66 



against the tube, it should return to its position 

 when released. 



If a new instrument does not meet the con- 

 ditions here set down for testing a fine or coarse 

 adjustment, it may be put down as of faulty con- 

 struction, no matter by whom made or how well 

 made it may appear in other respects. 



Draw-Tube. While this part of the instrument 

 may be an advantage when judiciously used, it 

 may have a pernicious influence when abused. It 

 will give both short and long tube standards and 

 should be provided with a mark to indicate each 

 length, or should have divisions by which the 

 standard can be read off. It should not be over- 

 looked, that when a double nose-piece is used the 

 heighth of this is added to the optical length and 

 suitable allowance should be made for it. In the 

 cheaper instruments the draw-tube slides in the 

 inner surface of the outer tube, but in the better 

 instruments, a special 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. 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 downward, may force the 

 objective onto the object or by the compression of 



