48 



N. Clips.- -These are two springs which are 

 attached to the upper surface of the stage and 

 serve to hold down the object. 



Centering 1 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 con- 

 densing 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. 



j 



P. Mirror Bar.- -This carries the mirror and 

 swings in a circle around the object in order to 

 illuminate it from various directions. 



Q. Substage. --This is a ring or attachment 

 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. 



Substage Bar. This receives the substage 

 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 

 position they may be in, relative to the object, the 



