PRISM

, in Geometry, is a body, or solid, whose two ends are any plane figures which are parallel, equal, and similar; and its sides, connecting those ends, are parallelograms.—Hence, every section parallel to the ends, is the same kind of equal and similar figure as the ends themselves are; and the Prism may be considered as generated by the parallel motion of this plane figure.

Prisms take their several particular names from the figure of their ends. Thus, when the end is a triangle, it is a Triangular Prism; when a square, a Square Prism; when a pentagon, a Pentagonal Prism; when a hexagon, a Hexagonal Prism; and so on. And hence the denomination Prism comprises also the cube and parallelopipedon, the former being a square Prism, and| the latter a rectangular one. And even a cylinder may be considered as a round Prism, or one that has an infinite number of sides. Also a Prism is said to be regular or irregular, according as the figure of its end is a regular or an irregular polygon.

The Axis of a Prism, is the line conceived to be drawn lengthways through the middle of it, connecting the centre of one end with that of the other end.

Prisms, again, are either right or oblique.

A Right Prism is that whose sides, and its axis, are perpendicular to its ends; like an upright tower. And

An Oblique Prism, is when the axis and sides are oblique to the ends; so that, when set upon one end, it inclines on one hand, like an inclined tower.

The principal properties of Prisms, are,

1. That all Prisms are to one another in the ratio compounded of their bases and heights.

2. Similar Prisms are to one another in the triplicate ratio of their like sides.

3. A Prism is triple of a pyramid of equal base and height; and the solid content of a Prism is found by multiplying the base by the perpendicular height.

4. The upright surface of a right Prism, is equal to a rectangle of the same height, and its breadth equal to the perimeter of the base or end. And therefore such upright surface of a right Prism, is found by multiplying the perimeter of the base by the perpendicular height. Also the upright surface of an oblique Prism is found by computing those of all its parallelogram sides separately, and adding them together.

And if to the upright surface be added the areas of the two ends, the sum will be the whole surface of the Prism.

Prism

, in Dioptrics, is a piece of glass in form of a triangular Prism: which is much used in experiments concerning the nature of light and colours.

The use and phenomena of the Prism arise from its sides not being parallel to each other; from whence it separates the rays of light in their passage through it, by coming through two sides of one and the same angle.

The more general of these phenomena are enumerated and illustrated under the article Colour; which are sufficient to prove, that colours do not either consist in the contorsion of the globules of light, as Des Cartes imagined; nor in the obliquity of the pulses of the etherial matter, as Hook fancied; nor in the constipation of light, and its greater or less concitation, as Dr. Barrow conjectured; but that they are original and unchangeable properties of light itself.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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PRESS
PRESSURE
PRIMES
PRINCIPAL
PRINGLE (Sir John)
* PRISM
PRISMOID
PROBLEM
PROCLUS
PROCYON
PRODUCING