REFRACTIVE INDICES. 85 
increase toward the center of the grains; in short, a bright fringe appears 
in this case to wander toward the center of the grain on raising the tube, 
while for Fig. 5 1 , a, the reverse takes place on raising the objective and the 
fringe of light recedes from the grain in all directions as a halo. On lower- 
ing the objective, the reverse phenomena are observed in both instances.* 
If pencils of obliquely incident light be used, the conditions represented 
in Fig. 53, a, b, will obtain. f In Fig. 53, a, the mineral has a lower refractive 
index than the liquid and the rays are concentrated along the left margin 
of the grain. When viewed through the objective focussed on the grain, 
this margin appears, therefore, much brighter than the opposite edge. If 
the mineral has the higher refractive index the rays are deflected as shown 
in Fig. 53, b. Obliquely incident light beams enable the observer to deter- 
mine at once whether the mineral grain has a higher or a lower refractive 
index than the liquid or medium which envelops it. 
FIG. 53. 
The best methods for obtaining obliquely incident rays are (i) dark- 
ground illumination from a part of one quadrant only, (2) by use of the 
movable substage iris or stop diafram, (3) shading half the field by inserting 
the forefinger between the reflector and polarizer (Fig. 38), and thus cutting 
out half the incident cone of rays. This last method is extremely simple 
in its application and is satisfactory in practical determinative work. 
In case the mineral grain and liquid have the same refractive index for 
some color, as yellow or yellow-green, interesting effects are produced which 
enable the observer to determine the refractive index of the grain with con- 
siderable accuracy, provided that of the liquid be known. As a general rule 
liquids have a greater dispersion than minerals. This is shown graphically 
in Fig. 54, in which the refractive index curves^ for w and e of quartz are 
plotted and also those for the refractive liquids benzoylchloride, bitter almond 
oil, apiol, ethylenebromide. Under these conditions the liquid will have a 
higher refractive index for the waves at the blue end of the spectrum and a 
*O. Maschke, Pogg. Ann. Phys. Chem. US, 568, 1872; Wied. Ann. Phys. Chem.. II, 723. 1880; A. Bran. 
Arch. Soc. Phys. and Math.. Geneve, 33, 3. 1894. 
(Compare Schroeder von dcr Kolk, Zeitsch. Wissen. Mikros. 8, 456-458. 1892; F. Becke. S. W. A.. 
I. Abt. 102, 358. 1893. 
^Refractive indices taken from the tables of Landolt-Borastein. 
