Hutton's eyepiece? (4)
I believe the answer is:
lens
This clue could be a double definition.
'hutton's' is the first definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'eyepiece?' is the second definition.
(eyepiece is a kind of lens)
(Other definitions for lens that I've seen before include "Transparent optical devices" , "Bit of a looker" , "Piece of glass, usually curved" , "Device to converge or diverge light" , "Part of telescope" .)