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Reading stone
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A reading stone was an approximately hemispherical transparent object placed on top of text to magnify the letters so that people with presbyopia could read the text more easily. Reading stones were among the earliest common uses of lenses.
Reading stones were said to be invented by Abbas Ibn Firnas, in 8th century Córdoba, Spain, who had devised a way to finish sand into glass; which until this time, was secret to the Egyptians. These glasses could be shaped and polished into stones used for viewing - known as reading stones.
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Encyclopedia
A reading stone was an approximately hemispherical transparent object placed on top of text to magnify the letters so that people with presbyopia could read the text more easily. Reading stones were among the earliest common uses of lenses.
Reading stones were said to be invented by Abbas Ibn Firnas, in 8th century Córdoba, Spain, who had devised a way to finish sand into glass; which until this time, was secret to the Egyptians. These glasses could be shaped and polished into stones used for viewing - known as reading stones. These spread to the rest of Europe from approximately the 11th century onwards. The function of reading stones was replaced by the use of spectacles from the late 13th century onwards, but modern implementations are still used. In their modern form, they can be found as rod-shaped magnifiers, flat on one side, that magnify a line of text at a time; or as a sheet in the form of a Fresnel lens placed over an entire page. The modern forms are usually made of plastic.
Early reading stones were manufactured from rock crystal (quartz) or beryl as well as glass.
The Visby lenses may have been reading stones.
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