86 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
lower index for the red rays than the mineral. On raising the objective, 
the red rays wander toward the center of the grain, as in Fig. 51,6, while the 
blue rays recede from the center toward and beyond the margin, as in Fig. 
51,0.. with the result that the mineral grain is tinged pale orange red and 
appears set in a blue background. On lowering the objective the reverse 
phenomena take place and the grain is colored pale blue and is surrounded 
by a pale reddish or orange-colored halo. By properly adjusting the refrac- 
tive index of the liquid, it is possible to obtain clear color reactions of this 
kind and thus to determine definitely when the mineral and liquid have the 
same refractive index for a color as yellowish-green midway between red 
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FIG. 54. 
700 
and blue or about the center of the visible spectrum. The greater the 
relative dispersion of the liquid the more pronounced the colors observed, 
but also the less sensitive the determination. If the dispersion were the 
same in both liquid and mineral, no such color phenomena would result, 
because the rays would then pass through the mineral undisturbed and no 
fringes, colored or colorless, would be observed along its edges on raising or 
lowering the objective. For satisfactory work it is essential, therefore, as 
Maschke was the first to emphasize, to choose liquids whose dispersion is 
sufficiently strong to produce distinct color phenomena in mineral grains, 
but still low enough so that these colors do not appear on mineral plates 
whose refractive indices are noticeably different. 
