F. E. Wright— Measurement of Extinction Angles. 375 



character, and after total reflection in the upper prism pair is 

 again brought back to the common Held of vision and viewed 

 by the Kamsden ocular and upper nicol. The nicol prisms or 

 b'irefracting plates fit in collars and can simultaneously be 

 revolved about the axis and in opposite directions so that the 

 angle 6 can be made to vary from +90° to -90° in each plate 

 and at any instant 6 of the first plate is equal and opposite to 



Fig. 11. 



Fig. 11.— Bi-nicol ocular; consists of the following parts : Two pairs of 

 reflecting prisms, P x and P 2 , (ground specially for the purpose by Steeg & 

 Renter of Homburg v. d. Hohe, Germany) ; two tapering revolving brass 

 holders, M, into which either two Thompson prisms, N, or birefracting plates 

 are introduced. These conical brass carriages are revolved in opposite 

 directions and through equal angles by means of the worm thread S and 

 grooved wheels T, the angle of revolution being read off directly on the head 

 H. The bi-nicol ocular fits in the microscope as an ordinary ocular and into 

 it in turn a Ramsden ocular is introduced and above this the cap nicol. 



of the second plate. In the mechanical construction of this 

 apparatus, special care has been taken to make the angular 

 movements of both plates exactly equal and opposite. 



With this revolving bi-nicol ocular, it is thus possible to 

 allow any proportion of the light incident on the crystal plate 



