: an optical device that inverts and reverses right and left an image viewed through it, that consists usually of a pair of isosceles right-angled prisms so arranged that a beam of light entering the hypotenuse face of one is totally reflected twice before emerging at the same face and entering the second prism at its hypotenuse face and being again doubly reflected, and that makes possible a shortening of the physical length of the instrument in which it is used see prism binocular
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Etymology
after Ignazio Porro †1875 Italian engineer
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