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 stone in Archeon, a historical theme park

The regular use of reading stones began around 1000 AD.[1][2] 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.

References

Rubin, Melvin L. (1986). "Spectacles: Past, present, and future". Survey of Ophthalmology. 30 (5): 321-327. doi:10.1016/0039-6257(86)90064-0.
Quercioli, Franco (2011). Alberto Diaspro (ed.). Optical Fluorescence Microscopy: From the Spectral to the Nano Dimension. Springer. p. 2. ISBN 978-3-642-42281-2.

"A good illustration of a reading stone in use". Zeiss Optical Museum, Oberkochen. Retrieved 2011-03-06.

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[1] Rubin, Melvin L. (1986). "Spectacles: Past, present, and future". Survey of Ophthalmology. 30 (5): 321-327. doi:10.1016/0039-6257(86)90064-0.

[2] Quercioli, Franco (2011). Alberto Diaspro (ed.). Optical Fluorescence Microscopy: From the Spectral to the Nano Dimension. Springer. p. 2. ISBN 978-3-642-42281-2.