PETROGRAPHY AND METALLOGRAPHY. 1^9 



B'a and B'b. A'a and B'a are parallel to the plane of the 

 original illuminating ray A, and are suppressed. A'h 

 and B'h lie in the plane at right angles to this and pass 

 freely. As they unite, however, their waves, are, in differ- 

 ent phases, due to the retardation of B' in passing through 

 the mica plate, and if the amount of this retardation be 

 right, interference phenomena will be set up and very 

 beautiful color effects produced. The partieular color 

 varies with the thickness of the plate examined; but at 

 the two opposite positions of the analyzer at which most 

 light passes, the colors which appear are complementary. 

 Using the notation adopted above for illustration, in one 

 case A'a and B'a, in the other, A'h and B'h, pass; and, 

 since the sum of all these four rays originally produced 

 white light, it is obvious that the complementary relation 

 must exist. 



3. The Identification of Minerals. — Besides the general 

 microscopical appearance of crystals noted above, it is 

 possible with the polariscope to study one character of 

 perhaps more practical importance than any other, the 

 optical structure as indicated by its effect on polarized 

 light. We have seen that crystals of the first or isometric 

 system are isotropic. All other crystals are anisotropic, 

 but in those belonging to the tetragonal and hexagonal 

 systems there is one axis about which the structure of the 

 crystals is homogeneous, and along this axis light passes 

 unaffected. Such crystals are called uniaxial, while 

 crystals of the orthorhombic, monoclinic, and triclinic sys- 

 tems possess two such axes, and are called biaxial. 



Examination with crossed Nicols makes it possible at 



