The power of doubly refracting and polarizing is not possessed by all crystalline bodies, but only those belonging to other than the cubic system; crystals belonging to this system neither doubly refract nor polarize light. In all doubly refracting crystals there are one or more lines or directions in which the light is not doubly refracted. These are called the optic axes, and sometimes they coincide with the geometric axis of the crystals, at others they do not; and they may be regarded as positions or directions of equilibrium of certain molecular forces existing within the crystal, which, acting in opposition, neutralize each other. If light, polarized by the polarizer, be transmitted through thin doubly refracting crystals, and analyzed by the analyzer, splendid colours will become visible; and on rotating separately either the polarizer or the analyzer, at each quarter rotation the colours will change, being complementary to those at first visible, or such as are requisite with the first to make white light. We have seen (fig. 19) that white light consists of seven coloured rays, or of three primary colours--red, yellow, and blue, which, by superposition, form the others; and thus red is complementary to green, which consists of blue and yellow, the two sets of complementary colours appearing and vanishing as the light and darkness did when the crystals were not used. These colours are produced by interference. The compound rays of white light (fig. 30 _l_) passing through the polarizer (_t_) are all polarized in one plane; the crystal (_d_) depolarizes this light, _i. e._ doubly refracts and resolves it into two sets of rays polarized in planes at right angles to each other, forming the ordinary, O, and the extraordinary ray, E. Each of these two sets of rays is resolved by the analyzer (_s_) into two other sets, polarized in planes at right angles to each other; so that in all there are four sets, two in one plane and two in the other; and, the primary rays of the two sets in each plane being in different stages or phases of undulation, in consequence of the retardation of the extraordinary rays, the undulations of certain coloured rays check and annihilate each other, while the remainder or complementary conspire and pursue their course, producing the appearance of colour, this effect being reversed at each quarter-revolution of the analyzer. An idea of what is meant by phases of undulation may be obtained by reference to fig. 29, in which the undulations, _a_, _b_, are in similar states or phases, and so conspire in action, while the wave _c_ is in a different phase and half an undulation behind the others; hence it would check or interfere with either of the other waves (_a_, _b_), the etherial molecules of the two, which vibrate perpendicularly or at right angles to the direction of the wave, acting to the same extent and in opposite directions.