Petrocjraphic Microscope WorJc. 



73 



135 the reverse is the case, the zone of maximum illumination 

 receding from the center of the grain on raising the draw tube.* 

 In figs^ 12<?, 13<? the effect of oblique illumination on the 

 higher refracting mineral grain is indicated. Under such con- 

 ditions opposite margins of the mineral grains appear unequally 

 illuminated, the rays from the margin on the right emerging at 

 such a large angle that they fail to enter the objective (fig. 17, 

 grain A), while from the margin on the left the rays do enter 

 the microscope. The left margin appears, therefore, brightly 

 illuminated while the right margin is relatively dark. The 

 greater the difference in refractive index between liquid and 

 mineral grain, the more pronounced is this contrast in illumina- 



Fig. 17. 



OBJECT-PLANE 



tion and the more the grain stands out in relief. In figs. 12d 

 and 13c7 the mineral grain has the lower refractive index and 

 dark and light margins appear reversed, the right margin being 

 now bright and the left margin dark. These relations are 

 indicated in fig. 15 and in fig. 17 (grains A and B), and are 

 sufficiently obvious without further comment ; they enable the 

 observer to determine at once whether the mineral grain has a 

 higher or lower refractive index than the enclosing liquid. 



In case the mineral grain and liquid have the same refractive 

 index for some wave length near the center of the visible spec- 

 trum (about 550/Lt/x), the greater color dispersion of most liquids 



*0. Maschke, Pogg. Arm. Phys. Chem., cxlv, 568, 1872 ; Wied. Ann., xi, 

 722, 1880; F. Becke, Sitzungsber. Akad. Wien, I Abt., cii, 358, 1892; 

 Min. petr. Mitt., xiii, 385, 1893 ; A. Brim, Arch. Soc. Phvs. et Math., Geneve, 

 xxxii, 3, 1894. 



