102 



REPORTS ON inve;stigations and projects. 



List of standard inciting points. 



In addition the following temperatures were incidentally obtained : 



(21) The methods of petrographic-microscopic research: Their relative accuracy and 

 . range of application. Fred. Eugene Wright. Publication No. 158, Carnegie 

 Institution of Washington. (In press.) 



During the past six years the work with artificial silicate preparations in 

 the Geophysical Laboratory has imposed new and difficult problems to be 

 solved by the microscope. Not only are such preparations very fine-grained, 

 but the degree of accuracy of each measurement must be definitely known if 

 it is to be applied without reserve to geophysical problems. To meet these 

 new conditions, it has been necessary to devise new methods, involving ex- 

 tensive alterations in the microscope, and also to test the different methods 

 available for the determination of the optical constants of minerals in the 

 thin section and to ascertain their relative accuracy and general applicability. 

 As a result of these tests, the methods best adapted for work with artificial 

 and all fine-grained preparations are now fairly well established and their 

 application has become in large measure a matter of routine. 



Minerals are determined under the microscope by means of their crystallo- 

 graphic 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. The optical properties thus made use of in the practical 

 determination of minerals under the microscope are, briefly: refractive in- 

 dices, birefringence, optic axial angle, optical character, extinction angle, 

 color, and pleochroism. By means of these properties alone, it is not only 

 possible to ascertain the crystal system to which a given mineral belongs, but 

 also by a short process of elimination to determine definitely the mineral in 

 question. This process has been carried to such refinement in certain in- 

 stances, as, for example, in the isomorphous series of the plagioclase feld- 

 spars, that by optical properties alone the actual chemical composition of the 

 particular plagioclase under observation can be obtained with considerable 

 accuracy. 



