POLARIZED LIGHT. 107 



definition is sacrificed (this, however, is scarcely 

 perceptible if the prism is of good quality). 

 Having fixed the polarizing apparatus to the Micro- 

 scope, we may now proceed to test its effects on 

 various objects. Some will be tinted with all the 

 colours of the spectrum, whilst others are either 

 not affected by the altered condition of the light, 

 or are merely black on a white ground, or white 

 on a black ground. The last-named objects are 

 best seen with a film of selenite placed beneath. 

 This is sometimes mounted between two ordinary 

 glass slides and placed below the object. The 

 selenite should, however, be mounted in such a 

 way that it can be revolved independently of the 

 object. This is done in several ways; the best 

 contrivance is perhaps the revolving selenite stage. 

 The following diagram represents one of the 

 simplest forms of revolving stage. 



Fig. 27. 



With these stages a set of selenites is usually 

 supplied ; these separately give the blue, purple, 

 and red, with their respective complementaries 

 orange, yellow, and green. 



These discs generally have engraved upon 

 them the amount of the retardation of the 



