Petrographic Microscope Wo?*k. 



The Uses of Oblique Illumination in Petrographic 

 Microscope Work. 



71 



Determination of the relative refractive indices of adjacent min- 

 eral grains or of mineral grains immersed in liquids. 



Let us first consider the principles on which the immersion 

 method for determining the refractive indices of mineral grains 

 is based. This method has been frequently described, but it 

 is so useful that it cannot be too strongly emphasized. It is 



Fig. 12a. 



Fig. 126. 



Fig. 12c. 



Fig. 12d. 



founded primarily on two physical facts, the refraction of light 

 and the relative color dispersion between liquids and solids. 



The relations which result from these facts are illustrated in 

 figs. 12 to 17, in which the mineral grain is considered immersed 



Fig. 13a. 



Fig. 13&. 



Fig. 13c. 



Fig. 13c?. 



in liquids of different refractory indices. Figs. 12$, 12c, 13#, 13c 

 are drawn for the case that the refractive index of the mineral is 

 1*70 and that of the liquid 1*50. In figs. 11a, b, the refractive in- 

 dices of the mineral are : 1*512 for the wave length 650yu/x ; 1*516 

 for X= ddO/jl/x ; 1*552 tor A = 150/^/z, while for the liquid they 

 are 1*535 for A = 650/x/z ; 1*516 for X = 550/z/n ; 1*570 for X = 

 4:50fjL/jL ; the refractive indices of mineral and liquid are equal 

 for the wave length X = 550/*/ii, but the liquid has a lower 



