ing power by a slight pressure against the tube, it 

 should return to its position when released. 



If a new instrument does not meet the conditions 

 here set down for testing a fine or coarse adjustment, 

 it may be put down as of faulty construction, no matter 

 by whom made or how well made it may appear in 

 other respects. 



A fine adjustment should fullfill the following con- 

 ditions : 



The screw must work freely and smoothly, and 

 without any side motion or play. 



The adjustment should act promptly without the 

 least particle of hesitation or lost motion. 



There should not be the slightest displacement of the 

 object in the field when the screw-head is worked back 

 and forth. 



Draw-Tube. While this part of the instrument 

 may be an advantage when judiciously used, it may 

 have an injurious influence when abused. It will give 

 both short and long tube standards and should be pro- 

 vided with a mark to indicate each length, or should 

 have divisions by which the standard can be read off. 

 It should not be overlooked, that when a double nose- 

 piece is used its thickness is added to the optical tube 

 length and the draw-tube should be shortened an equal 

 amount. In the cheaper instruments the draw-tube 



49 



