The Stanhope, or "optical bijou", was the name given to the items created with microphotographic images inside them. Ranging from letter openers, to knives, to sewing tools such as tape measures, to jewelry, the Stanhope revolutionized the perception of photography and the souvenir industry beginning in 1860's France. About that time, Rene Dagron (1819-1900), a portrait maker in Paris created the first Stanhopes by affixing a micro-image to a lens. From Stanhopes.info: "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."
To use a Stanhope peep-hole viewer, one would put the tiny hole in the object up to one's eye while holding the object up to the light, and the image could be seen. Popular subjects included monuments, royalty (especially Queen Victoria!), world's fairs, cities, towns, shrines, and landscapes. Stanhopemicroworks.com has an extensive gallery of Stanhope objects.