# 1911 Encyclopædia Britannica/Prism

**PRISM** (Gr. πρίσμα, properly a thing sawn, πρίζειν , to saw),
in geometry a solid enclosed by plane surfaces, two of which,
termed the ends, are parallel, equal, similar and similarly
situated polygons, and the faces connecting the ends are parallelograms,
equal in number to the sides of the polygon. If the
faces be perpendicular to the ends the prism is a “right prism,”
and the faces are rectangles; otherwise the prism is “oblique.”
The *axis* is the line joining the centres of the ends. It may be
generated by moving a plane (corresponding to an end or base)
parallel to itself. A prismoid differs from a prism in having for
its ends two dissimilar parallel figures. For illustrations see
Crystallography, and for the mensuration see that article.
In optics the word denotes a triangular prism, *i.e.* one having
a triangle for base, used to decompose white light. (See
Refraction and Dispersion.)