1.4 - The Craft of Lens-making
It was in the late 13 th century that the first eyeglasses with convex lenses to correct presbyopia came into use. And toward the middle of the 15 th century the first eyeglasses with concave lenses to correct myopia made their appearance. During this century, Florence became one of the major centres for the production of eyeglasses, rivalling the German cities of Nuremberg and Regensburg. From the end of the 13 th century, parallel to the diffusion of eyeglasses, an exceptionally transparent type of Venetian glass, with few flaws and almost colourless, the so-called "crystal", came into widespread use.
The lenses in the first eyeglasses were made by cutting discs out of balls of blown glass. The concave surface was ground to make lenses for the farsighted and the convex surface to make lenses for the nearsighted. In the late 15 th century the spectacle-makers of Nuremberg began however to cut the discs out of plates of flat glass and then grind one of the surfaces to either a concave or a convex shape. Through this procedure, lenses with more accurate spherical curvature could be produced. It was the spread of this perfecting technique that made the invention of the telescope possible to those spectacles-makers who thought of combining concave and convex lenses of high quality.
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