rendered the wearer invisible. Gyges, the Lydian, is the person to whom Candaules showed his wife naked. According to Plato, Gyges descended into a chasm of the earth, where he found a brazen horse; opening the sides of the animal, he found the carcase of a man, from whose finger he drew off a brazen ring which rendered him invisible, and by means of this ring he entered into the king's chamber and murdered him.
“Why, did you think that you had Gyges ring. Or the herb that gives invisibility [fern-seed]?” Beaumont and Fletcher: Fair Maid of the Inn, i. 1.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894