Glasses
Have you had your eyes tested? If you have, you will have been asked to look at a chart with letters on it. The chart may have been at the end of a long room or, if the room was small, it may have been above your head and you will have looked at it through a mirror. Wherever eye charts are placed for a test, they all have the same appearance. There is one large letter at the top, two smaller letters beneath it, three smaller letters below them, four smaller letters in the next row and so on. In the test you would have been asked to read the letters on as many lines as you could. If you struggled with the first few lines and could only see the lower lines as blurs, the optician probably said that you were short-sighted. If you could see the first few lines but struggled with the lower ones and found those at the bottom blurred, the optician probably said that you were long-sighted.

Before we think about what is happening inside a short-sighted eye or a long-sighted eye, we will see what happens in a normal eye. An eye is like a hollow ball filled with jelly. At one side it has a transparent window called the cornea. This has a curved surface, which bulges outwards and bends the paths of light rays that strike it so that they can travel further into the eye. Once a light ray has passed through the cornea it travels across some transparent liquid and enters a black hole surrounded by a coloured ring. This hole is the student. After the light ray has passed through the student it strikes the curved surface of the lens. This is a transparent disk-like structure, which has two bulging surfaces. One bulges towards the student and the other bulges towards the back of the eye. When the light ray has passed through the lens it travels through a transparent jelly to the back of the eye. It joins with other light rays to form a picture of the view on the inside surface of the wall of the eye. This surface is called the retina and is packed with light sensitive cells. They send electrical signals along a nerve at the back of the eye to the brain. The brain uses the signals from both eyes to build up a picture of the view - the view we see.

When we look around us light enters our eyes from objects that are near and objects that are far away. The light rays coming from a nearby object spread out as they enter the eye but the lens has a special feature which can help focus them into a picture. The lens can change its shape. There is a muscle, which forms a ring around the lens. When the muscle contracts, the lens is made to bulge even more. When the muscle relaxes, other parts of the eye, such as the transparent jelly, squash the lens flatter. When the eye is dealing with light from a nearby object, the lens makes a highly curved, bulging surface so it can bend the paths of the light rays and make them focus on the retina. Light rays from distant objects are much less spread out. They are almost parallel. This means that the lens does not have to be highly curved to make them focus on the retina so the muscle relaxes and the lens flattens.

What happens inside an eye that is short-sighted? The light rays pass through the cornea, student and lens and come together at a focus point but it is not on the retina. It is somewhere in the transparent jelly in front of the retina. This means that the light rays run through the point of focus and start to spread out again by the time they strike the retina. Instead of a clear picture forming on the retina, as in a normal eye, a blurred picture is formed. Short sight is corrected by wearing glasses with dish-shaped or concave surfaces. When light rays pass through these lenses they are more spread out as they hit the cornea and are gradually brought together by the lens in the eye. This pushes back the focus point inside the jelly until it reaches the retina where a clear picture is made.

What happens inside a long-sighted eye? The light rays which pass into the eye fail to come to a focus point on the retina. They behave as if they would come to a focus point at some place behind the eye. This results in the light rays making a blurred picture on the retina. Long sight is corrected by wearing glasses, which have bulging or convex lenses. When light passes through these lenses they are brought closer together as they hit the cornea and are brought together even more by the lens. This pulls back the focus point from behind the eye until it reaches the retina where a clear picture is made.

If you have normal sight just think of the way the light rays are passing from this screen to your retina. If you have long or short sight think of how the lenses in your glasses are helping bring the light rays to focus on your retina so you can see this clearly.

What is it like to have short sight?
If you are short-sighted, you can see things clearly which are close to your face but as you look further away all objects become blurred. It is possible that you cannot read any of the letters on an optician's chart without glasses.

What causes short sight?
Short sight can be caused in two ways. It can be caused by the eyeball being longer than normal. This means that when the lens brings the light rays to a point of focus where the retina should be they actually meet in the transparent jelly. A second way in which short sight is caused is by having a cornea which bulges out more than normal. This also makes the point of focus form in the jelly instead of on the retina.

What is it like to have long sight?
You can see distant objects clearly but objects which are close to you are blurred.

What causes long sight?
Long sight can be caused in two ways. It can be caused by the eyeballs being short. When this happens, the lens brings the light rays to a point of focus as it would in a normal eye but because the eyeball is too short this point is actually behind the retina instead of on it. A second way in which long sight is caused is when the lens can no longer change shape. This happens to many people when they become older. The lens can no longer bring the light rays to a point of focus on the retina. If it were possible they would meet at a point behind the retina.

You sometimes see older people holding newspapers away from their face to read them. Is this long sight?
Yes, it is. This is caused by the lens losing its ability to bulge and focus light from nearby objects on the retina. The person compensates for this by holding the newspaper further away where the light from the words can be focussed onto the retina.

Can you tell by looking at glasses what kind of sight they help?
Yes, you can. You hold the glasses at arms length and look at an object like a picture on a wall. If the glasses make the object look smaller than it really is, they are for correcting short sight. If the glasses make the object look larger than it really is, they are for correcting long sight.

When did people start to use glasses?
The first pairs of glasses are thought to have been invented in China some time before 1270. They were first used in Europe between 1270 and 1290.

Have glasses always looked as they do today, with metal or plastic sides which hook over the ears?
No. In about 1400 glasses were made from two lenses which had a hinge joining them together. The hinge was placed over the nose and closed so it pinched the nose and held the lenses in front of the eyes. In the late sixteenth century the lenses were held together by a curved bar which rested on the nose and string was used to tie the lenses to the ears. In the middle of the eighteenth century you could hold the lenses in place by having a metal strip going from the bar between the lenses over the top of the head. Alternatively you could have the sides of your glasses stopping before they reached your ears, holding the lenses in place by gripping the sides of your head.