F. E. Wright — Petrographic Microscope Work. 133 



Art. XIII. — The Index Ellipsoid (Optical Indicatrix) in 

 Petrographic Microscope Work ; by Feed. Eugene Wright. 



During the past half century petrologists have given con- 

 siderable thought and attention to the development of petro- 

 graphic-microscopic methods with the result that the methods 

 at present available are in general satisfactory and easy of 

 application. The microscope in petrography is simply a use- 

 ful tool which enables the observer to ascertain certain details 

 about rocks and rock-making minerals that would otherwise 

 escape his attention. The working petrographer observes the 

 different optical phenomena under the microscope and uses 

 them, when properly interpreted, as important diagnostic fea- 

 tures. In order that he be able to interpret the phenomena 

 readily and with the least effort, it is essential that the under- 

 lying principles be presented in clear and tangible form, as 

 little encumbered with involved geometric conceptions as 

 possible. So far as the writer's experience goes, this can be 

 done best by considering the wave-front normals alone (with- 

 out reference to the rays) and by using the index ellipsoid 

 (optical indicatrix) to express their special relations in proper 

 form. This view-point, however, has not, apparently, been 

 taken by English and German writers of text-books on petrog- 

 raphy, who prefer the historical method of treating the subject 

 and favor, as a result, several old terms and conceptions such 

 as "axes of elasticity,'' a, b, c, or X, Y, Z, which are superflu- 

 ous and tend only to burden and confuse the mind of the 

 observer. These expressions are not employed to any extent 

 by French petrologists, whose usage in this respect might well 

 be universally adopted. In the following paragraphs this situa- 

 tion is presented in greater detail and the advantages of the 

 index ellipsoid emphasized. 



Light waves on entering a crystal pass within the influence 

 of its crystallographic forces, which are vectorial in character 

 and modify the light waves in a definite manner. To describe 

 in a systematic way the phenomena which are thus produced 

 and to explain them satisfactorily is the object of crystal 

 optics, which is, as its name implies, a border-land subject, 

 requiring for its elucidation two sciences, crystallography and 

 physics. The relations between the optical phenomena observed 

 on a crystal plate and its crystal system are so intimate and 

 fundamental that it is possible to determine, as Brewster was 

 the first to show, the crystal system of the crystal plate from 

 its optical properties alone even though the plate itself be 

 irregularly bounded and afford no outward indication of crys- 



