delicate and great care should be exercised in using 

 the adjustment and keeping the parts free from dust. 

 Lubrication should never be necessary between the 

 teeth of the rack and pinion. When applied to the 

 spindles of the pinion it should be only a very small 

 quantity of the best oil, and when lubricating the 

 sliding parts, wipe these with cambric to which a drop 

 of oil has first been applied. A surplus amount of 

 oil acts as a dust catcher. 



Fine Adjustment. While this is constructed in 

 numerous ways, in all it depends upon a screw for the 

 propelling power. It is sometimes called the slow 

 motion, as one revolution of the screw seldom gives more 

 than 1-50 inch motion. This screw is also called the 

 micrometer screw. The brass button by which it is 

 rotated is called its head and, when it is provided with 

 equal divisions upon its upper surface, it is the grad- 

 uated head. In this case a stationary index is fastened 

 to the arm. 



The fine adjustment, although it should be delicate 

 and sensitive, must, nevertheless, be rigidly constructed. 

 Its bearings must be large and still free so as to be 

 responsive to the movement of the screw, and these 

 conditions must be maintained with an endless amount 

 of use. 



While the fine adjustment, even more than the 

 stage, will show displacement with moderate magnify- 



48 



