22 SELECTION AND USE 



tube in which these lenses are secured is in general movable, 

 and the best microscopes are furnished with several eye-pieces 

 of different powers. We may here remark that where a micro- 

 scope is furnished with several eye-pieces, the shortest eye-piece 

 gives the greatest magnifying power. 



The Object- Glass or Objective is the lens or lenses which is 

 placed next the object. The term is frequently applied to the 

 glass plate or slide upon which the objecb is placed, but this 

 use of the word is entirely wrong, and tends to produce con- 

 fusion. 



A Draw-tube is a secondary body which receives the eye-piece, 

 and slides within the main body like the draw of a telescope. 

 It enables us to increase the distance between the eye-piece 

 and the objective, and thus to change the magnifying power, as 

 explained in a previous paragraph. 



The objective is moved to or from the object by two methods, 

 which are calltd respectively the coarse and fine adjustments. 



The Coarse Adjustment is used for bringing tl.e objective ap- 

 proximately but rapidly into focus. To effect this the body 

 either slides through a tubular collar or is attached by means 

 of dovetail t-lides to the arm, and in either case it is moved up 

 and down either by a rack and pinion (or some substitute there- 

 for) or directly by hand. 



The Fine Adjustment is employed for bringing the object 

 exacty into focus. It usually consists of a fine screw, which 

 moves either the entire body or the lower part of it. In 

 some cheap stands, the fine adjustment is effected by moving 

 the stage towards the objective. 



The Mirror reflects the light, and causes it to pass through 

 the object aud the body of the instrument. 



A Sub-stage is furnished with some instruments. It is used 

 for holding and centering various means of illumination. 



Clips are springs attached to the stage for the purpose of 

 holding in place the glass slide or plate carrying the object. 



The Object is that which is subjected to examination. It is 

 usually mounted upon 



A Slide, or plate of glass, which is laicl upon the stage. 



