296 



PROCEEDINGS OF THE AMERICAN ACADEMY 



Removing the Nicol prism, this polarimeter becomes a very sensitive 

 polariscope. The plane of a polarized ray may be found by turning it 

 until the bands disappear, when the plane of the prism will be inclined 

 45° to the plane of polarization. 



In the paper referred to above, it was shown that the polarization 



P = 



A —B 



A-\- B 



^=: COS 2 V, 



in which A and B represent the intensity of the two components of 

 the polarized beam, and v is the angle between the position in which 

 the two images are equal and that in which one of them disappears. 

 Of course the bands must first be placed parallel, or perpendicular to 

 the plane of polarization of the ray. It is best to set the Nicol prism 

 in the four positions in which the bands disappear. Subtracting the 

 first reading from the second, and the third from the fourth, and add- 

 ing the differences, gives 4 i\ The observations are thus rendered 

 symmetrical, the uncertain point of disappearance of the images is 

 eliminated, and the errors of observation are reduced one half as com- 

 pared with a single setting. 



Table I. serves to reduce the observations. The argument gives the 

 sum of the two differences found by subtracting the first reading from 

 the second, and the third from the fourth. The tabular number gives 

 the corresponding polarization. If the sum of the differences exceeds 

 180°, it must be subtracted from 360°. This may be avoided by 

 beginning with the second reading. 



TABLE I. — Eepuction Table. 



