Reading stone
Encyclopedia
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.
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
that was placed on top of text to magnify
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...
the letters so that people with presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...
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
Abbas Ibn Firnas
Abbas Ibn Firnas , also known as Abbas Qasim Ibn Firnas and عباس بن فرناس , was a Muslim Andalusian polymath: an inventor, engineer, aviator, physician, Arabic poet, and Andalusian musician. Of Berber descent, he was born in Izn-Rand Onda, Al-Andalus , and lived in the Emirate of Córdoba...
of a process for converting sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
into glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
. 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
Fresnel lens
A Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...
placed over an entire page. The modern forms are usually made of plastic.
Early reading stones were manufactured from rock crystal (quartz) or beryl
Beryl
The mineral beryl is a beryllium aluminium cyclosilicate with the chemical formula Be3Al26. The hexagonal crystals of beryl may be very small or range to several meters in size. Terminated crystals are relatively rare...
as well as glass.
The Visby lenses
Visby lenses
The Visby lenses are a collection of lens-shaped manufactured objects made of rock crystal found in several Viking graves on the island of Gotland, Sweden, dating from the 11th or 12th century...
may have been reading stones.