138 F, F. Wright — Petrographic Microscope WorJc. 



110 it is c:y' = 33°. In view of the fact that, except in 

 French literature, the Greek letters a, /3, 7 are used to express 

 the refractive indices, and these letters are easier to write and 

 to pronounce than the French symbols n p , n m , n gi or n a , n*, n , it 

 would seem preferable to adopt the Greek letters throughout. 

 No confusion is introduced by using these letters to designate 

 (1) the principal axes of the ellipsoid and (2) to express their 

 actual lengths ; this is evident from the French petrographic 

 literature and the occasional usage of a, j3, 7 in this sense in the 

 literature of other nations. As noted above, the primary pur- 

 pose of the present paper is to direct attention to this situation 

 and to urge the consistent adoption of this method of expres- 

 sion. From its very nature it is not a matter which can be 

 adopted at once, but it behooves those interested in the devel- 

 opment of petrology as a science to make the petrographic 

 microscopic part of the subject as simple and easy of appli- 

 cation as possible. 



Summary. 



In the foregoing paragraphs the importance of presenting the 

 subject of microscopical petrography consistently from the view- 

 point of the index ellipsoid as applied to wave-front normals is 

 emphasized. The various optical properties employed in prac- 

 tical petrographic microscopic work can be best described and 

 explained systematically by means of the index ellipsoid. The 

 use of the so-called '■' axes of elasticity" a, b, c or X, Y, Z in 

 this connection is confusing and only adds to the difficulties 

 encountered by the observer in mastering the subject. They 

 should accordingly be abandoned and the French usage of nam- 

 ing the principal axes of the index ellipsoid a. /3, 7 (or n p , n m , n g ) 

 adopted. This applies in particular to the different modes now 

 in vogue for expressing extinction angles. For a given crystal 

 face an extinction angle is simply the angle between a definite 

 crystal lographic direction on the face and one of the axes, a' or 

 7', of its optic ellipse, and this fact should be indicated in the 

 expression for the extinction angle. To introduce " axes of 

 elasticity" (a', c' or X', Z') in this connection is not only needless 

 but less direct, as it introduces entirely new conceptions which 

 experience has shown only tend to bewilder the student. Clear, 

 concise modes of expression and simple methods of attack are 

 as essential in petrology as in other sciences whose development 

 is often directly dependent on the care and attention given by 

 its workers to these features. 



Geophysical Laboratory, 

 Carnegie Institute of Washington, 

 October, 1912. 



