WRIGHT: A NEW HALF SHADE APPARATUS 



311 



indicated in figure 2, care being taken to have the prism edge 

 perpendicular to the axis of rotation. 



The axis of rotation is placed inan E - W position parallel to 

 one of the principal nicol planes. In this position the glass 

 plates make equal and opposite angles with the N ~ S principal 

 nicol plane; the angles of rotation for the different angles of tilt- 

 ing are equal and of opposite sign and are listed in Table 1 (com- 

 puted by the Fresnel equation), and are represented graphically 



in figure 3. 



TABLE 1 



Table showing angular rotalion of plane of vibration of light wave transmitted through 

 half shade apparatus for different angles of rotation about horizontal axis. The 

 values of the angular rotation are listed for the adjacent quadrants (glass plate 

 I and glass plate II, fig. 2) both for a single glass plate and also for two superim- 

 posed glass plates of refractive index n = 1.515 and n = 1.920. 



The values given in Table 1 are the angles computed from 

 Fresnel' s equation above. The actual situation is a little more 



