Reading stone

A reading stone was an approximately hemispherical lens that was placed on top of text to magnify the letters so that people with presbyopia could read it more easily. Reading stones were among the earliest common uses of lenses.

Reading stones were developed in the 8th century, following the development by Abbas Ibn Firnas of a process for converting sand into glass. Glass could be shaped and polished into stones used for viewing.

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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Wikipedia