Beginning at the end of the 1850s, tiny image viewers could be made as charms for watch chains and bracelets, or built into almost any device. While the miniature photographs were often scenes of favorite buildings or religious icons, many were erotic or even pornographic.
Here is a link to a wonderful thesis by Sol Legault that will explain more than you may wish to know about Stanhopes, and if you look for it, lots of entertaining nakedness.
https://digital.library.ryerson.ca/islandora/object/RULA%3A2406/datastream/OBJ/view
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Dark Lantern Tales
Dark Lantern Tales 