26 BACTERICLCGICAL APPARATUS 
become gummed and corroded from exposure to the atmosphere and 
reagents of the laboratory, and particularly from the perspiring or 
dirty hand of the operator in focusing. In this condition the tube 
works hard and unevenly, so that one is apt to force an objective into 
the slide. The tube and sleeve can generally be cleaned by rubbing 
with a coarse towel after moistening with xylol. Do not grease the 
tube or sleeve because it gums and corrodes the tube, and is very 
unsightly. Should the tube be too loose in the sleeve remove the tube 
and press in the top edge of the sleeve on either side of the one or more 
slots running down from the top edge. 
All of the better microscopes are provided with the rack and pinion 
coarse adjustment. The bearings must necessarily fit very closely. 
Any foreign matter on them interferes seriously. Do not strain the 
teeth of the rack and pinion by forcing the bearings back and forth 
over one another when they are not clean. A little xylol or chloro- 
form rubbed on the surfaces will clean them. Do not use emery in 
any form. When the bearings are perfectly clean, oil them slightly 
with a good acid-free lubricant (paraffin oil or watch oil). If the bear- 
ings become so loose that the tube will not stay in place tighten the 
little screws at the back of the pinion box. All makers have a provision 
here for taking up lost motion and wear. Do not fill the teeth of the rack 
with paper paraffin or any other foreign substance. If anything should 
accumulate in these teeth clean it out. 
Fine Adjustment. The fine adjustment is necessarily of limited 
range and delicate in its mechanism. If, when looking into the eye- 
piece, no change of focus is noticed by turning the micrometer head or, 
if the micrometer head ceases to turn, the adjustment has reached its 
limit. Turn the micrometer back to bring the fine adjustment mid- 
way within its range. When the fine adjustment head stops do not 
force it. Allof the better microscopes are made so that the head stops 
at both ends of the range and so that the micrometer threads cannot 
be removed from their bearings. Other microscopes do not have this 
safeguard, and great care should be exercised not to remove the thread. 
If by chance it should be removed, exercise great care in replacing it 
to see that the threads are started properly so that they do not “ run.” 
Do not force them if they run at all hard. If they are started to “ run- 
ning”? they must go to a machinist. On some microscopes these 
threads are “ left-handed,’’ which should be noted in trying to start 
the threads. 
In some microscopes a little steel pin is fitted loosely into the hollow 
end of the micrometer thread. Be careful to see that this little pin 
