Polarisation of Light. When light is transmitted through, or reflected from, certain substances, it is changed in a peculiar way, and is said to be polarised. Light is caused by ether vibrations, which take place in all directions in a plane at right angles to the direction of the ray, and if light falls upon a surface - say, of glass - it is found that a portion is transmitted through the glass and a portion is reflected. A part of the reflected portion has the peculiarity that its vibrations are all in a plane parallel to the glass, the vibrations in other planes having been transmitted through the glass; and if the beam of light makes an angle of 58° with the normal to the surface, practically all the reflected light will be thus modified. Plane polarisation consists in this quenching or diverting of all the vibrations save those in one particular plane. Some substances, such as tourmaline, will only transmit light whose vibrations are in one plane. Iceland spar, which has the property of double refraction (q.v.), splits up ordinary light into two portions; by cutting a rhomb of spar diagonally and cementing the halves together with Canada balsam these portions may be separated. These two rays, called the ordinary and extraordinary rays, are diffracted in a different ratio by Iceland spar and Canada balsam, from which it results that the ordinary ray is totally reflected [Total Reflection] and passes out through one side of the rhomb, the extraordinary ray passing straight through; the latter portion is completely polarised. This arrangement, known as Nicol's prism, is the most perfect device for polarising light. If polarised light falls upon a second polarising body, it may or may not be absorbed; for if we quench the vibrations which are not in one plane (i.e. polarise the light) and then further quench those vibrations which are not in a second plane, it is clear that, if these two planes are coincident, the light will not be affected; but if the second plane makes an angle with the first, the light will be more or less extinguished, and will be completely so if the two planes are at right angles. In this case the first Nicol prism or other device is called a polariser, and the second an analyser. Many substances exercise a peculiar influence on polarised light. If a thin film of selenite or mica is introduced between the polariser and analyser, and viewed through the latter, it will in some positions restore the light after it has been extinguished by crossing the prisms; and may, moreover, exhibit brilliant colours, which at first disappear as the analyser is turned; when it has moved 90° from its former position, colours complementary to the former are seen. Many fine effects may be projected on a screen by a lantern by using designs built up of plates of selenite of varying degrees of thickness; the colours change as the analyser is rotated. In these experiments the polarised light is split up into two components vibrating in two planes at 45° to that of the polarised beam; these two are retarded unequally by the film of selenite, so that, when recombined into a single beam by the analyser, the difference in phase of the two sets of vibrations produces interference (q.v.). Analogous effects are produced by non-crystalline bodies when in a state of strain due to unequal heating, mechanical pressure, or otherwise. Some bodies, such as quartz and sugar solutions, rotate the plane of polarisation - that is, if introduced between two crossed Nicol prisms, the light will be restored, and will not again be extinguished until the analyser has been turned in one direction or the other. If a tube of carbon disulphide or a piece of Faraday's heavy glass is placed between the polariser and analyser, and is subjected to the action of a powerful magnetic field, the plane of polarisation will be rotated, the direction being reversed on reversal of the magnet. A similar effect is produced by subjecting carbon disulphide to electrostatic strain.