A MIRROR — and in the first century a disc of polished BRONZE, never glass. Corinth was a centre of their manufacture, which is very likely why Paul reached for one writing to Corinth. The KJV's 'through a glass, darkly' gave English a great phrase and a wrong picture: you do not see through a bronze mirror, you see a dim reflection in it. The NWT says 'metal mirror' outright. → first discussed at 1 Corinthians 13:12