Prism

(prizm). " A solid whose bases or ends are any similar, equal, and parallel plane polygons and whose sides are parallelograms." (Cent. Diet.) Made of gla , it is used for refracting the sun's rays and forming a spectrum, and to make prismatic lenses. Prismatic lenses refract the light toward the base of the prism. Their chief use in ophthalmology is in cases of unbalance of the external ocular muscles. P., Amice, one consisting of a triangular prism of heavy flint-gla wedged in between two triangular prisms of crown-gla , the apical edges of the crown-gla prisms pointing toward the base of the flint-gla prism, i. e., the apical edges of the crown and flint-gla prisms point in opposite directions. The flint-gla gives the dis persion or separation into colors, while the crown gla makes the emergent rays approximately parallel with the incident rays, so that one looks directly into the prism along the axis of the microscope. P.diopter, P. -dioptre, or P.-dioptry, a standard of prismatic refractive power consisting in a prism that deflects a ray of light one centimeter on a tangent plane situated at a distance of one meter. Ps., Enamel, the irregular columns of from four to six sides composing the enamel of teeth, closely packed together and generally vertical to the surface of the underlying den tine. P., Nicol, one consisting of two somewhat elongated rhombs of Iceland spar cut obliquely and cemented together with Canada balsam. These are mounted in such a way that the light pa es through them lengthwise, and in pa ing is divided into two rays of plane polarized light.