OPTICAL PROPERTIES OF CRYSTALS ISO 



ami finally a shadow will appear, or darkness, at which point the 

 difference between the slow and fast rays of the section is exactly 

 equal to that of the quartz wedge, and the wedge is said to com- 

 pensate the section; for this reason the wedge is often termed a 

 compensator. When compensation occurs, if the section is re- 

 moved from the stage, the quartz will show the original color, 

 due to the double refraction of the mineral section before the 

 wedge was inserted. At the point of compensation, if the wedge 

 is pushed farther through, the colors will rise in the scale unin- 

 terruptedly to the end of the wedge. 



When t he direction of the c axis in the section can be determined, 

 either from cleavage cracks or crystalline edges, and the vibration 

 plane of the slow ray is known, then the optical sign of the section is 

 also known ; for when the c axis is parallel to the long edge of the 

 quart 7. wedge and the slow ray parallel to the slow ray of the quartz 

 wedge, the optical sign is the same as that of quartz (+) ; when the 

 slow ray is at 90 to the slow ray, as marked on the wedge, the sign 

 is opposite to that of quartz ( ). 



Pleochroism is the unequal absorption of light waves of different 

 lengths. In the case of tourmaline, when the section was thick 

 enough it absorbed all the light vibrating parallel to the basal 

 section and therefore the section appeared dark for light polarized 

 and vibrating only in this direction. When one color or wave of 

 one length is absorbed more than another, the color of transmitted 

 light will change with the direction, or plane of vibration. Miner- 

 als in which pleoehroism is well marked will appear differently col- 

 ored according to the vibration plane of the transmitted light. 

 The absorption reaches a maximum when the vibration planes are 

 parallel to the planes of symmetry of the indicatrix. In uniaxial 

 crystals there can be only two directions, parallel to the c axis, and 

 parallel to the basal section. Crystals of this class can show only 

 two maximum absorption directions and are said to be dichroic. 

 In biaxial crystals there are three maximum directions possible, 

 and these are said to be trichroic or pleochroic. 



To test a section for absorption or pleoehroism, it is placed on 

 the stage and revolved to extinction, then the analyzer is removed 

 and the color of the section noted ; it is then revolved 90 and the 

 color again noted. Any difference in color is due to the unequal 

 absorption of the two rays vibrating in the section, as in the first 

 position one vibration plane of the section is parallel to the vibra- 

 tion plane of the polarizer and transmits the light, while in the 



