The contact lenses are worn directly on the cornea of the eyes to correct any defects of vision. All the defects that are corrected by regular eye glasses or others that cannot be corrected thus can be rectified by contact lenses.
The first contact lenses to be used as an eye aid were made by A.E. Fick in 1887. These early lenses were first made by blowing the glasses and then by grinding and polishing bottoms of glass test tubes. These lenses were not successful and for a long time remained just a subject of academic study. However, concrete progress in this direction was made in 1938 when the plastic (methyl methacrylate) contact lenses were developed. From 1938 to 1950 many lenses were made by taking impressions of the eye and forming the lens on this mould. 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 a layer of tears. In this case it is not necessary to make an impression of the eye as the curvature of the cornea can be measured by optical instruments. Such lenses are usually 7 to 11 millimeters in diameter and 0.1 to 1 mm in thickness and can be worn all day without removing.
To fit the contact lenses, the eyes are first tested for the vision defects just as they would be in case of spectacles. Then the radius of curvature of the eye surface is found out by using a device called keratometer. After deciding the diameter and power of the lens, the prescription goes to the manufacturer for making the lens.
To make the contact lens the plastic rod is first sawn into sections and then turned on a lathe to make button-shaped tablets known as bonnets. Then they are given the right curvature with the help of machines for obtaining the desired power and then polished finally. The lenses are then examined to see whether they fit the eye well, or not. Finally they are worn on the cornea. With increasing acquaintance these lenses can be worn comfortably by most people for 12 hours at a stretch.
Besides being invisible, contact lenses provide a much wider field of vision than the ordinary spectacles. They are more useful in active sports since they are not easily lost or broken and can even be tinted to protect against the sun. But contact lenses are not effective in all cases of eye trouble. They are also expensive and some people find difficulty in wearing them.
As the research continues, even smaller and more flexible lenses are being developed in order to make them less irritating to eyes. Soft lenses of hydroxyethyle are used in modern contact lenses.
