F. E. Wright — Measurement of Extinction Angles. 349 



Aet. XL. — On the Measurement of Extinction Angles in 

 the Thin Section ; by Fred. Eugexe Weight. 



1. The Measurement of Extinction Angles of Minerals in the Thin 



Section. 



2. The Adjustment of the Petrographic Microscope with Special 



Eeference to the Measurement of Extinction Angles. 



3. A Device for Holding Minute Crystals. 



1. The Measurement of Extinction Angles of Minerals in the 



Thin Section. 



The petrographic microscope as an instrument has under- 

 gone many changes and modifications since its introduction 

 nearly forty years ago, but from the very first, each improve- 

 ment has tended to increase its efficiency in such a way that 

 the optical features of minerals in the thin section can be 

 ascertained more readily and more accurately. In the hands of 

 geologists, the microscope is merely the means to an end — an 

 apparatus to aid in recognizing the minerals composing a given 

 rock (mineral composition) and the relations of such rock 

 components to each other (rock texture) ; and for such purposes 

 the modern petrographic microscope is admirably adapted, 

 especially since, as a rule, approximate results only are required. 

 But with the increased knowledge of rocks thus attained, the 

 demand for data which are precise and quantitative in character 

 rather than qualitative has become more imperative, with the 

 result that, at the present time, one expects to find in a thorough 

 petrographic investigation accurately determined optical con- 

 stants of each of the rock-forming minerals examined, and in 

 critical points, the probable, error of each determination given. 

 This passage from qualitative to quantitative work implies 

 consequences of profound importance; an additional burden is 

 placed on the working geologist, and the time and energy re- 

 quired for the investigation of a given problem is much greater 

 under the present regime than formerly ; at the same time, 

 this transition indicates that, in one phase of geology, at least, 

 the step from the lower first plane of preliminary reconnaissance 

 work to the higher level of precise and detailed work is being 

 taken. 



The optical properties which are made use of in the practical 

 determination of minerals under the microscope are, briefly: 

 refractive index, birefringence, optical axial angle, optical 

 character, extinction angles, color and pleochroism. By means 

 of these properties alone it is possible to ascertain the crystal 

 system to which a given mineral belongs, and by a short pro- 

 cess of elimination to determine definitely the mineral in ques- 



Am. Jour. Sci.— Fourth Series. Vol. XXVI, Xo. 154.— October, 1908. 

 25 



