POLARIZATION. 183 



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 equili- 

 brium 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 con- 

 sists of seven coloured rays, or of three primary 

 colours red, yellow, and blue, which, by superposi- 

 tion, form the others ; and thus red is complementary 

 to green, which consists of blue and yellow, the two 

 sets of complementary colours appearing and vanish- 

 ing 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 /) 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 



