biprism
    n 1: an optical device for obtaining interference fringes

Biprism \Bi"prism\, n. [Pref. bi- + prism.]
   1. A prism whose refracting angle is very nearly 180 degrees.
      [Webster 1913 Suppl.]

   2. A combination of two short rectangular glass prisms
      cemented together at their diagonal faces so as to form a
      cube; -- called also optical cube. It is used in one
      form of photometer.
      [Webster 1913 Suppl.]