Prism

Prism is a, solid whose ends are any equal and parallel plane figures and whose sides are parallelograms. When its ends are triangles (and this is the most usual form of prism), it is extensively used in practical optics for producing refraction and dispersion (q.v.), and may be conveniently used for finding the refractive index of a substance. An "erecting" prism is frequently used when pictures of experiments done in a lantern are thrown upon a screen. The object-glass would cause the top of the apparatus to appear at the bottom of the screen; and, to avoid this somewhat confusing effect, a carefully-polished prism (of definite angular form) is placed in front of the object-glass.