16 MINERALOGY AND LITHOLOGY. 
vertical axis at right angles to one another ; and, as the orthodiagonal is an axis of elas- 
ticity, in this position the crystallographic axes fall together with the axes of elasticity, 
and, on bringing these directions to correspond with the hair lines in the ocular, the 
section will be dark between crossed Nicols, as would a section of an orthorhombic 
mineral cut parallel to any pinnacoid. This relation is shown by Fig. 24, which rep- 
resents a section of augite cut parallel to the orthopinnacoid. On examining Fig. a, 
we see that one of the optic axes cuts the face of the orthopinnacoid at nearly a right 
angle. If, then, while examining section 4, we remove the ocular and replace the 
analyzer, thus producing convergent light, we shall see this optic axis, not in the centre 
of the field, for it does not pierce the face ata right angle, but we shall see it off on 
the side, as represented inc. The rings will be traversed by a black bar, which will 
revolve in the opposite direction to that in which we revolve the section. These three 
figures will make plain all that has been said in regard to the microscopic examination 
of crystals with polarized light. 
Sections of monoclinic crystals differ, therefore, from those of orthorhombic crystals, 
in that some sections (those that contain the orthodiagonal) will be dark between 
crossed Nicols when a crystallographic axis is parallel to the plane of vibration of the 
light, while the others will not. 
Triclinic Crystals. The light also passes through triclinic crystals parallel to three 
axes of elasticity, which are at right angles to one another; but in no case does 
one of these axes correspond with a crystallographic axis. Therefore no section of 
a triclinic mineral is dark between crossed Nicols when a crystallographic axis falls 
together with the plane of vibration of the light, but all become dark when revolved 
from this position a certain number of degrees, dependent on the mineral and the rela- 
tion of the section to the axes of the crystal. This uniform behavior of triclinic crys- 
tals serves well for their identification. 
Absorption and Pleochroism. Dependent upon the difference in the elasticity of the 
ether in different directions, through uniaxial and biaxial crystals, are the phenomena 
of absorption and pleochroism. For example: the rays of light passing through a 
beryl vibrating parallel to the vertical axis may have some vibrations absorbed, 
which will give to the emergent light a certain color, as, for example, blue, while 
that vibrating at right angles to the prism may have no rays absorbed, and thus 
emerge white. Again: the light, as it emerges from a section of a biaxial crystal, may 
be of three different colors or degrees of intensity, according as it vibrates parallel to 
one or the other of the three axes of elasticity. Therefore it is plain that sections of 
the same mineral may be quite different in color, depending upon their relation to the 
axes. For example: the hornblende in the schists of the Connecticut valley transmits 
light of three different colors. The color of any given section will therefore depend 
upon the resultant of the two sets of vibrations that pass through the section. If, 
now, we insert the polarizer without the analyzer, we can fix the plane of vibration of 
the light in any desired relationship to the crystal, and see what colored light is trans- 
mitted parallel toa given axis. In the case of this hornblende, we find that the light 
