Wright — Two Microscopic-Petrographical Methods. 385 



Art. XXXYII. — Two Microscopic-Petrographieal Methods; 

 by Fred Eugene Wright. 



1. " The Determination of the Relative Index of Refraction of 

 Minerals in Thin Section."* 



The relative index of refraction of two adjacent minerals in 

 thin section can be determined under the microscope in three 

 different ways. (1) By their relative relief, and also by the 

 degree of apparent roughness of their surface (shagreened 

 surface). (2) By the method of Becke's line,f whereby under 

 high power a narrow bright band of light can be seen to 

 travel back and forth, toward and away from one of the 

 two minerals at their intersection as the microscope tube is 

 raised and lowered : the rule then holding good that Becke's 

 line moves toward the mineral with higher index of refrac- 

 tion on raising the microscope tube. This method is sensi- 

 tive and extremely simple. The only objections to it are 

 that in certain cases two such lines can be seen moving in 

 opposite directions, whereupon it is difficult to decide as 

 to the correct one, and that a certain amount of time is 

 expended in changing from low power to high power. (3) By 

 the method described below, which does not exhibit the two 

 lines occasionally seen in the Becke's method, is equally sensi- 

 tive and does not require the change from low to high power. 



The method is based on principles developed by J. L. C. 

 Schroeder van der Kolk in his recent "Tabellen zur mikro- 

 skopischen Bestimmung der Mineralien nach. ihreiri Brechungs- 

 index,"^; and is but an application of the same to minerals in 

 thin section. 



Mr. Schroeder van der Kolk determines the index of refraction 

 of a mineral by submerging small fragments of the same in a 

 liquid whose index of refraction can be lowered or raised by 

 addition of other liquids until it coincides with that of the 

 mineral, and then measuring the index of refraction of the 

 mixture on a total reflectometer. The method is exact to the 

 second decimal place. Mr. Schroeder van der Kolk gives in his 

 Tabellen a list of over 300 minerals arranged according to 

 their refractive index, and also a list of sixteen or more liquids 

 of variable refractive index suitable to be used in connection 

 with his method. 



In actual practice, small fragments more or less lenticular in 

 shape are taken and placed in one of the fluids. If its refrac- 



* Compare Tschermak's Miner. Petrogr. Mittheil. , vol. xxi, page 238, 1901. 

 f Sitzungsberichte cL Kaiser. Akademie d. Wissensehaften., vol. cii, page 

 358-376, 1893. 



% Wiesbaden, C. W. Kreidels Verlag, 1900. 



