56 



the half revolution and as the stage is brought 

 around to the first point again comes into focus. 

 This may be due to poor fitting of the parts, or to 

 the fact that the stage is not square in all direc- 

 tions with the body ; in either case a serious 

 defect. 



Glass Stage. This with the slide carrier 

 (Fig. 22) is a device for moving the object more 



Fig. 22. 



steadily and smoothly than can be done directly 

 on the stage. It is made detachable and consists 

 of a glass plate in a metal frame upon which rests, 

 on four points, the slide carrier. At its ends the 

 spring arms are passed around the glass plate and 

 press against its lower surface, thus offering the 

 minimum of friction, with sufficient resistence to 

 make an easy movement. 



Mechanical Stage. This is a very important 

 form in which the movements are mechanical in 

 two directions and at right angles to one another, 



