<P> The magnifying glass is an icon of detective fiction, particularly that of Sherlock Holmes . </P> <P> The earliest evidence of a magnifying device was a joke in Aristophanes's The Clouds from 424 BC, where magnifying lenses to start kindling were sold in a pharmacy, and Pliny the Elder's "lens", a glass globe filled with water, used to cauterize wounds . (Seneca wrote that it could be used to read letters "no matter how small or dim"). Roger Bacon described the properties of a magnifying glass in 13th - century England . Eyeglasses were developed in 13th - century Italy . </P> <P> The magnification of a magnifying glass depends upon where it is placed between the user's eye and the object being viewed, and the total distance between them . The magnifying power is equivalent to angular magnification (this should not be confused with optical power, which is a different quantity). The magnifying power is the ratio of the sizes of the images formed on the user's retina with and without the lens . For the "without" case, it is typically assumed that the user would bring the object as close to one eye as possible without it becoming blurry . This point, known as the near point, varies with age . In a young child, it can be as close as 5 cm, while, in an elderly person it may be as far as one or two metres . Magnifiers are typically characterized using a "standard" value of 0.25 m . </P> <P> The highest magnifying power is obtained by putting the lens very close to one eye and moving the eye and the lens together to obtain the best focus . The object will then typically also be close to the lens . The magnifying power obtained in this condition is MP = (0.25 m) Φ + 1, where Φ is the optical power in dioptres, and the factor of 0.25 m represents the assumed near point (1⁄4 m from the eye). This value of the magnifying power is the one normally used to characterize magnifiers . It is typically denoted "m ×", where m = MP . This is sometimes called the total power of the magnifier (again, not to be confused with optical power). </P>

Where should an object be placed in front of a convex lens which signifies it as a magnifying glass
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