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A reading stone is an approximately hemispherical
A reading stone is an approximately hemispherical lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
that can be placed on top of text to magnify
Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...
the letters so that people with presbyopia
Presbyopia is physiological insufficiency of accommodation associated with the aging of the eye that results in progressively worsening ability to focus clearly on close objects. Also known as age-related farsightedness (or age-related long si ...
can read it more easily. Reading stones were among the earliest common uses of lenses.
The invention of reading stones is often credited to Abbas ibn Firnas
Abu al-Qasim Abbas ibn Firnas ibn Wirdas al-Takurini ( ar, أبو القاسم عباس بن فرناس بن ورداس التاكرني; c. 809/810 – 887 A.D.), also known as Abbas ibn Firnas ( ar, عباس ابن فرناس), Latinized Armen ...
in the 9th century, although the regular use of reading stones only began around 1000 AD. Early reading stones were manufactured from rock crystal
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
(quartz) or beryl
Beryl ( ) is a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring, hexagonal crystals of beryl can be up to several mete ...
as well as glass, which could be shaped and polished into stones used for viewing. The Swedish Visby lenses
One of the Visby lenses in a silver setting
The Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal (quartz) found in several Viking graves on the island of Gotland, Sweden, and dating from the 11th or 12th cen ...
, dating from the 11th or 12th century, may have been reading stones.
The function of reading stones was replaced by the use of spectacles
Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples o ...
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 large dome magnifier
A dome magnifier is a dome-shaped magnifying device made of glass or acrylic plastic, used to enlarge words on a page or computer screen. They are plano-convex lenses: the flat (planar) surface is placed on the object to be magnified, and the co ...
s which magnify a circular area of a page. Larger Fresnel lens
A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships."
The design allows the c ...
es can be placed over an entire page. The modern forms are usually made of plastic.
References