Eyesight
About one person in 30 is colour blind. More men than women are affected by colour blindness. Healthy eyes are so sensitive to light that a candle burning in the dark can be detected 1.6 kilometers (one mile) away.
Eyesight problems can be due to various reasons. The eye can have four types of defects, which can be corrected by using spectacles.
* MYOPIA
It is caused by a case wherein the ciliary muscles attached to the eye lens do not reflex sufficiently. Thus hampers its ability to reduce the lens' converging power. Due to the greater converging power of the eye lens in myopic eye, the image of a distant object is formed in font of the retina and hence the eye cannot see it clearly. This defect can be corrected by using concave lenses.
* HYPERMETROPIA
This happens when the ciliary muscles attached to the eye lens become weak and, as a result, cannot expand the eye lens to increase its converging power. Thus, an eye suffering fro this defect has low converging power of the eye lens. The image of the nearby object is formed behind the retina and hence the eye cannot see it clearly. In this case, convex lenses are used to correct the defect.
* PRESBYOPIA
It is the defect of the eye due to which an old person cannot read clearly without spectales. It is corrected by using spetacles having convex lenses.
* ASTIGMATISM
In this case, the person cannot focus on both hoizontal and vertical lines simultaneously. The defect can be corrected by superimposing cylindrical lenses on the spherical shape of the spectacle lenses.
* CATARACT
It develops when the eyelens of a person becomes hazy (or even opaque) due to the formation of a membrane over it.
Nero, the Roman emperor who lived in the first century BC, used to look at objects through a crude lens made from an emerald and a ruby. Many centuries passed before a complete set of spectacles was put together. Although, it's exact origin is unclear, they seem to have to have been in Florence, Italy, sometime in 13th century by Alexander of Spina.
The use of spectacles soon became widespread, as is seen in the portrait of Hugh of Provence, painted in 1352 by the artist Thomas of Modena, which shows the nobleman wearing spectacles. The first spectacles were used to correct long-sightedness, a defect which makes it difficult to focus on objects at close range. With convex lenses, this is made easier. Concave lenses to correct short sightedness were available only from the 16th century. These were the type of lenses won by Pope Leo X in the portrait painted by the artist Raphael in 1517.
It was Benjamin Fanklin, American inventor and hero in the independence war against England, who made the first pair of bifocal lenses in 1775. These were a combination of concave and convex lenses, which corrected both short and long sightededness.
There were two basic types of lenses - convergent and divergent. Convergent lenses have convex sides and curve inwards so that the rays of light are concentated, helping a shortsighted person to focus images exactly on the retina. Divergent lenses are concave, so they have sides curving outwards, which spread the rays of light like a fan and correct long sightedness. That's how spectacles help us see clearly.
last updated on 10-July-2004
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