The effects produced by passing a beam of convergent or divergent light through crystals are still more striking. If plates of crystalline material, placed between the prisms of a polari scope which is provided with a lens for the pro duction of a conical beam of light, are viewed through the analyzer, symmetrical patterns in color the form of which depends upon the char acter of the crystal and the plane, with refer ence to its axis, in which the layer has been cut, are seen. Any detailed description or ex planation of these phenomena lies beyond the scope of this article, nor can any idea of the beauty and intricacy of the color-patterns thus produced be conveyed to the mind by words.
Isotropic substances, such as glass, show temporary double refraction, which manifests itself by restoration of light and display of colors between crossed Nicols, when subjected to distortion or stress. Such effects in glass, for example, are produced by compression under a clamp, by bending or twisting, by sud den unequal heating or cooling, or even by setting up standing sound-waves within the substance. Permanent strains, which show themselves in the same way, may be formed in glass by sudden chilling. Blocks of glass thus treated, and known, in French, as verre trempe, are among the most beautiful of objects under the polariscope.
Circular and Elliptical Polarization.— When the vibratory motion to which a beam of light is due is capable of being resolved into two components at right angles to each other, of equal amplitude, and differing in phase by a quarter period, the beam is said to be circularly polarized.
The actual motion of the ether particles in such a case, could we observe it, would be found to consist of rotation in a circular whit, either clock-wise or counter clock-wise (look ing in the direction from which the wave comes) hence the names, right-handed circular polarization and left-handed circular polariza tion.
Circular polarization may be produced by sending light through a crystalline layer of such thickness that the difference of phase of the two rays on emerging is x 2 (a quarter period). Such a layer is caller a. quarter-wave plate.
When the difference of phase of the two components differs from ir 2 the emerging beam is said to be elliptically polarized.
Rotation of the Plane of Polarization.— Certain substances have the property of rotat ing the plane of polarized light which passes through them. Quartz cut perpendicularly to the optic axis possesses this property to a re markable degree, and many organic liquids possess rotatory power. Such substances are said to be optically active. A plane polarized ray upon entering such a medium is converted into two circularly polarized rays, respectively right-handed and left-handed. The resultant
of the superposition of these two rays is a plane polarized ray the plane of polarization of which varies with the depth within the layer at which the angle is taken. We may, therefore, think of the plane of polarization as rotating uniformly Zbout the path of the ray.
The rotatory power of solutions of optically active substances depends upon the strength of the solution and this fact has been utilized, in the case of sugar, for determining the amount of saccharine matter present. The instruments employed in this process are special forms of polariscope of great sensitiveness. They are known as saccharimeters.
Magnetically Active Substances.— When a beam of plane polarized light passes through a magnetic field in the direction of the lines of force, the plane of polarization is turned.
Looking along the lines of force in the direction in which, by convention, they are as sumed to run, the rotation is always clockwise to the observer, whether the light travels with or against the lines. The angle of rotation is proportional to the strength of the field, and to the distance through which the light passes. It depends also upon the medium through which the light moves; being nearly four times as great in carbon-disulphide than in water and more than five times as great in glass as in water. These relations may be expressed by means of the equation a= y 1H; where a is the angle of rotation, 1 the distance traversed, H the strength of the field and 7 a constant (known as Verdet's constant) which depends upon the character of the medium.
This rotatory power of the magnetic field was discovered by Faraday (1845). To ob serve it a tube containing carbon-disulphide or water (Fig. 6) is placed between crossed Nicol prisms.
The tube is surrounded by a coil of wire of many turns.
A beam of light is plane polarized by the prism (p), traverses the nix of liquid (1), but cannot pass a, the polarizing plane of which is at right angles to that of p.
When a strong current is sent through the coil, lines of force of the magnetic field thus produced traverse the tube from end to end. Some light now passes through a and the angle of rotation may be measured by turning that prism until the light is again cut off. Upon reversing the current in the coil the direction of rotation is reversed.
Bibliography.— For popular discussions consult Spottiswoode, 'Polarisation of Light' ; Tyndall, 'On Light' ; Stokes, 'On Light) ; Tait, 'On Light); Wright, Lewis, 'On Light,' etc. For a very complete modern statement of the theory see Drude, 'Theory of Light' (trans lated by Mann and Millikan). See LIGHT; PHYSICAL CRYSTALLOGRAPHY.