is I 



SUMMARY OF CURKKNT RESEARCHES RELATING TO 



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 safety-razor blades mounted 

 in a horizontal position to a vertical connecting-bar, which in turn is 

 attached to the side of the stage-support of the Microscope. These 

 blades are so adjusted that as the lower blade swings into position below 



low power objective: 



LOWER STOP IMAGE 



MINERAL GRAIN 



Fig. 19. 



the condenser, the upper blade is brought to rest in the conjugate image- 

 plane above the condenser 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. In case these two edges just meet, the 

 entire field of view appears very weakly illuminated and is almost dark. 

 The path of the rays is indicated in fig. 19. The effect of the refract- 

 ing mineral grain is to disturb the path of the transmitted rays, so that 



