PETROGRAPHY AND METALLOGRAPHY. 159 
B’a and B’b. A’a and B’aare parallel to the plane of the 
original illuminating ray A, and are suppressed. A’b 
and B’b 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 particular 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’b and B’b, 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 
