a small. handheld magnifying glass; a Ioupe.

Merriam-Webster Online Dictionary
hand lens (noun)
a magnifying glass to be held in the hand
hand lens (Wikipedia)

A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to create a hot spot at the focus for fire starting.

Text seen through a magnifying glass
Jim Hutton as detective Ellery Queen, posing with a magnifying glass
A plastic Fresnel lens sold as a TV-screen magnifier

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.

The magnifying glass is an icon of detective fiction, particularly that of Sherlock Holmes.

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