170 
The arm earrying the maguifying glass (L) should be down to the 
1.3 end of the scale. Then by lcoking through the eyepiece (I*), focusing 
the cross-hairs into plain view, the arm (IL) is moved until a bright color 
appears in the lower half of the field. By turning the milled screw (M) 
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 
