672 PHARMACEUTICAL BOTANY 
3. Never touch the objective or ocular lenses with fingers or 
cloths. 
4. Never change from lower to higher power objective with- 
out first ascertaining that the body tube has been raised suff- 
ciently to allow the high-power objective to be slipped into place 
without injury to the objective or mounts. 
5. Never clean the microscope lenses or stand with cloths 
that have been used for removing surplus of alkali, acid or other 
reagent from slides. 
6. Note whether the front lens of the objective is clean before 
attempting to use it. If soiled, breathe on the lens and gently 
wipe with an old, clean, soft handkerchief or lens 
paper. If the lens be soiled with balsam or some 
Cnedeheted other sticky substance, moisten the handkerchief 
or lens paper with a drop of xylol, taking care to 
wipe it perfectly dry as soon as possible. 
7. Do not let the objective remain long near 
corrosive liquids, such as strong solutions of 
iodine, corrosive sublimate, or mineral acids. 
Never examine objects lying in such fluids without putting ona 
cover glass. 
8. Never lift the slide from the stage, but, after raising the 
objective, slide it off the stage without upward movement. 
9. Never allow the stand (microscope without lenses) to be 
wetted with such substances as alcohol, soap, etc., which dissolve 
lacquer. 
10. Keep the microscope covered when not in use. 
Fic. 500.—Ocular 
micrometer. 
MICROMETRY 
The unit of length used in microscopic measurement is the 
micron (u) which is one-thousandth part of a millimeter (0.001 
mm.) or one ‘twenty-five thousandth part of an inch. 
In measuring microscopic objects it is necessary to make use 
of a micrometer of some kind. That pretty generally used is the 
ocular micrometer (Fig. 500).° Itis a circle of glass suitable for inser- 
tion within the ocular with a scale etched on its surface. The 
scale is divided to tenths of a millimeter (0.1 mm.), or the entire 
