8 4 
METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
on the mineral grain are then deflected by it after the manner of a divergent 
lens, as shown in the figure; the aperture of the transmitted cone is increased. 
If now the objective be raised and focussed on successive parts of the grain, 
the distribution of the intensity of illumination varies noticeably in the dif- 
ferent planes a, b, c, as illustrated in the figure, the effect of the mineral 
FIG. 51. 
grain under these conditions being that of a divergent lens on the incident 
rays. If, on the other hand, the mineral grain has a higher refractive index 
than the liquid (Fig. 51, b), it will tend to converge the rays and to bring 
them to focus. From Fig. 52, a, b, it is also apparent that total reflection 
may occur and tend likewise to modify the aperture of the incident cone. 
FIG. 52. 
On viewing the grain of Fig. 51, b, under the microscope and raising the 
objective from the position of exact focus on the plane indicated by the 
dotted line b-b to that on the plane a-a the intensity of light will appear to 
