170 



The arm carrying the magnifying glass (L) should be down to the 

 1.3 end of the scale. Then by looking through the eyepiece (F), focusing 

 the cross-hairs into plain view, the arm (L) is moved until a bright color 

 appears in the lower half of the field. By turning the milled screw (M) 



Fig. l. 



of the compensator the line of color is made more distinct; viz.. there is 

 sharpness of line dividing the light and the dark field. By moving the 

 arm farther this line is brought up to a point where it coincides with the 

 intersection of the cross-hairs. At this point, the index of refraction is 



