190 



MINERALOGY 



second position the second vibration plane is parallel to the plane 

 of the polarizer and transmits the light. 



If it is darker when the extraordinary ray is passing than when 

 the ordinary is passing, and the difference is not marked, then it is 

 noted, absorption or pleochroism is weak, < o>. If there is a change 

 of color, this is also noted thus : = green, o> = bluish ; such is the 

 case for beryl. 



The dichroscope is an instrument by means of which the color 

 of the two rays vibrating in planes at right angles to each other 

 may be directly compared, Fig. 338. It is constructed of a cleav- 

 age piece of calcite, c, long enough to separate the two images of the 

 square orifice o when viewed from the opposite end, where the lens 

 1 magnifies them. One image is due to the ordinary ray and vi- 

 brates parallel to the long diagonal of the rhombic section of the 



FIG. 338. Diagram showing the Construction of the Dichroscope. 



calcite; the other is due to the extraordinary ray and vibrates 

 parallel to the short diagonal. The ends of the calcite are squared 

 up with the two wedge-shaped glasses G, G'. 



Possibly the most remarkable example of pleochroism is shown 

 by thick sections of the mineral iolite, also known as cordierite 

 and dichroite, an orthorhombic mineral, in which therefore three 

 limiting absorption directions are possible, one parallel to each 

 axis. If a smooth surface of iolite parallel to the brachypinacoid is 

 held over the orifice, o, of the dichroscope, and the transmitted light 

 viewed through the lens, one ray will vibrate parallel to the c axis 

 and the other parallel to the brachyaxis. The first will be yellow, 

 the second gray a very marked contrast. If the dichroscope be 

 revolved and the section held stationary, then after a revolution of 

 90, the two images will reverse in color. If a section parallel to 

 the base be used, one image caused by the vibrations parallel to the 

 brachyaxis is gray, the same as it was in the first section, but the 

 image caused by the vibrations parallel to the macroaxis will be 

 blue. In the description of a mineral these conditions are expressed 

 for iolite as X = yellow, Y = gray, Z = blue, absorption Y < Z < X. 



