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A reading stone is an approximately hemispherical lens that can be placed over text to magnify the letters, making it easier for people with presbyopia to read. Reading stones were among the earliest common uses of lenses.
The magnifying power in this case is roughly MP = d o Φ. [9] A typical magnifying glass might have a focal length of 25 cm, corresponding to an optical power of 4 dioptres. Such a magnifier would be sold as a "2×" magnifier. In actual use, an observer with "typical" eyes would obtain a magnifying power between 1 and 2, depending on where lens ...
The constant 25 cm is an estimate of the "near point" distance of the eye—the closest distance at which the healthy naked eye can focus. In this case the angular magnification is independent from the distance kept between the eye and the magnifying glass.
Walmart Neighborhood Market, former also known as "Neighborhood Market by Walmart" or informally known as "Neighborhood Walmart", [158] is Walmart's chain of stores ranging from 28,000 to 65,000 square feet (2,600 to 6,000 square meters) and averaging about 42,000 square feet (3,900 square meters), about a fifth of the size of a Walmart ...
Grandma's Reading Glass is a 1900 British silent trick film, directed by George Albert Smith, featuring a young boy who borrows a huge magnifying glass to focus on various objects. The film was shot to demonstrate the new technique of close-up .
There are claims that single lens magnifying glasses were being used in China during the Northern Song dynasty (960–1127). [ 28 ] [ 29 ] Robert Grosseteste 's treatise De iride ( On the Rainbow ), written between 1220 and 1235, mentions using optics to "read the smallest letters at incredible distances". [ 30 ]