14 MINERALOGY AND LITIIOLOGY. 



Nicol prisms, and consequently no colors will be seen. However, if such a section is 

 examined between crossed Nicols in convergent light, as in the case of the uniaxial 

 crystal, the optic axis will be surrounded by a series of colored rings ; but in this case 

 the ring system will be intersected by one black bar, and not by a cross, and when the 

 crystal is revolved, this bar, instead of remaining stationary, as does the cross of uni- 

 axial crystals, revolves, but in the opposite direction to that in which the section is 

 revolved. 



The properties of orthorhombic crystals, when microscopically examined, are, then, 

 these : Any section is dark between crossed Nicols, when the direction of a crystal- 

 lographic axis corresponds with the plane of vibration of the light, as indicated by the 

 hair lines in the ocular. In every other position the sections are colored, with the 

 exception of sections cut perpendicular to an optic axis, which act as if isotropic. Such 

 a section can, however, be distinguished from an amorphous or isometric body, because 

 it will be colored when the ocular of the microscope is removed in order to render the 

 light convergent, and it will be distinguished from an uniaxial section, because the field 

 will be traversed by a black bar that revolves as the stage revolves, and not by a sta- 

 tionary black cross. It may be remarked, that, when examining thin sections, the bar 

 may be seen when the section is so thin that the rings have disappeared, for the number 

 of rings that surround an optic axis depends on the thickness of the plate ; and the plate 

 may be made so thin that one ring may spread over the whole field, and then the field 

 will be more or less light, but will still be crossed by a distinguishable bar. 



Hlonoclinic Crystals. Monoclinic crystals have three axes of elasticity at right angles 

 to one another, but they differ from orthorhombic crystals in that, with one excep- 

 tion, these axes do not correspond with the crystallographic axes. One of them 

 always corresponds with the orthodiagonal ; hence, sections in some directions through 

 a monoclinic crystal follow the laws laid down for orthorhombic crystals, and others 

 not. Sections parallel to the base, or the orthopinnacoid, or any other sections in 

 this zone, will be dark between crossed Nicols when the crystallographic axes, indi- 

 cated by crystalline edges or cleavage, correspond to the plane of vibration of the light 

 that illumines them ; but all other sections will not be dark under these conditions, and 

 must be revolved a certain number of degrees before they become dark, and this angle 

 through which they must be revolved corresponds to the angle that the axes of elas- 

 ticity make with the crystallographic axes in the given section. The measurement of 

 this angle is often serviceable. For example : suppose we have sections of augite and 

 hornblende cut parallel to the clinopinnacoid ; when placed between crossed Nicols, 

 with the vertical axis indicated by the cleavage placed parallel to the plane of the 

 lower or upper Nicol, the sections will be colored, proving them to belong to an inclined 

 system. In order to make them black, the augite must be revolved 39° and the horn- 

 blende 15°. 



As the alternations of light and darkness furnish but an inexact method of measure- 

 ment, its delicacy can be much increased by laying a calcite plate, cut perpendicular to 

 the vertical axis, between the ocular and the analyzer ; thus the interference figure of 



