THE MIcroscopr. 331 
rattles and gnashes its teeth, or one that makes the image 
change its position by throwing the body out of centre, should 
be rejected. The only place for such a thing is a shelf in a 
museum of microscopical antiquities. The action should be 
noiseless, perfectly smooth, and the bearings so firmly in place, 
that the microscopist shall have no fear that a heavy objective 
may force the body to run downward by its weight and the ab- 
sence of resistance in the coarse adjustment mechanism, an acci- 
dent that has happened. This undue looseness, however, may 
oceur after constant and prolonged service, and a remedy is 
usually provided by the optician, who places two screws in such 
a position on the arm or elsewhere, that by tightening them the 
pinion bearings are tightened, and the trouble is corrected for a 
time. Every stand, even the best, is liable to this annoyance in 
a greater or lesser extent. 
Some of the cheaper stands have no coarse adjustment. The 
body is then encircled by a collar through which it moves when 
actuated by the hand, the focus being obtained by pulling and 
pushing on the tube. This is a very inconvenient and undesir- 
able 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 the circumstances the body has the 
obnoxious habit of suddenly slipping further than the micros- 
copist 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 adjust- 
ment 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, a nuisance, 
and a peril. 
THE FINE ADJUSTMENT. 
When the objective has been imperfectly focussed by the 
coarse adjustment, its position must be further changed until 
the image becomes clear and bright, and the outlines as distinct- 
ly and sharply defined as the lines in the best steel engraving. 
This is accomplished by means of the fine adjustment screw, 
