i88 



ELEMENTARY CHEMICAL MICROSCOPY 



with oil, n > liquid, the emergent rays are converging. In Fig. 

 107 the solid line arrows indicate the direction of the moving 

 mirror, while the dotted line arrows that of the corresponding 

 direction of movement of the disk of light in the image. These 



FIG. 107. Oil Globule and Air Bubble illuminated with Oblique Light. (Gage.) 



diagrams indicate the behavior of the light rays, but in the image 

 in the microscope positions and directions are reversed; hence 

 the phenomena observed are those described above. 



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. Ob- 

 lique light 1 is obtained by swinging the mirror to one side when 

 no condenser is employed, or by sliding a piece of black paper 

 or card just below the condenser or by holding a finger just below 

 the condenser so as to cut off about one-half the lower aperture. 



In the chemical microscope slide a piece of stiff black paper 

 between the condenser and the ring attached to its lower part. 

 The preparation on the stage will then be illuminated by oblique 



1 See Wright, Oblique Illumination in Petrographic Microscopic Work; Amer. 

 J. Sci. (4) 36 (1913), 63. 



