A monocle turned up among my grandfather's belongings.
He was reading the documents with a monocle.
眼鏡 means glasses (spectacles) with two lenses, whereas 片眼鏡 is a single lens for one eye.
Standard kanji spelling for a monocle.
Mixed kanji-kana spelling; less common but still encountered.
Formed from 片 “one; single” and 眼鏡 “eyeglasses,” hence a single-lens eyeglass.