Monocle

A monocle is a type of corrective lens used to correct or enhance the visual perception in only one eye. It consists of a circular lens placed in front of the eye and held in place by the eye socket itself. Often, to avoid losing the monocle, a string or wire is connected to the wearer's clothing at one end and, at the other end, to either a hole in the lens or, more often, a wire ring around its... (Wikipedia)