Emily Dickinson Lexicon

FOIL, n.³ [Fr. feuille; It. foglia; Port. folha; Sp. hoja; L. folium; Gr. φυλλον.]

1. A leaf or thin plate of metal used in gilding.
2. Among jewelers, a thin leaf of metal placed under precious stones, to make them appear transparent, and to give them a particular color, as the stone appears to be of the color of the foil. Hence,
3. Any thing of another color, or of different qualities, which serves to adorn, or set off another thing to advantage. Hector has a foil to set him off. Broome.
4. A thin coat of tin, with quicksilver, laid on the back of a looking-glass, to cause reflection. Encyc.