The Collaborative International Dictionary of English v.0.48:

Dioptrics \Di*op"trics\, n. [Gr. ? ?: cf. F. dioptrique.]
   (Optics)
   The science of the refraction of light; that part of
   geometrical optics which treats of the laws of the refraction
   of light in passing from one medium into another, or through
   different mediums, as air, water, or glass, and esp. through
   different lenses; -- distinguished from catoptrics, which
   refers to reflected light.
   [1913 Webster]