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Prism. In optics a body, of glass, quartz, fluor spar, rock-salt 

 or other transparent substance, bounded by three planes 

 which intersect in three parallel straight lines, used for 

 deflecting light rays either by reflection or refraction. [1, 3, 

 8, 10, 12, 13, 14, 15, 20, 21.} 



Prism, Ahrens. A form of polarising prism invented by Dr. C. 

 Ahrens, giving about four times the aperture of the Nicol 

 prism (q.v.). [2, 3, 22, 26.] 



Prism, Amici. A tetrahedral reversing prism which bends the 

 optic axis through 90. [2, 3, 14, 20, 26.] 



Prism, Achromatic. A combination of prisms of different sub- 

 stances designed to obtain deviation without much dis- 

 persion. 



Prism Binocular Glass. An improved form of binocular glass 

 (q.v.) with double reflecting prism, giving a higher power 

 with larger field. [2, 3, 5, 9, 10, 13, 15, 16, 17, 18, 20, 21, 

 26, 27, 28.] 



Prism, Constant Deviation. A compound prism so designed that 

 the deviation, i.e., the angle between the incident and the 

 emergent beam of light, is constant. [3, 10, 13, 14.] 



Prism, Diatom. A triangular prism used for illuminating by 

 oblique light small objects in the field of a microscope. 

 [2, 3.] 



Prism, Direct Vision. A compound prism of crown and flint 

 glass giving dispersion without deviation, i.e., an undeviated 

 spectrum. [2, 3, 14, 20, 26.] 



Prism, Double Image. A prism of Iceland spar or other doubly- 

 refracting crystal, giving a double image of an object. [2, 

 3, 13, 14, 15, 20, 22, 26.] 



Prism, Erecting. A prism with angles of 45, 45, 90, giving, 

 by internal reflection of the beam of light, an erect image 

 where, otherwise, there would be an inverted image in the 

 optical projection of an object. [1, 2, 3, 10, 13, 20, 26, 27.] 



Prism, Foucault. A compound prism in which a layer of air 

 takes the place of the cement. 



Prism, Fresnel. A prism with an obtuse angle slightly less than 

 1 80 for producing interference of light images. [3, 14, 26.] 



Prism Glass. Any form of telescope, monocular or binocular, 

 in which a double reflecting prism is used. [3, 9, 10, 13, 15, 

 18, 20, 21, 26, 27.] 



Prism, Nachet's. A form of prism used for conveying an oblique 

 pencil of light upon a microscopic object. [2, 3, 26.] 



Prism, NicoPs. See Nicol Prism. [2, 3, 20, 22, 25, 26.] 



Prism, Pentagonal. A five-sided prism, used for range-finding, 

 whose angles are, respectively, 90, 112^, 6o, 66, 112^, 

 silvered on two surfaces, to deflect a beam of light at a 

 constant angle when the prism is partially rotated. [1, 2, 

 3, 8, 10, 12, 13, 14, 18, 20, 26, 27.] 

 Prism Photometer. See Photometer. 



