[Gk. prisma, something sawn off; prizein = priein, to saw.] A piece of wood, metal, glass, etc., the ends of which are parallel, and equal in size and shape, and the sides parallelograms. Prisms of different forms are often named from the figure of their bases, as triangular, hexagonal, etc.-In optics, a three-sided piece of glass with two equal and parallel triangular ends, used for separating the colors in a ray of light, and in refraction, etc. (See Spectrum,)

REFRACTION, THROUGH A PRISM.