged in groups, several sharing one nerve. The cone-shaped cells, on the other hand, have individual pathways to the brain: that is, they have one nerve each. In all there are hundreds of thousands of nerves which carry electrical messages from the eye to the brain. They are bunched together at the back of the eye and taken deep inside the brain in a sort of transatlantic cable, which is called the optic nerve. Mostly the flow of information is one-way, but there are signals that travel in the opposite direction, from the brain to the retinae.

The rods and cones may be described as small chemical factories. They make molecules of pigment that are sensitive to light (rods) and sensitive to colour (cones). The rods can be sensitive to very low levels of light, and they are the receiving cells for night vision. The cones, which are chiefly situated at the centre of the retina, are equally sensitive to light, but they also distinguish colour. Because of the nerve structure the cones give a ‘one-to-one’ account of themselves to the brain and therefore provide the ‘detailed’ focus. The rods, on the other hand, because there may be several sharing one nerve fibre, give much coarser information. They tell us where we are, both by day and night, and help to keep us orientated to our environment. The cones, on the other hand, enable us to do our work. Both kinds of cell, however, function on the same broad principles. Light and colour coming into the eye trigger minute electrical discharges which are conveyed along the nerves, or relays of nerve fibre, until they reach the visual cortex at the back of the head. These tiny impulses disturb the brain’s normal electrical rhythms, and the ‘picture’ is created. (It is now possible to record the eye’s electrical discharges, which is of some diagnostic value. The electrical changes in the visual cortex of the brain can also be collected from the scalp and be used to measure vision accurately, although this method is still chiefly of research value. The functions of the retinae can be interpreted crudely, and sometimes in detail, by their electrical behaviour, which can be measured from the front of the eye or adjacent tissues.)

The ability to see clearly, therefore, is determined not only by how well the image is in focus, how good are the lenses and how clear the fluids of the eye, but also by how active are the cones and how well their impulses travel through the brain. When the messages reach the back of the brain the image is reformed in a new electrical energy pattern, and other parts of the brain interpret this as a picture. For the developing baby and child, many patterns will be seen for the first time, and memory of them stored until reinforced and eventually identified when the image is next seen again. The acquisition of visual sense is, in this respect, similar to spoken language. Everything depends upon repetition. If we do not remember the original turmoil of birth then that may well be because it only happens once.

The rods, far more numerous than the cones, work at many different levels of light intensity. Thus, when the light becomes very dim, they still manage to transmit signals back to the brain. But because they are arranged in groups they are less useful in forming pictures. They are, however, vital for visual orientation. They tell us where objects are in space, what is our immediate and distant environment. Without this system, which with two eyes can roughly locate any object in space, we would not be able to move where we want with any confidence. Not only do the rods direct the eye muscles via the brain, but, through the same route, they also co-ordinate all the limbs.

The presence of both cones and rods make the primate’s eye exceptionally sophisticated. Elsewhere in the animal kingdom the power of vision is not so well developed. Many insects (the bee is the most familiar example) have a polygonal or faceted cornea and each polygon forms a separate optical system (like a tube or tunnel) to the retinal cell at the back of the eye. Such an eye is designed to work all round the creature without movements. Hence the difficulty of swatting a fly. Whichever way you come at it, it can see you. But the actual quality of vision is nowhere near as good as ours. At best a fly’s or a bee’s acuity can only be like that of a newspaper photograph, hundreds of small dots, while we can see in superchrome. Some birds have an optical system that can change the position of the crystal lens, and some birds have a supplementary lens in a third eyelid that can be flicked across the eye when necessary in order to find small insects. But these creatures do not have binocular vision: the pictures delivered by each eye are not synthesized.

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.

Society does not permit the sort of experiment carried out on cats, apes and monkeys to be performed with humans. Even so, there is at least one instance of a medical or social custom that has some of the features of an experiment, and which sheds some light on the development of our sight. For some time before, and for a little while after, World War II there was a trend among a minority of the Japanese population to perform surgical operations on the eyes of their baby girls. These operations consisted in making cuts on the eyelids, so that when the child grew up she would be ’round-eyed’. The real reason for doing this was almost certainly cosmetic, even though some doctors tried to justify what they were doing on the medical grounds that the oriental eyelid restricts the natural movements of the eye.

This operation was usually carried out when the child was between one and two years of age. Unfortunately it was very rare that both eyes were treated simultaneously. First one eye would be done, and then it would be kept covered for a week or two; then the other eye. The consequence was that the eye treated first nearly always became visually dominant later on.

The upshot of all this is that, although humans develop their visual mechanisms reasonably quickly, it is clear that the process of secondary ‘programming’ continues well into early childhood.

This corresponds with what we know about the physical growth of the organ itself. At birth the human eye is about two-thirds the size it will eventually attain in adulthood. But by the age of four the eye is almost fully grown. To put this in perspective, compare it to the growth-rate ofyour body. At birth you are one-third or less of your final height, at four perhaps not half. To reach its final limits the body takes fifteen to twenty years, but not so the eyes. The head and brain likewise mature early—it is the skeleton that lags behind. From an early stage the mechanisms are set to make the child a good receiver of sensations, many of which are stored as memories to be used later by the adult when he/she takes on all the functions of a fully mobile animal.

The eye is developed to provide you with a field of vision almost a full 180 degrees to the frontal plane of your body. Unlike many birds and reptiles you cannot see behind you without turning your head. Instead your eyes are set in such a way that they can work together. You are one of the few animals that can fuse the two pictures, or sets of information, coming from your two eyes. This is called stereoscopic, as opposed to simple binocular, vision. But because the two pictures are slightly different, their fusion can only occur using slightly different levels of perception in the brain. The result is interpreted by the brain as depth. The greater the differences between the two pictures the greater will be the sense of depth, or space. Hence the very precise impression of space and distances between objects that are close to, and a correspondingly vaguer sense of distances when you look at a horizon. If for any reason the pictures are too dissimilar to be synthesized, the developing child will not have single fused vision. The six muscles of each eye must work not only as a team to point the eye accurately to wherever the brain wants it to look, but they must also work together with the muscles of the other eye. Co-ordination of this kind has generally occurred by the age of six months.

In the first year of life, changes are happening to the inside of the eye. The optical system of the eye may be compared to the lens system of an expensive camera. Light is refracted not just through one lens, but through many different lenses, each with its own particular function. Biologically a lens may be defined as any part of the optic system through which light passes before reaching the retina. As light passes through the human eye there are four main changes in optical density, hence four main lenses. These are, from front to back:

1. The cornea: the outer surface of the eye where that surface is transparent, and by far the most powerful of the four lenses, accounting for as much as two-thirds of the average eye’s refractive capacity.
2. The area between the cornea and the ‘crystal’ lens, filled with fluid, but as a lens extremely weak.
3. The crystal or crystalline lens, accounting for almost a third of the eye’s refractive power, and capable of increasing its power when the eye is being used, for example, for reading.
4. Thevitreous lens, from the back of the crystal lens to the retina, a jellyish fluid of no significant power.

Between birth and the age of four these lenses are rapidly changing shape so as to provide the child with the required visual functions. The front lens, or cornea, is circular in shape. When you look at an eye, either your own in a mirror or someone else’s, the coloured part is the iris, and the black hole in the middle of the iris is the pupil. These in fact lie just behind the cornea. The coloured iris, when looked at closely, reveals a very delicate and intricate pattern, peculiar to each person and race. In most cases the colour of the baby’s iris will change as it grows older, becoming darker. The reason for this is that, at birth, the darker pigments have not yet been laid down in the iris. In albino children there are no dark pigments, and so light will make their eyes look pink. What in fact happens here is that red light, or the red end of the colour spectrum, is reflected forward from the back of the eye.

But biology, though it may have changed man’s understanding of organic creation, has not in any way diminished the pre-eminence attached to sight. Rather the reverse: for biology shows us that the eye is, quite literally, a part of the brain, the place, if you like, where the brain rises to the surface. We see with, not through, our eyes. But because they are such complex mechanisms they are peculiarly liable to defect, breakdown and injury. And yet, because we use them continuously and in so many different contexts we insist that they perform to a very high standard. We have devised ways of measuring that performance with a precision that is entirely lacking in our measurement of hearing, taste, smell or touch. Not surprisingly, therefore, many people have ‘bad eyes‘: modern society is on the active look-out for them, in a way it is not on the look-out for poor hearing, poor smell, poor touch. Equally, modern civilization also places an unprecedented strain on the optic system of the ordinary person. Television screens, artificial lighting, abundant reading materials, bad diet, tobacco, alcohol, even some of the drugs used in medicine, can all contribute to individual cases of deteriorated vision.

But the industrial environment is by no means entirely to blame. In addition to the possible causes of bad sight just mentioned there are a host of natural agents of optic degeneration to be accounted for as well: genetic abnormalities, viruses, entropy. The wonder of it is, perhaps, that the number of people who suffer serious disorders is as few as it is. But the eye is not only intricate and delicate: it is also well-made, nature’s finest artefact, and comes equipped with several sturdy defence mechanisms.

We examine the different sorts of problem that may arise, and the different parts of the eye that may be affected. But before we turn to these matters it would be well to give a general account of the normal eye, and its passage from birth to old age.

In general terms organs develop in accordance with the needs of the host body. Thus, the newborn baby does not necessarily have good vision during the first hours or even weeks of its life. Because of the sacrosanctity of human life, not a very great deal of research has been done in this area, but studies of our nearest kin in the animal kingdom, the primates (apes and monkeys), have shown that good vision depends upon certain brain cells developing at a site best considered as a relay station. These ‘bells help process information gathered by the eye as it travels toward the ‘visual cortex’, i.e. that part of the brain which is responsible for recoding visual data (tiny electronic impulses) into ‘pictures’. If these cells are destroyed then the primate concerned will be blind. But the evidence suggests that the complete development of these cells, among primates, does not occur until a few minutes after birth, so that, even if at the moment of birth the eye itself were capable of perfect vision, the machinery for receiving its data is still on the assembly line.

As far as we know humans follow roughly the same schedule as apes and monkeys. It is known, however, that some other animals take a relatively longer time to establish ‘neural pathways’ from the eye to the cerebral cortex. Among cats, for example, there is usually a delay of around six weeks before vision is developed. Some interesting recent experiments with kittens shows that if they are kept ‘blindfolded’ for six weeks or less there will be no impairment in their subsequent visual acuity; but if they are blindfolded for more than six weeks there will be an impairment, and the severity of this will depend on the total period of deprivation. On the other hand, kittens that are blindfolded after the visual cortex has been fully activated, that is, after a period of about twelve weeks, are not seriously affected

The ideal sight for the non-athletic person is about —1.50 dioptres of myopia. The occasional use of spectacles or contact lenses will give excellent distance vision. For studying and other detailed close work no spectacles are required, even into advanced old age. Many young people, however, do not consider it this way, and ask for operations to correct this small degree of myopia. If such operations are carried out, the strong likelihood is that, when the patient reaches middle age, he will regret losing his ability to see near to without spectacles, which he would otherwise have retained.

Spectacles for short sight not only reduce the size of object- images; they also tend to make them brighter, unless the spectacle lens has been manufactured or coated with a tint. Many very short-sighted people prefer, and may even be advised by their practitioner, to use tinted glasses. Furthermore, the abnormal length of the eye, from the cornea to the back of the retina, which is usually the cause of myopia, often means that the jelly between the crystal lens and the retina is not as firmly attached as it should be. If the eye then changes its contour or volume, as it tends to from early middle age onwards, its internal structures (retina and vitreous jelly) are put under great pressure to adapt, which they cannot readily do, to the new shape. The only way they can adapt is by the retina being unduly stretched and the jelly becoming less solid.

The contact lens operates slightly differently from the spectacle lens. It is in contact (hence the name) with the cornea either directly or with a variable thickness of tear film in- between. Between the spectacle lens and the eye, on the other hand, there is a volume of air. Because the contact lens only has air in front of it, light must be bent to correct short sight almost entirely by its front surface. The reason for this is that, where the fitting of the contact lens permits, the eye and tear fluids have almost as much negative refractive power as the artificial lens itself. Therefore, after light has passed through the contact lens, it will not be further bent until it reaches the lens inside the eye. The contact lens becomes, in effect, the new front of the eye, the new cornea. But because of its proximity to the natural optic system, and because of the materials used in its manufacture, the contact lens is necessarily much thinner than a spectacle lens. For short-sighted eyes, the contact lens also has an additional advantage: it tends to correct any non-spherical problems of the cornea, since the power of the cornea has now been negated (by the additionally thick layer of fluids). The image formed on the retina will now be of a size either equal to that of a normal eye, or larger, and of course the correction itself curves with the eye. But although from an optical point of view the contact lens is significantly more efficient than the spectacle lens, there are some problems associated with the use of contact lenses.

The opposite of short sight is long sight. This occurs when the optical power of the whole eye is too weak to bring the image on to the retina, but instead projects it some distance beyond. The eye is simply too short for the focal length of its lens system. A small degree of long-sightedness can be corrected by `accommodation‘ of the inner lens to a higher power, but if more than two dioptres of accommodation is required in each eye there occurs a simultaneous stimulus for the eyes to converge. But if accommodation is thus required to correct long sight and obtain a clear image of distant objects, this synchronized convergence is of no value since it becomes no longer possible to direct both eyes in the correct direction. Therefore, when there is more than two dioptres of long sight, only one eye is held straight, whilst the other drifts into a corner. Such behaviour is known as strabismus or squinting of the eye. This condition is most common when long sight of high degree is present from birth.

Long sight is very often inherited (as is short-sightedness). It may be due to eyes that have failed to develop to a correct length as the rest of the body grows, or it may be due to a weakness in the cornea or crystal lens. A combination of both factors is also a possibility. Someone suffering from long sight, whilst able to focus easily on distant objects, may find the extra effort required for near vision just too much and consequently develop symptoms of eye-strain. As with short sight, substantial long sight can and should be corrected by the use of spectacles or contact lenses, only in this case the power of the artificial lens needs to be positive in order to bring the image forward on to the retina. Long-sighted spectacles increase the power of the individual’s optics.

When the eye is in a relaxed state and a distant object forms an image in front of (and not on) the retina, this is a state of myopia, or short-sightedness. The image quite literally falls short of the screen-receptor. If the distance between the object and the eye is decreased the position of the image in the eye will not change until that distance is less than six metres. The reasons for this are not too difficult to understand. Any light coming from a distance sends out light energy (particles) in all directions. Therefore, as they speed away from an object, they become increasingly divergent. But the eye’s opening (pupil) is only 2-10 mm in diameter. Thus an object has to be reasonably close to the eye for its emitted light rays to be more or less parallel when they reach the eye, and this distance, because of the fixed dimensions involved, happens to be at around six metres. This can be shown by drawing lines between the eye and the object concerned. Of course, light rays coming from an object more than six metres away will be ‘even more’ parallel; and within six metres the light rays will be more and more divergent. The normal eye can bend parallel rays to form a clear image on the retina. As the object draws closer, and its rays becomes increasingly divergent, the eye changes its focus and bends the light more and more.

A myopic or short-sighted eye, however, has difficulty in reducing its power when focused on far-off objects. It is not so much a question of weakness as of too much (optical) strength, relative to the size of the eye. The image of a distant object falls short of the retina. But as the object comes closer to the eye and the divergence of light increases there will come a moment when even in its relaxed state the natural power of the eye will compensate for the divergence and the image will fall on the retina. By establishing what this distance is one measures accurately the degree of myopia. It is the furthest point at which the myopic eye can see distinctly without the aid of an optical device such as spectacles. The less this distance is, the higher the degree of short-sightedness. Thus if you can only see detail at a distance of one metre you are a one-metre myope; and if the distance is 50 cm, you are a 50 cm myope. However, opticians, optometrists and ophthalmologists* do not use quite this terminology. The professional and technical term employed is a `dioptre‘. A dioptre is the reciprocal of a measurement in metre units. One dioptre corresponds to one metre, but two dioptres is one half of one metre, while half a dioptre is two metres. Thus if your natural focal length is one metre you are said to have one dioptre of myopia, while if you can focus on something 50 cm away you have two dioptres. ‘Perfect vision‘ means being able to focus in detail on something six metres away. Nowadays these metric measurements are standard throughout the world, but in the old days, when the metric system was not popular in English-speaking countries, measurements were made in feet and inches. Hence the expression ‘twenty-twenty vision‘, which meant that each eye could see in detail at a distance of 20 feet. Such good sight — or even better — is the good fortune of 75 per cent of the population.

There are very high degrees of myopia where the far point of good vision is only 5 cm from the cornea (20 dioptres). If the object is brought still closer to the eye it may still be seen in focus (`accommodation’); but in fact very highly myopic eyes do not generally have such powers.

The early, or young, short-sighted person, is difficult to detect since, under one dioptre of myopia, objects at a distance of between one and six metres away are subject to blurring. A slight contraction of the lids (`screwing up’ one’s eyes) will reduce the optical aperture of the eye (irrespective of the actual size of the pupil), and correct the focus. Indeed there are many people with a low degree of myopia who manage well enough without spectacles and do not even realize that they are shortsighted until they have their eyes tested. Such small degrees of myopia can only be corrected with spectacles, as the margin of error in the manufacture of contact lenses does not permit very small powers of corrective refraction. But low-power spectacles can be prescribed, and are often used by individuals with a small degree of myopia when, for example, driving a car or visiting the cinema.

There are other surgical techniques for flattening the curve of the cornea, all involving removal of tissue, but all more serious in their complications than Radial Keratotomy. They are used only in correcting more substantial degrees of myopia, say from six to sixteen dioptres.

However, the cornea may be artificially flattened without recourse to surgery by using small, hard contact lenses. Unlike ordinary contact lenses they are purposely designed to bring sustained pressure on the cornea; but at best they can only correct up to two dioptres of vision, and in many cases the correction is only temporary, so that after a while it again becomes necessary to wear the lenses. There is also the risk of creating irregular curves on the cornea, which will cause visual distortions.

Other mechanical techniques of flattening the cornea have been attempted. Various ways of relaxing the outside eye muscles (which help to make the eyes `bulge’), including acupuncture and electrolysis, have had their advocates, though the results have been far from conclusive. There also exist techniques that combine the mechanical with the psychological. Of these the ‘Bates Method’ is the best known. Certainly it is the one for which the most exaggerated claims have been made.

William H. Bates was a New York ophthalmologist who practised at the beginning of this century. The title of his famous book speaks for itself: Better Eyesight without Glasses. In it he recommends the treatment of short-sightedness by non-medical and non-optical methods. (Glasses, wrote one of his disciples, are to be regarded as ‘eye crutches’.) Aldous Huxley, who was almost blind, is reputed to have gained relief by doing the exercises that Bates advises. The very short-sighted, Bates suggests, should be taught to make the best use of object outlines as a method of recognition. For those with a smaller degree of short-sightedness various eye relaxation and eye-muscle exercises are recommended, along with ‘palming’ (a way of applying pressure to the eyes using the hands), and ’swinging’, a callisthenic exercise that massages the eyeballs by rhythmically alternating the body’s centre of gravity. While, individually, none of these practices is likely to cause harm, and may even help (`palming’ certainly helps, if only because it means the patient is unlikely to read too excessively), in their collective presentation they amount to an attempt to make the patient think his way out of trouble. Other similar programmes involve homoeopathic medicine, stimulation to the neck muscles, even hypnosis. Together they constitute a field of practice where the suggestive personality is most likely to find help. None of the techniques is scientifically proven, but they gain adherents among sufferers who have failed to find succour from orthodox practitioners. However, although a mildly short-sighted person will almost certainly improve his or her sight without recourse to glasses or spectacles by learning how to relax the eyes and avoid all close work, little succour is offered to the busy city executive or eager student.

So much, then, for the cornea. We come briefly to the crystalline lens. It has been stated that if the inner lens could be made to flatten its curves, this too would reduce the total eye power and therefore bring the image back into place on the retina. Very simply, this can be done by looking in the distance and avoiding all close work. Not realizing the mechanics of this, it is surprising how often a patient will tell his doctor that he has been seeing better during a trip to the mountains. Eye-drops containing drugs which act upon the eye’s internal muscles of accommodation (the ciliary muscle), thus releasing tension in the ligaments that keep the crystal lens in position, can be very effective. But such drugs dilate the pupil and cause excessive sensitivity to light, and so are only useful when short sight is sudden in onset and related to a spasm induced by near-sight tasks. It is not possible to move the inner lens forwards or backwards by mechanical (surgical) means. The crystal lens can be removed, and this will correct very high powers of short sight (say —18 dioptres), but such removal is a procedure that is liable to weaken the eye in other ways.

The mechanical methods of correcting short sight are still limited, therefore, and it is still the case that, in nearly every instance, the preferred solution is to use optical aids, i.e. spectacles and contact lenses.

Meanwhile, as we discover how the genetic programming of our body operates, it seems more not less remarkable that the eye grows from a ball 17 mm in diameter to one 24 mm in diameter without being noticed. The cornea and crystalline lenses make the necessary changes without any disruption to `normal services’, so that the image falls consistently on the film at the back of the eye. Is there some method by which eye- growth is controlled from the nervous system in such a way that the nervous system ‘knows’ how the image is formed? In other words, if the image goes out of focus, even fractionally, does the nervous system plot the necessary realignments in all parts of the optical system so that a good picture can be swiftly restored? The alternative would be to say that the pattern of development is pre-set and will operate irrespective of any one part being out of step with the rest, that each part will grow to its required size and optical function of its own accord. While both possibilities are perfectly compatible with the diverse ways in which genetic coding operates, the second theory seems to provide a better explanation of the occurrence of visual defects, in so far as those defects are not caused by environmental factors. But as yet the genetic machinery is not fully understood; when it is, clinicians will be better able to discover ways of diagnosing early malfunctions and therefore be in a better position to suggest ways of correcting, and even preventing, them.

Counselling apart, genetic ophthalmology as a practical science belongs to the future. Abnormal developments must therefore be discussed as they occur, not as they ‘would otherwise’ occur. In general there are three areas in which optical malfunctions may take place, the three related optical components of the eye: the cornea, the crystalline lens, and the retina. Aberrations may affect any or all of these, individually or in combination. If the cornea is too steeply curved, its power will be too great and the image will fall in front of the retina, thus causing shortsightedness. This state of affairs can be theoretically corrected by reducing the length of the eye, or by moving the crystalline lens backwards, or by reducing the power of the crystalline lens in some other way (e.g. replacing it with a weaker lens). In normal circumstances it is possible to lose up to one dioptre of power by stopping reading and all other close work and by applying drugs that relax the eye for distant vision. Alternatively, if the cornea is flattened, then its power can also be reduced. This can be done in more than one way. Since the cornea is maintained in its curvature by pressure and the elasticity of its own tissues, deep cuts in its surface, allowing the tissue to expand, will cause a flatter curve to be formed. Such an operation is called Radial Keratotomy. It is relatively simple to carry out, but as it only corrects up to between three and four dioptres of myopia it is of little help to patients whose problems are more acute. However, since in most patients the scale of myopia is only between one and eight dioptres, Radial Keratotomy is of great interest.

It is not uncommon for a patient to ask his practitioner whether he would recommend surgery, almost as though such surgery was an obvious solution which the practitioner had momentarily overlooked. What are the risks? The answer is that, in the long term, we don’t know yet. The operation produces several scars on the cornea in areas away from its centre. While clear vision is not affected, in theory at least it should slightly decrease the cornea’s ability to collect light rays from its periphery.

In tribal societies a child’s education involves learning just as much as it does in post-industrial societies. What differs is the technique of knowledge acquisition. While there are no exact figures as to the number of separate bits of knowledge a member of a tribe must learn, it must nevertheless be considerable and compare with what a ‘civilized’ child must learn. But this is done by word of mouth and by example, not through books. It is also most usually, in terms of a teacher—student ratio, a one-to- one process. Children learn from their parents or tribal elders all they need to know about survival, and they are told, either directly or through ritual, about their culture. The ‘civilized’ child must complete a very different course. Oral and performative instruction still occurs, and perhaps to a much greater degree than some modern pedagogues are ready to admit, but the emphasis is on book-learning, both as regards developing survival skills, and as regards culturization. The cultural background studies require proof of acquisition of knowledge by memorizing reading matter. Thus from an early age children of our times will probably find themselves reading print, and progressively smaller print, for several hours a day. This accommodation to close vision, together with sustained convergence of the eyes, will, in some instances, not relax sufficiently when the reading period is over. The power of the eyes remains set for close vision, and if this continues as an habituation the child will develop near sight. This is slightly simplified, but essentially it is what may happen. Also to be taken into account is the effect near-focusing has on the internal system of eye pressures, and the effect those pressures in turn have on the length of the eye. Children who read too much, or read in circumstances where they have constantly to ’strain’ the eyes, are likely to increase the physical length of the eye artificially, and, this is the main cause of short sight. But it does not happen to alt children who thus use, or overuse their eyes, and so such ‘acquired’ short sight needs to be seen as the result of an interaction between genetic composition (tendency) and environment.

Cause and effect are simple in theory. But explanations become more complicated as we increase the number of factors that are taken into consideration. Primitive society is less well equipped to detect small degrees of short-sightedness, and this in itself may be one reason why there are more people with short sight in post-industrial communities. There may even be a problem of language involved. It is possible, for example, that in some bygone societies the difference between being a ‘good hunter’ and being an average or even bad one may have been a difference, inter alia, in visual performance without actually being referred to as such. Again, it may be that certain children have an innate desire for the acquisition of knowledge, or at least looking at small objects, while other children may be naturally more extrovert. The questions arise: are personality traits inherited? and, are there more important considerations than short-sightedness? Some people will only become short-sighted if the pattern of close work is present, while others (a small minority) will become short sighted whatever the circumstances. (Acquired short-sightedness cannot of course be passed on, but a hereditary tendency to become short-sighted is eminently likely.) Yet the acquisition of knowledge from books is related to intelligence. No two children, given the same set of materials to read, are likely to ‘learn’ identically. (But there again, by the time a child is able to read, he or she has already been sufficiently socially conditioned to make it very hard indeed to judge how much intelligence is innate, and how much has been developed through environmental influence.) Furthermore, among social groups more given to reading than others (teachers for instance) interbreeding is likely, again making it difficult to distinguish between genetic factors and environmental influence.