2 THE MICROSCOPE. 



fixed. The body is moveable up and down by one 

 or two large milled heads, connected with a grooved 

 rod or pinion, which works in the teeth of a rack 

 fixed to the back of the body, or of the arm which 

 supports the body. The large milled heads form the 

 " coarse movement/* as it is called. 



On the top of the arm, or on the front and lower 

 part of the body of the microscope, is placed the 

 ' ' fine movement/' consisting of a small milled head, 

 with a fine screw, for moving the body through very 

 small distances. 



Next is the " stage," or flat plate, upon which the 

 objects to be viewed are placed. This is often so 

 arranged that, by turning two milled heads, the ob- 

 ject can be moved backwards and forwards, or from 

 side to side ; it is then a " moveable stage/' 



The eye-piece slides into the upper end of the 

 body ; and the object-glass screws into its lower end. 

 Beneath the stage is the mirror, which reflects 

 the light through the object, the object-glass, and eye- 

 piece to the eye. 



Object-glasses. The object-glasses are the most 

 valuable parts of the instrument. There are gene- 

 rally three or more of them ; and, by means of an 

 " adapter," any object-glass can be made to fit any 

 microscope. Great care is required in their use, 

 especially to avoid scratching the lower surface of 

 the glass, which is sometimes accidentally done by 

 pressing the surface against any hard body, or allow- 

 ing such a body to fall upon it. When not in use, the 

 object-glasses should either be put away in the brass 

 boxes or covered with a small bell-glass, to prevent 

 their receiving any injury. 



The object - glasses possess various magnifying 

 powers, according to the distance at which they re- 

 quire to be placed from the object for distinct vision : 

 this is not, however, absolutely correct, yet may 

 serve as a general expression. Thus we have a 



