In Act I Sc 2 Cassius subtly flatters Brutus hoping to make him a co-conspirator in his plan to assasinate Caesar. In order to entice him, Cassius tells Brutus that Brutus by himself cannot realise the many priceless virtues which lie hidden within himself (Brutus). This is because as Cassius prompts Brutus to remark, "the eye sees not itself." So, Cassius tells Brutus that he will become his mirror or looking glass to reflect all his virutes so that Brutus can then become aware of his hidden virtues and realise his full potential: "And it is very much lamented Brutus/That you have no such mirrors as will turn/Your hidden worthiness into your eye." "And since you know you cannot see yourself/So well as by reflection, I your glass/Will modestly discover to yourself/That of yourself which you yet know not of."
The metaphor which Cassius uses as a means to flatter Brutus in order to make him a co-conspirator in his plan to assasinte Julius Caesar is that of a 'mirror' or a 'looking glass.'