65 



It must be free from lost motion when working 



^> 



with the highest powers. 



The slide must be so perfectly fitted, that it 

 shall show no play when the tube is moderately 

 forced from one side to the other. 



Fine Adjustment. While this is constructed 

 in numerous ways it depends in all upon a screw 

 for the propelling power. It is sometimes called 

 the slozu motion, as one revolution of the screw 

 seldom gives more than -^ inch motion. This 

 screw is also called the micrometer screw. The 

 brass button is called its Jiead and when it is pro- 

 vided with equal divisions upon its upper surface, 

 it is the graduated head. In this case a stationary 

 index is fastened to the arm. A fine adjustment 

 which will not fulfill the following conditions may 

 be considered as faulty. 



j 



. The screw must work freely and smoothly and 

 without any side motion or play. 



The adjustment should act promptly in the 

 forward and backward motion without the least 

 particle of hesitation or lost motion. 



There should not be the slightest displacement 

 of the object in the field when the screw is worked 

 forward and backward. 



While the fine adjustment, even more than the 

 stage, will show displacement with moderate 

 magnifying power by a slight exertion of force 



