Why 18th-century lovers exchanged portraits of their eyes

Apparently there was a trend — beginning in the late 18th century, and continuing for a few decades — of lovers exchanging tiny portraits of one of their eyes, and wearing them mounted on jewelry.

There's a fascinating short piece on it here in JSTOR Daily, which notes:

While miniature portraits were already popular in eighteenth-century England, they were often private objects viewed solely by the wearer.

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