312 



WRIGHT: A NEW HALF SHADE APPARATUS 



complex than one might infer from the equation alone, because of 

 internal reflections and surface film effects but the values listed 

 in Table 1 indicate the order of magnitude of the angular rotation 

 of the vibration plane of a plane-polarized light wave for different 

 angles of inclination of the half shade apparatus. Since the 



effects of such fac- 

 tors, as internal 

 reflection and sur- 

 face films, affect 

 both glass plates 

 equally, they do not 

 appreciably disturb 

 the sensitiveness of 

 the apparatus. 

 The values are 

 listed for angles of 

 rotation of the ap- 

 paratus up to 85°; 

 in actual practice, 

 however, rotation 

 angles above 60° 

 are not used and 

 hence are not repre- 

 sented in figure 3. 

 Instead of rotat- 

 ing the apparatus 

 through large an- 

 gles, it is simpler 

 and better to 

 mount two superimposed plane-parallel glass plates to each side 

 of the right angled prism. The angles of rotation with this 

 arrangement are listed in Table 1. 



With this apparatus, which is applicable both to white and to 

 monochromatic light, the half shade principle with variable sensi- 

 bility is attained. In petrographic microscope work extinction 

 angles can be measured by its use, when it is placed in the lower 



15^ 



z 

 o 



< 



q: 



mo' 



> 



O 



u 

 z 

 < 



_J 



Q. 



O 



z 

 o 



I- 

 < 



I- 

 o 



o° io° 



ANGULAR 



20" 30" 



ROTATION 



40" 



OF 



50" 60^ 



PRISM 



Fig. 3. Curves showing angular rotation of the plane 

 of vibration of a beam of plane-polarized light on pas- 

 sage through glass plates of half shade device for dif- 

 ferent angles of rotation of apparatus about horizontal 

 axis. 



