230 
ELEMENTARY CHEMICAL MICROSCOPY 
diagrams indicate the behavior of the light rays, but in the image 
in the microscope positions and directions are reversed; hence 
Fig. 136. Oil Globule and Air Bubble illuminated with Axial Light. (Gage.) 
as we move the mirror to one side the disk of light in an air bubble 
appears to move in a direction opposite to that of the mirror, 
while in an oil globule the bright disk appears to move in the 
same direction as the mirror. 
It thus appears that under oblique illumination the contour 
bands are heavier or darker on one side of the image of the object 
than on the other, the particular side which is darker depending 
upon the difference in the indices of object and mounting medium 
and the direction of the illuminating rays. Advantage is taken 
of these facts to determine by means of oblique light whether an 
object whose refractive index is sought has a higher or lower 
index than that of the test liquid in which it is immersed. Oblique 
