200 



MINERALOGY 



a red of the first order. In measuring the angle of extinction the 

 crystal section is revolved until this even tone of red is not affected, 

 when there will be no double refraction due to the section and the 

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

 nicols, and the section will be in the position of extinction. If 

 there is the slightest double refraction, the red of the first order will 

 change to blue or yellow, or the tone of red will be changed as the 

 case may be, and the vibration planes of the section will not be 

 exactly parallel to those of the nicols. 



Interference figures in biaxial crystals. Let Fig. 348 represent a 

 section SS' cut perpendicular to the acute bisectrix, in which OCi 



FIG. 348. 



and O'Ci are the two optic axes. Here also, as in uniaxial crystals, 

 light transmitted in these directions is not doubly refracted, and 

 leaves the section vibrating in the same plane as it did on entering 

 that of PP', the vibration plane of the polarizer, and therefore when 

 viewed with the analyzer in position, the two points O and O' 

 marking the position of the optic axes in the section will be dark. 

 If the section is cut exactly perpendicular to the acute bisectrix the 



