32 BACTERIOLOGICAL APPARATUS 
only be reliably accomplished by considerable practice and experience. 
To obtain best position: 
Use a 16-mm. (2 in.) objective; focus upon the object; adjust con- 
denser until image of window-sash or flame is in the same plane with 
object. 
Practically all substages are provided with means for focusing 
the condenser! The rays of light should be focused sharply on the 
object. A little experience will indicate when the proper aperture is 
Fig. 27.—Showing Action of Different Parts of the Condenser. (After Bausch.) 
being used. The iris-diaphragm when used with dry lenses should 
not be more than half open. With the oil immersion objective, how- 
ever, the full aperture should be used. 
FinaL Hints 
“Sometimes the worker may have faithfully carried out all the 
directions heretofore given and been assured that his lenses possess the 
above-named qualities as they ought, yet be unable to obtain the desired 
results. He may be working with a water mount and dry objective 
become ‘immersed’ in some water which has worked to the top of 
the cover glass. His objective may be dirty from a previous ‘ immer- 
sion’ or it may have some other dirt upon the front lens. The field 
may be covered with specks which revolve when the ocular is turned. 
The field may be dim or hazy, due to dirt on the back of the objec- 
tive or a film on the inner surfaces of the lenses of the ocular, or because 
of moisture settling on the lenses because they have just been brought 
from a cold into a warm room. He may see great streaks on his field, 
which are due to his own eyelashes, or he may see small slowly moving 
bodies floating across the field. With the exception of this last, the 
ailment has only to be mentioned to suggest the remedy. The musce 
volintantes, as these last-named bodies are called, are little specks of 
