A Stanhope Cross takes various forms and shapes, usually decorative and often part of a necklace, with one special adornment: an integral magnifying lens.
This type of cross is named after Third Earl of Stanhope, a British statesman, scientist, mathematician and engineer. One of his inventions was a lens for use as a simple, one-piece microscope.
Because of its simplicity, and particularly after the development of photography, it was used as a popular novelty in the 19th and early 20th centuries. The lens was adopted into jewellery and other knickknacks including broaches, rings, needle cases, letter openers (letters were things people used before email), napkin holders, and particularly necklace crosses.
Through the lens one might see an image of saints or angels, or perhaps the entire Lord's Prayer. A further attraction of this item is that even without looking through the glass lens, the cross appears to be jewelled as a Glory Cross.
(Other crosses with a central jewel might be misnamed as Stanhope Crosses.)