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How optometrists test your eyes
An optometrist can get a good idea of your state of health by looking into your eyes. Closer examination can often determine your age within an astonishingly accurate couple of years, or even pick up the first signs of a serious disorder affecting another part of your body, such as a brain tumour or diabetes. This is because the eyes are the only part of the body where blood vessels and the end of a nerve (the optic nerve) can be seen clearly without surgery - and where slight changes in health can be picked up.
The optometrist's principal function is to assess - to a fine degree - how well your eyes work, to prescribe corrective spectacles or contact lenses if necessary, and to refer any more serious disorder for treatment by an eye specialist.
The starting point of an eye examination is still - more than 120 years after its introduction by Dutch eye specialist Dr Hermann Snellen - the familiar letter chart. The optometrist relies on the patient telling him just what can or cannot be seen with each eye. The letters are deliberately meant to confuse - similar shapes, like P and F, may be placed together, for example, so that the patient's ability to distinguish between the letters and the gaps between them is really put to the test. Cheating - by screwing up your eyelids to reduce the aperture and so improve your depth of field, for example - is possible, but it is not allowed.
The Snellen Chart gives only a rough idea of sharpness of vision. In other tests, which may be as simple as following a moving object with your eyes, the optometrist checks the strength of the eye muscles and whether both eyes are working together efficiently. He checks the reflex action of the pupils - that they respond to light - and that you have a good overall field of vision.
Many of the tests are used in tandem with a 'trial frame', like a heavy spectacle frame adjusted to the patient's exact eye measurement, in which the optometrist puts different combinations of lenses. It is a fine-tuning process; any weaknesses are identified by a process of elimination. From a tray of maybe 200 different lenses, each of which has a different function or strength, there are thousands of possible combinations, and the perfect one for a given individual can be found to a high degree of accuracy.
The optometrist backs up the tests by looking at the eye with a small, hand-held device called a retinoscope which shines a beam of light through the pupil onto the retina at the back of the eye. The retina contains special cells called receptors, which convert light waves into nerve impulses. These bounce back a pattern of light as the retinoscope scans the eye, giving the optometrist an idea of the shape. An eyeball which is too long from front to back is a sign of short-sightedness; one which is too short is a sign of long-sightedness. If the cornea, the transparent outer surface of the eye, is squashed, it means that images are distorted (astigmatism). If the beam of light doesn't reach the retina at all, there is probably some obstruction, such as the milky film known as a cataract.
The optometrist now has an idea of the eyes' shortcomings, but he needs to examine the interior of the eyes more closely to find any dangerous defect.
An ophthalmoscope - another handheld instrument - lights up the transparent centre and back of the eye with a fine beam of light directed through the pupil. This combined with a series of lenses that are adjusted by turning a small wheel at the side of the instrument, enables the optometrist to focus on different parts of the eye. If the eye is healthy, he will see a circle of clear pink-orange, crossed by a tracery of nerve tissues and blood vessels (this is the point where the general health of the patient's nervous and circulatory systems can be gauged). Any age-related changes will be detected too, for example if the fluid washing through the eye is being trapped.
The extent of any fluid blockage is gauged by measuring the pressure build up in the eyeball using a tonometer. The instrument is placed lightly against the eye, and gives a pressure reading in millimetres of mercury. Another type of tonometer releases a puff of air against the eye; the speed at which the air bounces back indicates the amount of pressure.
One of the earliest warnings of glaucoma can be seen with the slit-lamp microscope, which magnifies up to 50 times and is combined with a lamp that provides a narrow beam of light through a tiny slit.
The microscope is used to examine the front surface of the eye, where the angle between the iris (the coloured part of the eye) and the cornea can be seen clearly. It is here that the very first signs of fluid blockage can be seen. The optometrist can also detect the tiniest of scratches - caused perhaps by contact lenses - old scars and any damage to the cornea, iris or lens caused by a foreign body.
Examining the eye
When an optometrist examines an eye through an ophthalmoscope, he sees the retina at the back of the eye and can identify any damage or disorders which may result from disease. In the photograph at the bottom, the dark red spots and pale yellow areas are haemorrhages and deposits caused by diabetes.
How glasses sharpen your vision
If you have perfect vision, the light rays entering the pupils of your eyes will converge exactly on the retina at the back, and the sharply focused picture will be relayed to the brain.
Most people's vision is at its sharpest at about the age of one year. Problems often develop at around puberty. The eyeball grows too long from front to back, or not long enough, or it becomes misshapen. These are the three most common reasons why people need to wear glasses to correct the eye's focal length.
In early life, a person's potential sight problems may be compensated for by the strong action of the ciliary muscles attached to the iris of the eye's lens. These muscles increase or decrease the curvature of the lens so that it is possible to focus on things close up or far away.
But if the ciliary muscles weaken, as often happens in middle age, the lens can no longer be made thick enough to focus on close-up objects, such as small type on a printed page. A person who at the age of ten could focus on the tip of his own nose may suddenly find that he cannot read a book unless he holds it at arm's length.
The three main causes of blurred vision are long-sightedness, short-sightedness and astigmatism, and glasses of different types are used to correct them (see right). Tinted lenses help wearers whose eyes are sensitive to light or reflections from clear lenses. An anti-reflection substance can also be used to coat lenses and help to make vision sharper.Spectacle lenses are made of either glass or plastic. Glass is heavier but is also more resistant to scratching.
How contact lenses are made
Leonardo da Vinci described the principle of fitting an artificial lens directly onto the surface of the eye in 1508. Nearly 400 years later, in 1888, a German eye specialist, Dr Adolf Fick, made plaster-cast impressions of the eyes of corpses to use as moulds for the first glass contact lenses.
In 2015 today's contact lenses are tiny, transparent plastic discs which float on the film of moisture that covers the front surface of the eye. They are prescribed by an optometrist (also known as an ophthalmic optician) to fit the patient's eye size and shape precisely. Virtually invisible when worn, they actually move with the eye, giving a much more natural vision than that achieved with spectacles.
To make up a prescription for contact lenses, the optometrist determines what sight defects need to be corrected and makes detailed measurements of the eye's outer surface using a keratometer. This device registers a light image of the eye (rather like the reflection on a Christmas tree bauble), gives measurements to 0.0lmm and calculates the curve of the eyeball.
Modern contact lenses are made from plastic rather than glass and may be rigid or pliable. The material prescribed depends on the patient's eye sensitivity, allergic reactions and special needs or activities.
The sight defect is actually corrected by the shape of the front surface of the lens - a flat curve adjusts short-sightedness, a steep curve long-sightedness, and a distorted curve corrects the imbalance of astigmatism. The hard lens, which until the late 1970s was the most common but is now used by only about 10 per cent of patients, is hand-lathed to the precise size and shape required from small circular blocks of solid Perspex. These lenses are the most durable and easy to care for, but can cause dryness and irritation. To counter this problem, gas-permeable (or oxygen-permeable) hard lenses have fluorocarbons and silicon mixed in with the Perspex to give a more porous material. Oxygen can filter through to the surface of the eye and waste gases can escape.
Soft, or hydrophilic (water-loving), lenses are even more permeable - and comfortable to wear. They consist of almost jelly-like plastic with between 38 and 85 per cent water content. A mould of the prescribed shape and size is filled with the molten water-plastic mixture, and when this has cooled and solidified, the lens is removed and polished. Shapes do not vary a great deal between individuals - a range of only five different fittings is suitable for 80 per cent of patients - and because soft lenses are so flexible, they can often be bought off the shelf.
Tinted plastic is sometimes used to make contact lenses easier for the wearer to find when they have been taken out, to reduce sensitivity to light, or for cosmetic reasons - to make grey eyes look blue, for example. To ensure that the correct lens is fitted into the correct eye, tiny distinguishing dots can be marked on one or both lenses.
Short-sighted people cannot focus far away, because the eyeball is too long and the rays of light converge in front of the retina. Concave lenses alter the angle of the rays so they converge on the retina.
Vision is clear at distances but not close up because the eyeball is too short from front to back and light rays have not converged when they reach the retina. Convex lenses shorten the focal length.
Vision is out of focus on either the horizontal or vertical plane because the eyeball is irregular. Astigmatism is treated with lenses shaped like a lengthways slice from a tube.
A person with normal sight can focus on an eye chart at 20ft (6m) and closer. The eyeball is the right shape for light rays to converge exactly on the retina. It also reacts fast to changes in light.
How do blind people learn to read and write?
In 1829, a French population census listed 15-year-old Louis Braille, a pupil at the Paris Institute for the Young Blind, as `unable to read or write'. Yet in the same year, the young Braille published a new `language' which provided the key to reading and writing for blind people.He devised a completely new alphabet a code of raised dots (which are much easier to feel and identify than a continuous line). The dots are arranged in domino-like combinations to form characters that correspond with letters of the alphabet, punctuation marks and common words such as 'and' and 'the'.
The Braille 'language' is touch-read by running the tips of one or two fingers over the embossed text. In 1932, more than 100 years after the system was first published, Braille was adopted as the standard language for the blind in the English-speaking world.
The original, 63-character, letter-by-letter system has since developed into a more advanced, contracted form in which the dot symbols represent common letter combinations such as 'ow', `ing', and `ment', making it quicker to read and write, and less space consuming.
Now there are Braille adaptations for every major language in the world, and for music, mathematics and science.Braille can be 'hand-written', using a stylus to press out the dot characters onto a sheet of paper clamped into a metal frame. The writer works on the back of the sheet, from right to left, so that when the paper is turned over the dots are raised, and read from left to right in the normal way.
Braille typewriters and computers with Braille print-out facilities are in common use, and the printers' latest embossing machines can imprint both sides of a single sheet of paper without the dot patterns on either side conflicting.
Braille is used mostly by people who are born blind or who lose their sight at an early age. Many people, however, become blind in their 60s, after many years of reading conventional print - and may also, because of age, diabetes or arthritis, have reduced sensitivity in their fingers. For them, the adaptation to touch-reading dotted symbols can be difficult. Some depend totally on taped books. Others learn to touch-read the alternative `Moon' system of raised letters - the stronger, simpler outlines reflect standard letter shapes, and are easier to learn.
Invented in England in 1847 by Dr William Moon of Brighton, 'Moon' is a letter-by-letter system with nine basic characters, whose interpretation depends on which way up or round they are used. The lines of text are read alternately from left to right and then from right to left, in a continuous flow. Moon is used in the English-speaking world only, and is much less established and versatile than Braille. The range of texts is limited, as printing technology responded more readily to embossing the dotted characters of Braille than the Moon lines and circles.
Louis Braille invented Braille printing in 1829.
Music in Braille
Music can be transcribed into Braille by computer. There are no staves, as in written music, and instead of using the notes C to B, the Braille letters for D to] are used: the note C is represented by a Braille D, the note D by a Braille E, and so on. Notation differences, such as crotchets and quavers, are indicated by a system of dots.