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.
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.
The Visby lenses may have been reading stones.
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- 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.