indexicality locus

a. property of indexical signs, as defined by C. S. Peirce: signs bound to their object through cause and effect (smoke to fire, footprint to foot); b. basis of photography’s claim to record the real; c. trace, cast, imprint.

C. S. Peirce: “An Index is a sign which refers to the Object that it denotes by virtue of being really affected by that Object.” (“Nomenclature and Division of Triadic Relations,” 1903; The Essential Peirce 2:291)