1 6 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. 2 b, 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 b, 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 at a right angle, but we shall see it off on 

 the side, as represented in c. 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. 



TricUnic 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 cr3stallographic 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 Pleochroisjii. 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 to a given axis. In the case of this hornblende, we find that the light 



