Archaic KJV Word
Glass
Modern equivalent: mirror
What Was Lost
The quality of ancient seeing. Paul's glass was a polished bronze mirror -- you could see a shape, a suggestion, an impression, but not clarity. 'Through a glass darkly' means 'in a mirror, dimly' -- we see a blurred reflection of divine reality, not the thing itself. Modern readers picture peering through frosted window glass, which is a different metaphor entirely.
Closest Survivor in Modern English
looking glass (compound that preserves the mirror meaning, sounds like Alice in Wonderland)
Peak Usage (1611)
KJV 1 Corinthians 13:12 -- 'For now we see through a glass, darkly'; James 1:23 -- 'A man beholding his natural face in a glass'
Died ~1800 (the 'mirror' meaning faded as glass became the transparent material)
Narrowed from 'mirror/looking-glass' to the transparent material, making Paul's metaphor about imperfect reflection read as looking through a window.
What Replaced It
“mirror”
Modern mirrors are perfect; the glass Paul described was polished metal -- a dim, imperfect, suggestive reflection
“reflection”
Abstract; glass was a concrete object you looked into and saw yourself poorly