Monocle
A single lens held in one eye socket by the brow and cheek.
Also: eyeglass (single)
A monocle is one corrective lens, often with a rim and a cord, wedged between the brow bone and the cheek. It was fashionable among upper-class men in Britain, Germany and Russia in the nineteenth and early twentieth centuries, and became a cartoon shorthand for wealth. It corrects only one eye, so it suited people with different needs in each. A deep-set eye holds one most easily.