Archeon, a historical theme park A<unk>  is an approximately hemispherical lens that can be placed on top of text to magnify the letters so that people with presbyopia can read it more easily.<unk> s were among the earliest common uses of lenses. The invention of<unk> s are generally credited to Abbas ibn Firnas in the 9th century, although the regular use of<unk> s only began around 1000 AD. Early<unk> s were manufactured from rock crystal (quartz) or beryl as well as glass, which could be shaped and polished into<unk> s used for viewing. The Swedish Visby lenses, dating from the 11th or 12th century, may have been<unk> s. The function of<unk> s 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 large dome magnifiers which magnify a circular area of a page. Larger Fresnel lenses can be placed over an entire page. The modern forms are usually made of plastic.
Reading stone
Pulp stone
Goa stone
Scalding
Bladder stone
Reinke crystals
Keratic precipitate
Cyst