advantages of the rack and pinion seem to be generally 

 appreciated in this country and there are few instru- 

 ments sold and used without it. Dr. Stokes speaks of 

 the sliding tube adjustment as follows : 



" This is a very inconvenient and undesirable 

 arrangement. It is awkward, since the friction is often 

 so great that the whole stand will move out of position 

 before the body will budge, and frequently, more 

 frequently than not, even when the foot is heavy 

 enough to keep the instrument firmly on the table, 

 both hands are needed to manipulate the body. It is 

 dangerous too, since under certain circumstances the 

 body has the obnoxious habit of suddenly slipping 

 farther than the microscopist intends, stopping only 

 when it crashes against the slide, where it usually 

 grinds and crunches cover glass and objective with 

 apparently fiendish glee. A stand without a coarse 

 adjustment by rack and pinion is a good stand to be 

 permanently left with the optician. No fine micro- 

 scopical work can be done with an instrument whose 

 body slides through a friction collar. That arrange- 

 ment may be cheap, but it is also a torment and a peril." 



Rack and Pinion. This should be absolutely 

 smooth with no back-lash or lost motion throughout its 

 entire length, which can be determined by holding the 

 main tube and working the pinion buttons very slightly 

 but quickly back and forth. It should be perfectly 



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