Centering Screws are provided in some stands 

 for moving the stage in different directions to bring 

 the center of its revolving motion in the center of 

 the field. In a limited sense they offer means of 

 mechanical movement for the object. 



N. Clips, two springs attached to the upper 

 surface of the stage and used to hold the object in 

 position. 



O. Mirror, used for reflecting and condensing 

 light upon the object. As a rule there are two mirrors, 

 one plane and the other concave. The former gives a 

 comparatively weak light, while the latter concentrates 

 it and gives more intensity. 



P. Mirror Bar, carries the mirror and is pivoted 

 in order to illuminate the object from different 

 directions. 



Q. Substage, a tube or attachment below the 

 stage, of standard size, to receive various accessories 

 which may be required. It is sometimes immovably 

 fixed to the stage but in the best instruments is pro- 

 vided with an adjustment to vary its distance from the 

 object. 



S. Diaphragm, a provision for increasing or 

 decreasing the amount of light which illuminates the 

 object. 



33 



