SECT. n. TO CRYSTALLINE FOEM. 71 



other that plane revolves from left to right, although 

 the crystals themselves differ apparently by a very 

 slight and often almost imperceptible variety of forms. 



Thus polarization forms the most admirable connec- 

 tion between light, heat, and crystalline structure; 

 showing peculiar arrangements of the molecules in 

 regions otherwise unapproachable, and too refined for 

 our perceptions. Besides, the gorgeously coloured 

 images displayed by depolarization are splendid ex- 

 amples of the power of matter in decomposing light. 



The perfect correspondence of the properties of the 

 symmetrical, elastic, and optical axes of crystals with 

 light and heat is another instance of the connection 

 between the latter and crystalline form. 



The axis of symmetry is that direction or imaginary 

 line within a crystal, round which all the parts or par- 

 ticles are symmetrically arranged. A medium is said 

 to be elastic which returns to its original form with a 

 resilient force after being relieved from compression, 

 and the axis of elasticity of a crystal is that direction 

 in which it is most elastic. The optic axis is that linn 

 or direction through which light passes in one beam 

 according to the law of ordinary refraction. Crystals 

 may have one, two or more optical axes according to 

 their form. Doubly refracting crystals such as Iceland 

 spar have only one principal optic axis in which the 

 whole beam passes according to the ordinary law ; in 

 every other direction the beam of light is divided into 

 two polarized rays, one of which called the ordinary ray 

 passes according to the ordinary law, while the other, 

 known as the extraordinary ray, traverses the crystal in 

 a different direction, with more rapidity and according 

 to a different and more complicated law. The velocity 

 of this extraordinary ray is a maximum when at right 

 angles to the principal optical axis, and a minimum 

 when parallel to it. 



