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ng angle for any medium is the index of refraction for that medium. The phenomenon of double refraction, exhibited by Iceland spar (crystallized carbonate of lime), is a familiar one. If any small object be placed in contact with a piece of thy was discovered by Sir David Brewster. When Huyghens was occupied with the double refraction of light in crystals of Iceland spar, i. e. with the separation of the pencils of light into two parts, he also discovered, in 1678, that kind of polarie in examining the properties of polarized light. Its action depends on the doubly-refracting property of the material (Iceland spar) of which it is composed. On viewing a small object, such as a black dot on a sheet of paper through a piece of Iceland spar or of selenite, two images will be seen; on turning the spar around, one of these images will appear to revolve about the other. The fixed image is called the ordinary, and the other the extraordinary image; the line joining them is a
s ledgers to rest on the pullogs. 4. A pole lashed to a disabled carriage as a substitute for a wheel. 5. (Mining.) An earthy, lustrous mineral, which is often associated with ores, such as lead, copper, and tin, forming the gangue or matrix of the ore. It is known among miners by its characteristic color, as white spar, black spar, etc. The word has many interesting mineralogical significations, such as fluor spar, a beautiful, crystalline fluoride of lime, the double-refracting Iceland spar, etc., etc. Spar′a-drap. A cast-iron nail driven into soles of boots and shoes, and so called from its resemblance in shape to a sparrow-bill. (Pharmacy.) An adhesive plaster spread upon linen or paper. Spar-a-dra′pi-er. (Pharmacy.) A machine for spreading plasters. It is a table with two raised pieces, movable and furnished with points by which the cloth may be stretched, and a spatula for spreading the composition. Spar-deck. (Nautical.) Originally on
ce in yards of cavalry. By means of interior teeth b′ on the flange b engaging pinions c c on the screw-pins d d′, two wires are caused to approach or recede from each other; when these exactly coincide with the extremities of the object, the number beneath the index o, upon the scales b, indicates the distance if infantry be observed, or upon f if the object be cavalry. g g′ are the field-lenses, and h is the object-glass. Rochon's double-image telescope has within its tube a prism of Iceland spar a mineral possessing the power of double refraction), which divides the pencil of rays into two pencils diverging from each other at a determinate angle. This prism is, by means of a rack and pinion, movable from end to end of the tube. It is moved back or forth until the edges of the two images formed by the prism are in exact contact, when an index on the slide indicates the distance according to a scale engraved on the outer surface of the tube. The binocular marine-glass of M.<