The Stanhope lens was invented by Charles, 3rd Earl Stanhope (1753-1816). It was a rod-shaped hand viewer with two surfaces of unequal curvature, but later the design was adapted to incorporate a curved magnifying surface at one end, and a plane surface at the other. Lord Stanhope died many years before his invention was used in the manufacture of novelty souvenirs.
Microphotography was invented in 1839 by John Benjamin Dancer (1812-1887), a manufacturer of optical and scientific equipment in Manchester, U.K. In 1852 he used the "wet plate" collodion process to achieve a microphotograph on collodion film, mounting it on a glass microscope slide. Later he produced microphotographic slides commercially, issuing them on a series of different subjects. The main disadvantage of his invention was that a microscope was needed to view the images.
The microphotographic lens was developed by René Prudent Patrice Dagron (1819-1900) of France. He combined the Stanhope lens with a microphotograph to form a single miniature magnifying unit, which he patented in 1859. He inserted these "cylindres photomicroscopiques" in multitudes of cheap novelties, and established a "mail order" system to export his products around the world. His flair for publicity and considerable business acumen ensured that no other competitors achieved the same success.
In 1862 René Dagron established a small factory to make Stanhope lenses in the town of Gex, not far from the border with Switzerland. At least sixty local people were employed to produce the vast numbers of lenses needed to support the microphotographic industry in Paris. Eugène Reymond took over the business at the beginning of the twentieth century, followed by his son Roger. The last Stanhope lenses were made by Roger Reymond in 1972. After his death in 1998 the workshop was sold and the equipment dismantled. Although replica microphotographic lenses are being manufactured today, they are not Stanhopes made by the traditional methods.
Read about the fascinating life and adventures of René Dagron, and also the historical Stanhope industry at Gex in "Stanhopes: A Closer View", by Jean Scott.

Stanhopes: A Closer View by Jean Scott

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