F. E. Wright — Measurement of the Optic Axial Angle. 317 



Art. XXXIII. — The Measurement of the Optic Axial Angle 

 of Minerals in the Thin Section; by Feed Eugene Weight. 

 (With Plates I and II.) 



Dueing the past few decades the development of the petro- 

 graphic microscope and petrographic methods has been exceed- 

 ingly rapid and has now attained a state of such high efficiency 

 that nearly every optic constant of a rock mineral in the thin 

 section can be ascertained with ease and some degree of cer- 

 tainty. In many instances, in fact, the petrographer has at pres- 

 ent at his disposal two and even more methods for the determina- 

 tion of a single optical constant under the microscope. The 

 results obtained by these different methods are, however, often 

 not of the same value, nor are all methods equally applicable to 

 a given mineral section. Comparative and critical studies of the 

 relative merits and accuracy of such methods under different 

 conditions are not abundant in petrographic literature. It has 

 therefore seemed to the writer desirable that such a study 

 should be undertaken, and the microscopic examination of 

 the artificial mineral aggregates produced in the Geophysical 

 Laboratory has furnished a favorable opportunity. It has been 

 found necessary in this work to use most of the available pet- 

 rographic microscopic methods and to test them under different 

 conditions. 



Minerals are determined under the microscope by means of 

 their crystallographic and optic properties ; the more accurately 

 such properties or constants can be ascertained for any given 

 mineral, the more reliable and satisfactory is the determination. 

 Of the several optical constants which are thus made use of in 

 the identification of minerals, the optic axial angle, and with 

 it the optical character, whether positive or negative, are among 

 the most important. Different methods have been suggested, 

 from time to time, by means of which the optic axial angle 

 of any given biaxial mineral can be measured with greater or 

 less accuracy and more or less rapidly ; certain of these methods 

 are of general application and can be employed on all mineral 

 sections, while others can be used only on sections of fixed 

 orientation. In microscopic work, however, all possible sections 

 are to be encountered, and in each particular case, that method 

 is the best and should be selected which not only gives the 

 most accurate results, but also requires the least time. This is 

 particularly true of extremely fine-grained preparations where 

 it is frequently difficult to obtain results of any value whatever. 

 In the following pages, the methods for measuring optic axial 



Am. Jour. Sci. — Fourth Series, Vol. XXIV", No. 142.— October, 1907. 

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