Prism

(prizm.) [FromGr. prism a, a seeing.] (Geom.) Asolid whose upper and lower sides are equal and similar figures, and whose lateral surfaces are plane parallelograms. If the bases are triangular, it is called a triangular P.; if square, a quadrangular P. A right P. has its sides perpendicular to its ends; an oblique P. is that of which the two sides are oblique to the ends.(Opt.) A triangular-shaped piece of gla or other transparent medium with polished surfaces. The section may be either a right angle, an equilateral, or an isosceles triangle. The equilateral and isosceles prisms are employed for effecting the prismatic decomposition of light. When a ray of light falls obliquely upon one of its refracting surfaces, it pa es through and emerges at the opposite face, suffering at its ingre and egre two refractions in the same direction, whereby, unle the light be homogeneous, the ray is spread out into its component colors, forming a spectrum. [s. 733]