INTRODUCTION. 17 
vibrating parallel to the prism, emerges blue; that parallel to the orthodiagonal, green ; 
and that parallel to the clinodiagonal, yellow; hence, with the polarizer on the micro- 
scope in a section that contains several crystals, a crystal may have either one of 
these three colors, according to which axis corresponds with the plane of vibration of 
the light. For example, a basal section may be either yellow or green, and a prismatic 
section may be either blue, green, or yellow; and each one may be made to assume 
another color by revolving the section on the stage. If, now, we remove the analyzer, 
and observe with ordinary light, the basal sections will be greenish yellow, the result- 
ant of the two sets of vibrations parallel to the lateral axes, and the prismatic sections 
bluish green in the plane of the orthodiagonal, or green in the plane of the clinodiag- 
onal, the latter color being made by a union of the blue and yellow vibrations. This 
may be seen illustrated on Pl. 7, Fig. 2. 
The term pleochroism is reserved for the effect produced where certain colored rays 
are absorbed, as a beam of white light passes through a crystal, producing different 
colored emergent rays. An isometric crystal can possess no pleochroism. An uniaxial 
crystal may transmit two differently colored sets of vibrations, and can hence be di- 
chroic; a biaxial crystal may transmit three different colors, and hence may be trichroic. 
The term absorption, however, is reserved for that effect where much more light is. 
absorbed in one plane than in the other, producing, not a change in color, buta marked 
difference in the intensity of the light. This effect can also be best observed in the 
microscope when the polarizer and not the analyzer is affixed. Itis plain that a min- 
eral may exhibit both pleochroism and absorption at the same time, and that, with 
exactness, pleochroism is but a phase of absorption. 
All these principles are very concisely stated by Mr. Rosenbusch in the following 
form: 
I. The substance shows like optical properties throughout, or, if there are differences, 
the different parts are separated from one another by straight lines (twins).—A HoMmo- 
GENEOUS SUBSTANCE. 
1. All sections of the same substance, in all positions between crossed Nicols, 
appear dark. By revolving them on the stage of the microscope the light is not 
modified, and the interference figure of a calcite plate is not distorted.—Iso- 
TROPE. 
1*, The substance shows no traces of crystalline structure, neither in outline 
nor cleavage.—Amorphous. 
1>, The substance does show evidences of crystallization.—/sometric. 
2. All the sections, in all positions in a horizontal plane between crossed Nicols, 
are not dark, and may modify the calcite interference figure.—ANISOTROPE. 
28, The more or less regular quadratic sections behave like isotropic sections. — 
Tetragonal. Uniaxal. 
2». The hexagonal sections behave like isotropic sections.—Hexagonal. Uni- 
axial. 
VOL. IV. 3 
