18 



Centering Screws. These are provided for 

 moving the stage in different directions to bring 

 the center of its revolving motion in the center of 

 the field. 



O. Mirror. This is used for reflecting and 

 condensing light upon the object. As a rule two 

 mirrors are used, one plane and the other concave. 

 The first gives a comparatively weak light, while the 

 second concentrates it and gives it more intensity. 



P. Mirror-bar. This carries the mirror and by 

 a sliding arrangement allows the variations in dis- 

 tance of the mirror to the stage_; it also swings in 

 a circle around the object in order to illuminate it 

 from any direction. 



Q. Sub-Stage. This is a ring below the stage 

 to receive various accessories which may be required. 

 It is sometimes fixed to the stage but in the best 

 instruments it is separated from it and is provided 

 with an adjustment to vary its distance from the 

 object. 



R. Sub-Stage bar. This receives the sub-stage 

 and permits its adjustment. In modern American 

 instruments this, as well as the mirror-bar, is on an 

 axis in the plane of the stage, so that whatever posi- 

 tion they may be in, relative to the object, the dis- 

 tance from this to the sub-stage or mirror does not 

 vary, except when made to do so. 



S. Diaphragm. This is a perforated, revolving 

 disk, attached either to the stage or sub-stage. It 



