simply slides in the outer tube, but in the better instru- 

 ments a special spring sleeve is provided in which the 

 draw-tube operates. As both of these have the defect 

 incident to the sliding tube adjustment, a cloth lining 

 is preferable as the movement is smooth, while firm, 

 and will remain so for an unlimited time. 



Correct position of hands to operate draw-tube. 



The draw-tube may be used to vary the magnifying 

 power, but unless used judiciously may be the cause of 

 more harm than good. While this feature will be 

 touched upon again in another chapter, showing the 

 optical effect, it will suffice at the present to state that 

 it should be used only with objectives of low power or 

 with high powers only under well defined conditions. 



