67 



air in the tube, may force out the eyepiece. To 

 slide it properly, hold the main tube with one hand 

 and with the other grasp the draw-tube and move 

 it up or down by spiral movement. 



The draw-tube may be used to vary the magni- 

 fying power, but unless used judiciously may be 

 the cause of more harm than good. While this 

 feature will again be touched upon 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 when with high 

 powers, unly under well defined conditions. 



The draw-tube usually has at its lower' end a 

 diaphragm to prevent reflection from the inner 

 surfaces of the tubes and this is sometimes also 

 utilized as a society screw for attaching some very 

 low power objectives or accessories. 



Base. The judicious form and weight of the 

 base adds greatly to the stability of the micro- 

 scope and it is a too common fault that in many 

 instruments, even from reputable makers, this 

 essential feature is sacrificed from wrong motives 

 of economy, portability or compactness. While it 

 can hardly be expected that when the arm is 

 inclined to the horizontal position it shall be firm, 

 as it is never used in this position except for 

 photography and must then be clamped to the 

 table, it is but reasonable to demand that when 

 the instrument is upright or slightly inclined 



