in Petrographic Microscope Work. 137 



indices on the total refractometer or by use of refractive 

 liquids. The three principal refractive indices establish defi- 

 nitely the shape and size of the optic ellipsoid and with it the 

 principal birefringences, the optic axial angle, and the optical 

 character of the mineral ; the optical properties can also be, and 

 usually are, determined by other methods, but they have been 

 purposely listed together to show the fundamental character of 

 the principal refractive indices (the lengths of the principal 

 axes, a, ft, 7, of the index ellipsoid). Other methods are also 

 available for ascertaining the optic ellipsoidity of the mineral, 

 but their description would lead too far in the present paper. 



Turning to any given section of a birefracting mineral, 

 we find that the optical phenomena which it presents can 

 be adequately described and accounted for when the shape and 

 position of its optic ellipse are known. By determining this 

 feature, the "optic ellipsity" * of the section, we ascertain the 

 lengths of the major and minor axes 7' and a', of its optic 

 ellipse (the two refractive indices of the plate) ; their difference 

 7 / -a / (measure of the birefringence) ; and the positions of extinc- 

 tion of the plate between crossed nicols. These positions are 

 commonly referred to some definite crystallographic direction 

 on the plate, the angle between that direction and one of the 

 axes of the optic ellipse, a' or 7', being called the extinction 

 angle. In German literature these directions are expressed by 

 the symbols a, b, c, the so-called u axes of elasticity," whose 

 lengths are the reciprocals of a, /3, 7, the Greek letters being 

 reserved for the refractive indices alone. In this country the 

 symbols X, Y, Z are largely used as a substitute (suggested 

 by Iddings) for the German letters, which are less easy to 

 write. The French, however, use directly the lengths of the 

 optic ellipse a, ft, 7 (n p , n m , ?i g ), in their expressiou for extinc- 

 tion angles, and this seems to the writer by far the preferable 

 practice. It can only add confusion to introduce the concep- 

 tions of " axes of elasticity." The mechanical theories of light 

 based on an elastic ether have often been called in question 

 and have been largely superseded, since the time of Maxwell, 

 by the electro- magnetic theory. It would seem therefore advis- 

 able to drop the terms u axes of elasticity " altogether from 

 petrographic literature, especially as they do not aid but rather 

 detract from the real picture obtained from the use of the 

 index ellipsoid alone, which is valid regardless of the partic- 

 ular theory of light the observer favors. The French method 

 should be consistently adopted and extinction angles expressed 

 in terms of the axes of optic ellipse of the section ; thus for 

 diopside the angle of extinction on 010 is c : 7 = 39°, while on 



* F. E. Wright, Jour. Geology, xx, 488, 1912. 



Am. Jour. Sci.— Fourth Series, Vol. XXXV, No. 206.— February, 1913. 

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