REFRACTING telescopes require essentially two types of lenses concave and convex. The invention of telescopes is thus intimately connected with the development of lenses. The story of lenses is in turn connected with the development of reading aids spectacles.
The term telescope was coined on April 14, 1611, by Giovanni Demisiani, a Greek mathematician, at a reception organised by Prince Frederick Cesi where Galileo was demonstrating one of his instruments. The name was derived from the Greek tele (meaning far) and skopein (meaning to see).
The human eye has a notable lacuna; as we grow older it is rather difficult to read and write. With age, the eye progressively loses its power to accommodate and it becomes difficult to change its focus from far-away objects to nearby ones. This condition becomes noticeable for most people in their forties, when they can no longer focus on letters or books held at a comfortable distance from the eye, necessitating an appropriate aid for reading and writing. Until the medieval period, access to reading and writing were limited to a few. With the spread of literacy and improvement in the technique of copying books, such as printing technology, the act of reading became widespread and soon spectacles for reading became a necessity.
Aids were sought to be developed to meet the physical challenge. Magnifying glasses (bi-convex) became common in the 13th century for reading. Initially, lenses were made from glass blown by glass-makers, by cutting out discs of the desired shape and size. They were held in the hand to read and write. But this was cumbersome.
Soon the contraption of spectacles, lenses supported by the nose bridge, was invented. The earliest illustrations of spectacles date from about A.D. 1350, and spectacles soon came to be symbols of learning. By the 14th century, reading glasses were widespread throughout Europe. These spectacles were useful for reading, but did not help in case of near-sightedness. For that one needed concave lenses.
Technological advances made it possible to shape lenses from a glass block rather than cut from a solid glass ball. This was a significant step, for glass balls made by blowing invariably contained minute air bubbles and the resultant lenses were poor in quality. The glass block could be made with relatively fewer air bubbles and imperfections. Through this procedure, lenses with more accurate spherical curvature, including bi-concave lenses, could be produced by grinding.
Thus, by the 15th century bi-concave lenses were made and were used to correct near-sightedness. It was the invention of the two types of lenses that made the fabrication of the Galilean telescope possible.
T.V. VenkateswaranCOMMents
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