196 MINERALOGY 



i.e. makes a + sign. The crystal is then optically (+) and the 

 extraordinary ray is the slow ray; e > o>. When the two dark spots 

 are in the quadrants crossed by the arrow, i.e. are on the arrow, 

 and the line connecting them is parallel to the arrow, the crystal 

 section is optically ( ) and the extraordinary ray is the fast ray 

 and co > c. 



Circular polarization. When the interference figure of a sec- 

 tion of quartz of 3,5 mm. in thickness cut at 90 to the optic axis 

 is viewed in the microscope, it will be noted that the central por- 

 tion is not dark, as would be expected, but colored, and the dark 

 cross is not continuous through the central portion of the field, 

 Fig. 344. This is caused by a peculiar property of most crystals 



belonging to the holoaxial types, 

 of rotating the plane of polarized 

 light. When the light from the 

 polarizer enters such a section, 

 cut at 90 to the optic axis, it 

 is broken up into two rays cir- 

 cularly polarized in opposite di- 

 rections and one traveling faster 

 than the other. The two rays 

 on emerging from the section 

 unite to form plane polarized 

 light, but as one ray was faster 

 than the other the plane of the 



rence Figure of Quartz. resultmg pkne po l arize d ray is 



not the same as that of the ray on entering the section, but it has 

 been rotated through an angle, the size of which will depend upon 

 the thickness of the section, the specific rotating power of the 

 substance and the color of light used. The plane will be rotated 

 to the right (clockwise) in right-handed crystals and to the left 

 (anticlockwise) in left-handed crystals. 



If monochromatic light is used to make this observation, in the 

 ordinary section the central portion of the field is dark, as the light 

 passing is still vibrating in the plane of the polarizer and is ex- 

 tinguished by the analyzer ; but in case of quartz the central por- 

 tion of the field is illuminated, as the plane of polarization has been 

 rotated through an angle and the analyzer no longer extinguishes 

 it. In order to do so, the analyzer must be rotated through the 

 same angle, clockwise in right-handed crystals and anticlockwise 

 in left-handed crystals, through an angle, other things being 



