ol'TK'AI. I'KOI'KKTIKS < >| CRYSTALS 



I '.17 



FIG. 345. Airy 'a Spirals. 



equal, depending upon the wave length or color of the light. In 



the case of white light a right-handed crystal, when the analyzer is 



rotated clockwise, and to the 



right, the central portion of the 



field passes from red to orange, 



yellow, green, blue, violet, or 



will go down the scale of colors. 



In a left-handed section, this 



order of colors is yielded by a 



rotation of the analyzer to the 



left or anticlockwise. 



When a right-handed section 

 is superimposed on a left- 

 handed section, a very peculiar 

 interference figure is yielded, 

 Fig. 345, known as Airy's 

 spirals. These spirals are often 



yielded by sections of natural crystals, and are due to the 

 twinning of right- and left-handed forms. The rotating power 

 of a crystal decreases as the inclination of the section to the 



optic axis increases, until in a 

 parallel position it is nil. 



The indicatrix of biaxial crys- 

 tals is an ellipsoid, but not an 

 ellipsoid of revolution. The 

 two rays are both variable rays, 

 except in the planes of sym- 

 metry. It is in these three 

 planes of symmetry that one 

 ray is an ordinary ray or has 

 a constant index of refraction 

 and the wave front would be 

 a circle, that of the other an 

 ellipse. Figure 346 is a dia- 

 gram of the indicatrix con- 

 structed withr its three axes 

 proportional to the three indices 

 of refraction, OX = a, OY = P, 



OZ = Y- Then a ray passing through the crystal in the direc- 

 tion of OY will be divided into two rays, one vibrating in OY 

 with an index of refraction OY ; the other variable, and its index 



