310 



WRIGHT: A NEW HALF SHADE APPARATUS 



Fig. 1 



duces to cot B = — cos^ {% - r) . For a given angle of incidence the 

 angle of rotation produced by the glass plate in the second quad- 

 rant is accordingly equal in value to that in the first quadrant 



but opposite in sign. Thus in a glass plate of re- 

 fractive index 1.615 the angle of rotation of the 

 plane of vibration of plane polarized waves in- 

 cident at 40° under an azmuth angle 45° is 1°57' 

 while the rotation angle in a second plate for 

 the same light wave incident at 40° under an 

 azimuth angle -45° is -1° 57' (fig. 1). If now 

 two glass plates be so mounted that they meet 

 in a fine line they form a half shade apparatus 

 of a definite angle of rotation. On varying the 

 angle of incidence i by rotating the system, we can change the 

 rotation angle B and thereby introduce the principle of variable 

 sensibility into our system. 



Although there are a number of methods possible for rotating 

 the two glass plates simultaneously, the simplest method which has 



occurred to the writer is illus- 

 trated by figure 2. Into a brass 

 cylinder (telescope or microscope 

 tube) a diametral hole is bored 

 transverse to the axis and into 

 this a rod is fitted so that it can 

 be rotated about its axis. Two 

 plane-parallel thin plates are 

 mounted on the sides of a small 

 right angled total-reflecting 

 prism so that their ends project 

 beyond the prism any desired amount depending on the size of 

 field to be covered. The edges of these plates are beveled at 45° 

 and the ridge is cemented with Canada balsam, thus producing a 

 fine division line between the two glass plates. In case plane- 

 parallel polished glass plates are not available thin cover glass 

 plates may be selected and serve the purpose satisfactorily. The 

 glass prism is then cemented to the rotating bar in the tube as 



Fig. 2 



