Monocle

A monocle is a type of corrective lens used to correct or enhance the vision in only one eye. It consists of a circular lens, generally with a wire ring around the circumference that can be attached to a string or wire. The other end of the string is then connected to the wearer's clothing to avoid losing the monocle. The antiquarian Philipp von Stosch wore a monocle in Rome in the 1720s, in order to closely examine engravings and antique engraved gems, but the monocle did not become an article of gentlemen's apparel until the nineteenth century. It was introduced by the dandy's quizzing glass of the 1790s, as a sense of high fashion.

https://en.wikipedia.org/wiki/Monocle