nlTICAl, PROPERTIES >! CRYSTALS 1S7 



the other one \\a\e length in [Kissing the section, or any multiple 

 of whole wave lengths, oon will be opposed to tin- vil>r:it ions of OCn 

 and there will be darkness during a complete revolution of the 

 section. The conditions are the same as in the quartz wedge, but 

 here the half \\ave of the nicol is added. 



A- the vibration phmrs of every mineral section are absolutely 

 fixed, they may be determined, and if necessary their traces marked, 

 on the section ; if the section is revolved until there is a minimum 

 amount of light or darkness, as viewed through the analyzer, be- 

 tween crossed nicols, then the traces of the vibration planes of the 

 section will be parallel to the vibration planes of the analyzer and 

 polari/.er or to the cross hairs in the eyepiece. One of these planes 

 is t lie principal optic section or contains the optic axis, which in uni- 

 axial crystals is parallel to the c crystal lographical axis. It follows 

 that in all sections through the crystal parallel to the prism zone 

 one of the vibration planes of the section will be parallel to pris- 

 matic or pinacoidal cleavage cracks in the section, or at right 

 angles to them. Darkness will occur on viewing the section in the 

 microscope when one of the cross-hairs is parallel to the cleavage 

 cracks ; the section under these conditions is said to possess 

 parallel or straight extinction. 



The extinction angle of any section is measured by the cross 

 hairs in the eyepiece of the microscope. They are set parallel to 

 the vibration planes of the nicols ; then when extinction occurs on 

 revolving a mineral section on the stage, the vibration planes of the 

 section are parallel to the cross hairs. A reading is taken from the 

 graduated circle on the stage, then the stage is turned until the cleav- 

 age crack is parallel to the hair, when another reading is taken ; the 

 difference between these two readings will be the extinction angle of 

 the section. All sections of uniaxial crystals, not parallel to one axis 

 of the ellipsoid, extinguish at angles other than 90. The extinc- 

 tion angle will vary with the inclination of the section, but extinc- 

 tion is always symmetrical, or divides the angle between cleavage 

 ks equally. 



In basal sections of uniaxial crystals there are no definite vibra- 

 tion planes, and the light passed by the polarizer will pass through 

 the section unchanged, to be extinguished by the analyzer, and the 

 field will remain dark during a complete revolution, as if there were 

 no section at all between the nicols. 



Determination of the slow ray. The slow ray may be deter- 

 mined by means of the quartz wedge. This is cut from a crystal 



