178 



and pinion seem to be generally appreciated in this 

 country and there are few instruments 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 fre- 

 quently, 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 circum- 

 stances, the body has the obnoxious habit of sud- 

 denly slipping further than the microscopist in- 

 tends, 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 microscopical work can be 

 done with an instrument whose body slides through 

 a friction collar. That arrangement 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 through- 

 out 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 fitted in its bearings, so tha.t 



