What is "fresnel's rhomb"

Rhomb \Rhomb\ (r[o^]mb or r[o^]m; 277), n. [L. rhombus, Gr. "ro`mbos rhomb, a spinning top, magic wheel, fr. "re`mbein to turn or whirl round, perhaps akin to E. wrench: cf. F. rhombe. Cf. Rhombus, Rhumb.]

1. (Geom.) An equilateral parallelogram, or quadrilateral figure whose sides are equal and the opposite sides parallel. The angles may be unequal, two being obtuse and two acute, as in the cut, or the angles may be equal, in which case it is usually called a square.

2. (Geom.) A rhombohedron.

   Fresnel's rhomb (Opt.), a rhomb or oblique parallelopiped of crown or St. Gobain glass so cut that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing within the rhomb, at other faces, two reflections. It is used to produce a ray circularly polarized from a plane-polarized ray, or the reverse.
   --Nichol.