94 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
can be obtained by tilting the reflector itself. The results of shading part 
of the field by any of these methods are illustrated in Fig. 60, a, b, in which 
the refractive index of the mineral grain n\ is considered less than that of 
the liquid n, which, in turn, is lower than that of the second grain, w 2 - 
Fig. 60, a, is drawn on the assumption that the substage condenser has been 
lowered to some extent and the observations are made with a low-power 
objective. With the field thus half in shadow, the mineral section is observed 
b 
brighter 
brighter 
darker 
brighter darker 
condenser lens lowered 
darker 
brighter 
condenser lens raised 
FIG. 60. 
at the indistinct edge of the shadow which is purposely not in sharp focus. 
If the edge of the mineral section adjacent to or partially within the half 
shadow is brighter than the surrounding field, while the opposite edge is 
darker, the mineral has a higher refractive index than the liquid ; and vice 
versa, if the bright edge be on the opposite side of the grain away from the 
shadow the mineral has a lower refractive index. In case both liquid and 
mineral have the same refractive index 
for yellow light the edge of the grain 
next the shadow appears orange-red, 
while its opposite edge is pale blue 
(Fig. 61). 
It should be noted that, if the con- 
denser be raised so that the cone of 
rays impinging on the grain is con- 
vergent instead of divergent (beyond 
the point where the edge of the finger 
or stop appears in sharp focus), the 
above phenomena are reversed (Fig. 
60,6). This condition is convenient, 
since it allows the observer to reverse the phenomena quickly and thus to 
check his determination in uncertain cases (Figs. 60, a, b). Unless care be 
taken, however, to set the condenser correctly, false inferences as to the 
relative refractive indices can easily be drawn. After some practice the 
above rule is applied automatically, but, until then, it is well to test it on 
substances of known refractive indices. 
If high-power objectives be used the phenomena are less distinct unless 
it happens that the numerical aperture of the condenser exceeds that of the 
orange red 
pale blue 
condenser lens lowered 
FIG. 61. 
