The “glass” referred to using King James’s Early Modern English is. . . a mirror, a looking glass. 16th and 17th Century mirrors were neither as commonplace as today, nor as well-made, and often the reflective surface, most commonly made via a tin-mercury amalgam, with the mercury evaporated to leave the tin as a reflective surface, delaminated or otherwise became cloudy.
Mirrors were also viewed metaphorically as windows into another realm, so
“For now we see through a glass, darkly [view ourselves/see into another realm as in a cloudy mirror]; but then face to face: now I know in part; but then shall I know even as also I am known,”
Off topic, but maybe kinda related:
And then methought, divergently and NOT exegetically. . . If the eyes are the windows of the soul, maybe that explains why some people use reflective tinting and don’t even clean that. . .