biprism

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. ]