POLARISCOPE (from Neo-Lat. polaris, re lating to the pole + Gk. Cr KOlrell", skopcin, to view). An instrument which consists of any combination of it means of producing polarized light, or 'pollarizer,' and a device for testing the polarization at light, or 'analyzer.' In ordinary light (see LIGHT) the direction of vibration is changing millions of times per seemul. As a result of reflection, or refraction, the direction may be made to remain constant, and the light is then said to be 'polarized' plane, elliptically. or circularly, according as the motion of the ether particle is linear, elliptical. or circular.
When light is reflected (win the surface of a transparent medium, as glass, at the Ilidarizing angle? the tangent of which is equal to the index of refraction of the medium, it is found to be plane polarized, and the plane in which the incident and reflected rays lie is called the plane of polarization. If such a polarized beam of light is allowed to fall upon a second similar mirror at the polarizing angle, it will enter the medinin or be reflected according as the plane of this reflection is parallel or perpendicular to that of the first reflection. Polarization by re flection was discovered by Mattis in ISIO. As early as 1690 Huygens discovered that the two rays of light produced by the double refraction in calcite were polarized at right angles to each other.
Three methods of obtaining a beam of plane polarized light for experimental purposes are in general use. and are employed in polariseopes: (1) Reflection at the polarizing angle; (2) dou ble' refraction and the elimination of one of the beams. (a) by reflection (Nicol prism), (1) by sending it off to one side (Itochon prism. etc.) ; and (3) double refraction and the absorption of one beam, tourmaline plates. The Nicol prism (q.v.) is most generally employed in polariscopes and forms one of the best means for the produc tion and detection of polarized light.
In a polariseope the polarizer and analyzer may be similar, or not. In the form devised by Norremberg (Fig. 2) the polarizes is the mirror at the botom of the apparatus. and the analyzer is a Nicol prism in the eye-piece at the top, or it may be that a bundle of thin plates of glass are used, as shown in the figure. When the plane of polarization of both polarizer and analyzer is the Salle., the ray which emerges from
the polarizer will pass on through the analyzer, and they are said to he 'parallel' (Fig. 3) u but if the two planes are perpendicular, then the beam will all enter as the ordinary in the analyzer and be reelected and lost. and they are said to be 'crossed' (Fig. 4) : the eve looking in at the eye piece sees a dark field. If a plate of transparent material be introduced between the polarizer and analyzer, it may happen that the field of the crossed combination will beenme light or colored. This will mean that the material has in some way affected the ray emerging from the polarizer so that it is no longer entirely cut off by the analyzer. Such substances are in general said to be `double-refraeting.' Substances which do not so affect the polarized learn are called 'iso tropic' or 'single-refracting.' The double-refrac•t ing substances are generally crystalline mid fall into two broad (-lasses, uniaxial and biaxial. In certain double-refracting crystals there is one direction in which all light travels with the same velneity. and hence' no double refraction occurs. This direction is called the optic axis and such substances are uniaxial. In biaxial crystals there are two directions, or optic axes.
along which all light travels with the same velocity, and around which the optical properties group themselves. Substances are studied in the polaris•ope in either parallel or convergent (or divergent) light, and the double-refracting media in general produce two beams from the original beam coining from the polarize'', and these two new beams, traveling with different velocities. are made to interfere when their components are brought into the plane of the analyzer. As white tight is generally conployed at the polarizer, and as the kind of interference is a function of the wave length, colors are usually seen at the analyzer. A complete study of these color phe nomena, and their general behavior in the polari scope, make it possible to determine the identity of most minerals from their optical properties. This branch of optics has developed into micro scopic petrography, which has so supplemented the other tools of the mineralogist mid geologist that these sciences have been revolutionized.