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, 



FIG. 137. Oil Globule and Air Bubble illuminated with Oblique Light. (Gage.) 



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 



