In Julius Caesar, what metaphor dominates Cassius’ description of Brutus?
In act 1, scene 2 of this play, after Caesar has exited the scene, there follows an exchange between Brutus and Cassius which is dominated by imagery related to the eyes and seeing. First, Cassius says he no longer has "from your eyes that gentleness / And show of love as I was wont to have," meaning that he feels Brutus is no longer so fond of him as he once was. Brutus's response to this is that if he has "veil'd my look / I turn the trouble of my countenance / Merely upon myself." Brutus is saying here that if he appears changed, it is not because of a change in his attitude to Cassius, but because he is troubled in himself.
Later, Cassius asks, "can you see your face?" to which Brutus responds, "the eye sees not itself / But by reflection." The eye in this instance represents Brutus himself: he cannot see himself except through what Cassius has told him. It is at this point that the extended metaphor of Cassius as Brutus's mirror begins, with Cassius lamenting that "you have no such mirrors as will turn/Your hidden worthiness into your eye." Cassius explicitly offers to fulfill this function for Brutus, enabling him to understand himself without it causing him vexation: "I, your glass / Will modestly discover to yourself / That of yourself which you yet know not of."
It is notable that Brutus is imagined as an eye, suggesting that he is (in Cassius's description) clear-sighted and necessary; Brutus is not failing in being unable to properly perceive himself, as no eye can do this without support and "by reflection." Cassius's language indicates the need Brutus has for Cassius's support without indicating that this is because of any weakness on Brutus's part.
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.'