The two methods most in use for producing plane polarized light are to allow ordinary light to lie reflected from glass at the polarizing angle, and to allow ordinary light to pass through a `Nieol's prism.' This consists of two pieces of a crystal of Iceland spar cut in a definite man ner and cemented together by Canada balsam; Iceland spar is a uniaxial crystal and so the incident light is broken up into two train, of waves, the ordinary and the extraordinary; when these reach the layer of Canada balsam, the former sutlers total relleetion.while the latter is transmitted; thus the emerging light is plane polarized. It is found, too, that the light scat tered by fine particles, e.g. the blue light of the sky, is plane polarized.
It is pos-ible by several methods so to the] plane polarized light that all the vibrations in the ether as the waves pass are in circles or in ellipses; such light is called 'circularly' nr 'elliptically' polarized. Ordinary light such as tram tin• slut or a lamp can be considered :I S a mixture of all kinds of polarized light, plane, circular, and elliptical. (If plane polar ized light is retleetoil from a metallic surface it becomes elliptically polarized.) If ordinary light is incident upon a thin plate of any doubly refracting substance, it may be shown that two beams of plane polarized light— with their vibrations at right angle, to each otln•r—will be transmitted; and further. these beams will travel through the plate at different speeds. so that on emerging one will be slightly in advance of the other so far as the phase of the waves is eoncerned. If these two beams now enter a Nieors prism, only a component of each will be transmitted, for the prism allows to pass vibrations in a definite direction only, which direction is not in general that of the vibration in either of the two beams. If the light which
falls upon the thin plate is itself plane polarized, it will be broken up as before into two beams, and they will be recombined into a plane polar ized beam by the Nicol's prism. The two com ponent beams- are in different phases; and, if one is retarded by an amount equal to half a wave-length for any color or any odd number of half wave-lengths. then. when these beams are combined by the Nicol so as to have their vibrations all in one direction. the waves of this particular color will he absent, owing to inter ference, if the intensities of the two coinciding 11•111118 are the same. and if the incident light is white the emerging light will be colored. (If the incident light had not been plane polarized, there would not have been a definite relation between the intensities of the beams coinciding in the Nicol's prism.) This experiment offers a most delicate test of the double refraction of any substance: place two Nicol's prisms in line. and turn one around its axis until no light is trans mitted through the two—they are said to he `crossed now introduce between them the sub stance to he investigated. If it is doubly refract ing. light will now be transmitted through the Nieol's prisms. This is sometimes called 'de polarizat ion.' For a full description of the other properties of polarized light, some large treatise should he consulted.