vided. It is supplied in most of the better class of 

 instruments and is a convenience in almost all kinds 

 of microscopical work. As a slight deviation of the 

 center of revolving motion from exact coincidence with 

 the optical axis will cause the object to swing out of 

 the field, centering screws are provided by which this 

 error can be quickly corrected. These screws are 

 also convenient, within narrow limits, in providing a 

 mechanical method of moving the object in different 

 directions over the field of view. It sometimes occurs, 

 as the stage is revolved, that an object at the edge 

 of the field, which is in focus, gradually becomes 

 indistinct, showing poorest at 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 

 directions with the body; in either case a serious defect. 



Glass Stage. This with the slide carrier, Fig. 27, 

 is a device for moving the object more steadily and 



smoothly than can be 

 directly on 

 It is made 



done 



the 



stage. It is made de- 

 tachable from the micro- 

 scope and consists of a 

 glass plate, in a metal 

 frame, upon which the 

 slide carrier, resting on four points, moves. At each 

 end a spring clip passes around the glass plate and 



27. 



39 



