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

 phragm to prevent reflection from the inner surfaces of 

 the tubes, and this also sometimes has a society screw 

 for attaching very low power objectives or accessories. 



Care should be used in moving the draw-tube as a 

 too sudden movement upward may draw the main tube 

 with it and thus injure the rack and pinion, or down- 

 ward, may force the objective onto the object or by the 

 compression of air in the tube, may force out the eye- 

 piece. To operate the draw-tube, hold the main tube 

 with one hand and with the thumb and fore- finger 

 of the other grasp the milled edge of draw-tube and 

 move it up or down with spiral movement. 



Base. A judicious form and weight of base 

 adds greatly to the stability of the microscope and it is 

 a too common fault that in many instruments, even 

 from reputable makers, this essential feature is sacri- 

 ficed from wrong motives of economy, portability or 

 compactness. While it can hardly be expected that 

 when the arm is inclined to the horizontal position the 

 microscope shall be stable, 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 

 under ordinary manipulative operations, it should not 

 be required that the base be held with one hand while 

 the other makes the adjustments. We can imagine 



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