An excerpt from an interesting article in The Antiques Roadshow online site on eye miniatures:

“The images were painted by miniaturists, portrait painters who specialized in small, detailed images. In this case they focused on only the eye, often represented with eyebrow and lashes. A wisp of hair, the suggestion of sideburn or the bridge of a nose would hint at the owner’s identity but never reveal it. A border of clouds frequently encircled the image, further accentuating the mystery surrounding it.

Such portraits appeared between the 1790s and 1820s in the courts and affluent households of England, Russia, France and even, quite rarely, America. In all, Weber estimates that fewer than a thousand were produced.

Eventually, not just lovers but beloved family members were portrayed. One bracelet, for instance, is composed of four eyes, each belonging to a member of one family and bearing the initials of each individual. Mourning pieces contained the eye of a departed loved one, sometimes set in a frame of pearls which symbolized tears.”

 The full url link is:
http://www.pbs.org/wgbh/roadshow/tips/loverseyes.html

An example from the Antiques Roadshow article

An example of a British eye miniature c. 1840 I recently sold.

A rare American eye miniature c. 1815 I recently sold.