Lens

The term lens was firstapplied to any transparent refracting bodywhich had two spherical surfaces, on accountof its resemblance to a vegetable known as alentil. A lens is a transparent substance, crownor flint glass chiefly, ground with regular curvature on one or both of its opposite sides, but notparallel to each other, through which an objectmay appear to be increased or decreased in size,and may have either convex or concave spherical or cylindrical surfaces.

Source: Ophthalmic Dictionary including pronunciation, derivation and definition of the words used in Optometry and Ophthalmology by James J. Lewis, Oph. D. Professor of Optometry in the Northern Illinois College of Ophthalmology and Otology, Chicago.