MANIPULATION OF THE MICROSCOPE. 25 
oily, must never be allowed to touch the lens. Well- 
defined spots seen on looking through the microscope at 
a clear field are due to specks of dust upon the eyepiece, 
as may be proved by rotating that system of lenses. Not 
infrequently such specks are formed on the upper sur- 
face of the lower or field lens of the ocular, which must 
be unscrewed in order to remove them. Diffuse cloudi- 
ness is generally caused by dirt or moisture on the object- 
ive; and this may be removed by breathing on the glass 
and wiping it with lens-paper. Sometimes for stubborn 
smears of dirt or grease, 95% alcohol or xylol may be 
used, but in the latter case the solvent must be sparingly 
applied and promptly removed in order that it may not 
affect the setting of the lenses. 
2. Lighting.—After the objective has been screwed in 
at the bottom of the tube, as shown in Fig. 18, and the 
eyepiece has been inserted at the upper end of the draw- 
tube, the mirror should be so adjusted as to give the best 
available illumination. Direct sunlight must of course 
be avoided, but a clearsarea of sky or a white cloud fur- 
nishes an ideal sourcé of light; and sunlight reflected 
from a white wall or transmitted directly through a cur- 
tain may be used to advantage. For artificial illumina- 
tion electric bulbs with ground-glass shades are suitable, 
or the narrow edge of the flame from a flat-wicked oil- 
lamp may be substituted. A gas-flame, unless some 
incandescent mantle be attached to the burner, is not 
satisfactory on account of its unsteadiness. 
The flat surface of the mirror yields sufficient light 
under ordinary circumstances, but the concave surface 
