THE NICOL PRISM. 



239 



pletely, and transmitting but one of the rays, the other 

 being entirely suppressed. 



The Nicol prism is made from a rhombohedron of 

 Iceland spar, bisected in the plane which passes through 

 its obtuse angles, as shown in Fig. 207 by the letters 

 A B C D, the halves being joined again in the same order 

 by means of Canada balsam. 



FIG. 207. 



FIG. 208. 



When a luminous ray enters this prism the ordinary ray 

 undergoes total reflection, passing out of the crystal at O, 

 while the extraordinary ray emerges at E, Fig. 208. 



As attached to ordinary microscopes both polariser and 

 analyser are formed of Nicol prisms ; the former being 

 made to fix underneath the stage or into the sub-stage. 

 For this purpose it is mounted as 

 shown in C (Fig. 209), so that the 

 prism may be easily rotated by 

 means of the milled head. 



The analyser may be used in 

 several ways. In the first, the 

 Nicol prism is set in a mount 

 screwing into the lower end of 

 the microscope body, just above 

 the objective, or in a brass box 

 sliding into the side of the tube, 

 in a similar manner to the Wen- 

 ham prism of a binocular instrument. In another, the 

 prism fits over the eye-piece in the place of the ordinary 



