54 



If the coarse adjustment does not prove sensitive 

 enough to focus easily, adjust by the fine adjustment 

 by taking the head of the micrometer screw between 

 the thumb and first finger and move toward the right 

 or left as may be necessary. 



It may here be said in passing that the rack and 

 pinion should be so well fitted that they should per- 

 mit the adjustment of low power objectives with the 

 greatest ease, and should work without the slightest lost 

 motion with a \ or \ inch objective. This point is the 

 criterion of workmanship in an instrument, and if it 

 is found to have the least back-lash, or is not perfectly 

 smooth, it may safely be assumed that the instrument 

 is of inferior workmanship. 



If the fine adjustment does not act, the screw has 

 either come to its stop, or has "run out," and must 

 be brought into action again ; the range of movement 

 in almost all fine adjustments is quite short and con- 

 stant care must be taken to keep it at about a medium 

 point. If the object is found not to give a full view 

 or is not in the center of the field, it must be moved 

 on the stage, but it must be remembered that a move- 

 ment in one direction causes an apparent opposite 

 movement in the field. At first this movement will be 

 in jerks, but after a little practice the necessary sen- 

 sitiveness of touch is acquired to give it more steadi- 

 ness. 



Illumination. It should now be observed 

 whether the field is equally illuminated. Too 

 much stress cannot be laid on this point, 



