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 PI. 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, but a 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. It is 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 homo- 

 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. 



i^. The substance shows no traces of crystalline structure, neither in outline 



nor cleavage. — AniorpJwiis . 



i^. The substance does show evidences of crystallization. — Isometric. 



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. 



2=". The more or less regular quadratic sections behave like isotropic sections. — 

 Tetragonal. Uniaxal. 



2''. The hexagonal sections behave like isotropic sections.— I/exag07iaL Uni- 

 axial. 



