106 



A HALF-HOUR WITH 



revolving collar connected with the tube into 

 which the prism is fitted. By this cor^.nvance 



Fig. 25. 



the surface of the prism can be placed at any 

 angle with the analyzer. 



The prism used as the analyzer is sometimes 

 mounted in a brass cap, fitted over the eye-piece, 

 as in fig. 26 ; or in an adapter screwed on to the 



Fig. 26. 



nose-piece of the Microscope into which the object- 

 glass is screwed. 



By the first method the brightness of the field 

 and the definition of the object under examination 

 is not impaired, but the diameter of the field is 

 seriously diminished. By the latter plan the field 

 remains the same size, but a certain amount of 



