What are the Contact Lenses?

            The contact lenses are worn directly on the cornea of the eyes to correct defects in vision. All the defects that are corrected by regular eyeglasses and those which cannot be corrected with glasses can now be rectified by contact lenses.

Contact Lenses

            The first contact lens to be used as an eye aid was made by A.E. Fick in 1887. These early lenses were first made by blowing glasses and then by grinding and polishing the bottoms of glass test tubes. These lenses were not successful and remained just a subject of academic study for a long time. Progress was made only in 1938, when plastic (methyl methacrylate) contact lenses were developed. Form 1938 to 1950, the process adopted for making contact lenses was not simple. First the impression of the eye was taken, then a mould was prepared and then the lens was made out of it. Such lenses had a fluid under them and covered most of the eye.

            After 1950, smaller lenses were used that covered only the cornea, which is the front surface of the eye, and floated on layer of tears. It was no longer nec3esar to make an impression of the eye. Optical instruments could measure the curvature of the cornea. Such lenses are usually 7 to 11 millimeters in diameter and 0.1 to 1 millimeter in thickness and can be worn all day.

            To fit the contact lenses, the eyes are first tested for defects just as they would be in case of spectacle. Then the radius or curvature of the eye surface is found out by using a device called kerometer. After deciding on the diameter and power of the lens, the prescription is sent to the manufacturer for making the lens.

            Contact lens are made by taking a plastic rod which is first sewn into sections and then turned on a lathe to make button shaped tablets, know as bonnets. They are given the right curvature with the help of special machines for obtaining the desired power and then finally polished. The lenses are then examined to see whether they fit the eye. Finally, they are worn on the cornea. These lenses can be worn comfortably by most people throughout the day.

            Contact lenses save the bother of wearing spectacles; contact lenses provide a much wider field of vision than ordinary spectacles. They are better for active sports, since they are not easily lost or broken, and can even be used to protect against the sun. Contact lenses cannot correct all defects of the eye. They are also expensive and some people have difficulty in wearing them. In some cases people have experienced irritation after wearing them.

            As research continues, smaller and more flexible lenses are being developed in order to make them less irritating to the eyes. Soft lenses of hydroxyethyle are used today in the manufacture of contact lenses.

How do Spectacles Help in Clear Vision?

            We often come across people wearing spectacles. Wherever we go, it is possible to spot a person with spectacles within a moment. Anyone with weak eyesight uses spectacles. These days, there are spectacles available which help in viewing both near and far objects. Nowadays, lenses that fit externally over the cornea of the eyes are often used in place of spectacles. These are known as contact lenses. These lenses can be of different colors and are used by those who do not want to wear spectacles, especially the youngsters.

Lenses help the image to form on the retina

            The use of spectacles started over 700 years ago. In 1266, roger bacon of England used a piece of glass to magnify the words written in a book. This glass piece was cut out of a spherical ball of glass. Salvino D’Armate of Italy is credited with inventing the first wearable eyeglasses, around 1284.

            Cardinal Ugon’s portrait made in 1352 shows him wearing spectacles. This proves that spectacles were developed during the period 1266-1352. By the 16^th century, they had become very common. On 1784, Benjamin Franklin brought about a wonderful development in the field of spectacles by making bifocal lenses- a lens with two halves for both near and distant vision. You must be wondering how spectacles help us to see clearly.

            Our eyes act like a camera. The light rays enter our eyes through cornea. There is a convex lens inside the eye, and behind this les there is a light-sensitive screen, called the retina. The light rays coming from any object form an inverted image of the object on the retina with the help of this lens. This image is carried to the brain by the optic nerve. The brain converts these images as erect. This is how we are able to see an object correctly.

            If the eyes have no defect, the focal length of this lens gets automatically adjusted and the image of the object always fails exactly on the retina. But sometimes, the eyes develop some defects formed in front or behind the retina. Thus, the object appears blurred. Persons with such defects have to use spectacles. There are mainly there defects of vision that can be corrected by the use of suitable eyeglasses or spectacles.

            Myopia: persons suffering form this defect can see nearby objects clearly, but distant objects appear to be blurred. This defect causes the image of an object to form before the retina. This defect is corrected by using concave lenses in the spectacles, so that the image of a distant object is formed on the retina. Usually students suffer from this defect.

            Hypermetropia: people suffering from this defect cannot see nearby objects clearly, but distant objects are clearly visible. Nearby objects make their images behind the retina. This defect is corrected by using convex lenses in the spectacles so that the image of the object is formed on the retina. This defect is common after the age of 50 years.

            Astigmatism: in astigmatism, a person cannot focus simultaneously on both horizontal and vertical lines. This defect can be corrected by the use of cylindrical lenses.

            Sometimes, due to old age, a person may suffer from both myopia and hypermetropia. This defect is known presbyopia. Bifocal lenses, with both concave and convex lenses, are helpful for old people who are suffering form both these defects.

            This is how spectacles can help people to see objects clearly. Spectacles have proved to be a great boon for persons with weak vision. In addition, sunglasses with UV filtering or cooling lenses are used for protecting the eyes from the intense rays of the sun. Special glasses are used in UV sunglasses, which prevent the ultraviolet rays of the sun from entering the eyes. Ultraviolet rays given out by the sun are very injurious to the eyes, because they can damage the tissues due to their high energy content.