26 HISTOLOGY OF MEDICINAL PLANTS 
ture, and general arrangement or plan or structure if a section 
of a plant, or, if a powder, to note its striking characters. All 
the finer details of structure are best brought out with the 
high-power objective in position. 
In placing the high-power objective in position, it is first 
necessary to raise the body by the coarse adjustment; then 
open the iris diaphragm, and lower the body until the objective 
is within about one-eighth of an inch of the slide. Now raise 
the tube by the fine adjustment until the object is in focus, 
then gradually close the iris diaphragm until a clear definition 
of the object is obtained. Now proceed to make an accurate 
sketch of the object or objects being studied. 
In using the water or oil-immersion objectives it is first 
necessary to place a drop of distilled water or oil, as the case 
may be, immediately over the specimen, then lower the body 
by the coarse adjustment until the lens of the objective touches 
the water or the oil. Raise the tube, regulate the light by the 
iris diaphragm, and proceed as if the high-power objectives were 
in position. 
The water or oil should be removed from the obiectives and 
from the slide when not in use. 
After the higher-powered objective has been used, the body 
should be raised, and the low-power objective placed in position. 
If the draw-tube has been drawn out during the examination 
of the object, replace it, but be sure to hold one hand on the 
nose-piece so as to prevent scratching the objective and Abbé 
condenser by their coming in forceful contact. Lastly, clean 
the mirror with a soft piece of linen. In returning the micro- 
scope to its case, or to the shelf, grasp the limb, or the pillar, 
firmly and carry as nearly vertical as possible in order not to 
dislodge the eye-piece. 
ILLUMINATION 
The illumination for microscopic work may be from natural 
or artificial sources. 
It has been generally supposed that the best possible illumi- 
nation for microscopic work is diffused sunlight obtained from 
a northern direction. No matter from what direction diffused 
