WRIGHT: DETERMINATION OF REFRINGENCE 



391 



LOW POWER objective: 



LOWER STOP IMAGE 



by only =1= 0.001, the intensity differences in illumination are diffi- 

 cult to see, because of the relatively large amount of light in the 

 field. To reduce the field illumination, and thus to increase the 

 differences in relative intensity of illumination and to render them 

 more clearly visible, a double stop device has been found useful. 



This device consists 

 essentially of two safe- 

 ty razor blades mount- 

 ed in a horizontal po- 

 sition to a vertical 

 connecting bar which 

 in turn is attached to 

 the side of the stage 

 support of the micro- 

 scope. These blades 

 are so adjusted that as 

 the lower blade swings 

 into position below the 

 condenser, the upper 

 blade is brought to rest 

 in the conjugate image 

 plane above the con- 

 denser and between 

 the objective and slide. 

 The upper stop is so 

 adjusted that its knife 

 edge faces the knife 

 edge of the image of 

 the lower blade. Incase 

 these two edges just 

 meet, the entire field 

 of view appears very 

 weakly illuminated and is almost dark. The path of the rays is in- 

 dicated in figure 1 . The effect of the refracting mineral grain is to 

 disturb the path of .the transmitted rays so that instead of focus- 

 sing sharply in the image plane they are deflected (as indicated by 

 the two rays with arrows in figure 1 and are thus able to enter 



Fig. 1 



