<P> A sheet magnifier consists of many very narrow concentric ring - shaped lenses, such that the combination acts as a single lens but is much thinner . This arrangement is known as a Fresnel lens . </P> <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>

A convex lens used to magnify the image (usually between 5x and 30x)