lens locus

a. a shaped transparent body that refracts light to an image; b. a means of focusing or seeing; c. a lentil-shaped form; d. a body of one material lying within another (geological).

Getty AAT: Pieces of glass, crystal, plastic, or other transparent substance characterized by having two curved surfaces or one plane and one curved surface, serving to cause regular convergence or divergence of the rays of light passing through it. (AAT 300266807)