Swelling, not magnifying glass

Meaning

Lemma.

Loupe has its older bodily sense of a swelling or cyst beneath the skin in the country girl’s description. It obscures her left eye; it is not an optical magnifier she is using. The edition chooses the less diagnostic “swelling.” The remaining eye later brightens as she recognises the volunteer image, so the unusual noun contributes to the precise visual sequence rather than being only a grotesque label.