REFRACTION AND BIREFRINGENCE OF MINERALS 427 



Mineral 



Wernerite . . . 

 Wohlerite . . . 

 Wollastonite 



Zircon 



Zoisite 



Maximum 



Bi- 

 refringence 



0.030 

 0.026 

 0.015 

 0.062 

 0.006 

 0.006 



published in Rosenbusch-Wiilfing, and all that are shown in the 

 diagram. The first column gives n for isometric, e for uniaxial, or 

 a for biaxial minerals. The second column gives eo for uniaxial 

 or /S for biaxial minerals, as the case may be. The third column 

 gives 7 and the fourth gives the maximum birefringence. When an 

 index is lacking the fact is indicated by a question mark. 



The diagram illustrates very forcibly the need for some simple 

 microscopic means of measuring the indices of minerals in thin sec- 

 tions. Various means are available at present for finding the index 

 of mineral faces one or two millimeters in diameter, but no success- 

 ful apparatus which will serve to use as an attachment to an ordinary 

 petrographic microscope has been devised. The writer has had to 

 give up any idea of working on such a help for students, as his inter- 

 ests are directed into other branches of the science, but it is hoped 

 that this will fall into the hands of someone who will be interested 

 enough to solve the problem. 



