"She showed her mother a gold locket surrounded with seed pearls, containing the portrait of one human eye. Mrs. Belton was equally overcome by the masterpiece and congratulated Captain Hornby warmly."
"It is a very curious thing, Captain Hornby," she said, "that though there are dozens of these eye lockets about, a left eye is extremely rare.""
– from The Headmistress, first published in 1943, one of Angela Thirkell's many eccentric and under-appreciated novels (she reliably produced one per year for several decades in the middle of the 20th century).
Above, a selection of "right-eye" portraits. Below, a selection of "left-eye" portraits. Most of these are in the Victoria & Albert Museum.
Commissioning, exchanging and treasuring these peculiar tokens became fashionable among wealthy Europeans for a few decades at the end of the 18th century. Most eye portraits were made with watercolor on vellum. The practice continued into the early 19th century. After that, it died out altogether.