he ball of the eye is consists of three coats. There is a hard white outer coat called the sclera that we can see as the white of the eye. Inside this there is a coat rich in blood vessels called the choroid which supplies the nutrition for the eye, and, inside this is the retina, that is the part of the eye that responds to light, like the film in a camera. The retina sends its messages to the brain through the optic nerve allowing us to see. At the front of the eye the outer coat becomes the cornea, which is very transparent and the most sensitive part of the body. You just can't bear it to be touched.

n the same region the choroid becomes the iris, the coloured part of the eye that consists of the muscles that control the size of the pupil, which is the hole through which the light has to pass to be focussed by the lens in onto the retina. Covering the cornea there is a very thin membrane that extends over the front of the eyeball and the inside of the lids called the conjunctiva. If this gets infected its called conjunctivitis, and the eye gets red and sore. This can be caused by dirty contact lenses.

ight entering the eye it is bent, at the cornea, and passes through the pupil. In a normal eye, the light is focussed by the lens to a sharp focus on the retina at the back of the eye, and we see clearly.

hen you look at a nearer object the eye accommodates, that is, the lens changes shape so that its power is adjusted so that the incident light is still brought to a sharp focus on the retina. In age, accommodation becomes difficult, and so we will need reading glasses to see things close to when we get older.Instead of spectacles, however, this condition can now be corrected with bifocal contact lenses.

f the eye is too long for the power of the optical system light f rom a distant object is brought to a focus in front of the retina so that the retinal image becomes blurred. The eye is said to be myopic or short sighted.

or a myopic patient, while distant objects may appear blurred, near objects are often seen sharply and in focus.

n the other hand those people for whom the light entering their eyes comes to an apparent focus behind the eye are said to be hyperopic or far-sighted . This is a bit of a misnomer as for a hypemetrope the vision is clear neither for near nor far.

owever, younger hypermetropic people, that is, those for whom accommodation is still active can often make their vision quite clear by refocussing their lens by accommodating , although this excess of muscular activity can often produce symptoms of eyestrain and headaches.

he other common refractive error of the eye is astigmatism, where the light travelling in different meridians comes to a focus in different planes and the appearance of objects may be distorted out of shape. This is the condition that can be corrected by toric contact lenses. These so called refractive errors, myopia, hypermetropia and astigmatism, can all be corrected by spectacles or contact lenses.