28 ELEMENTS OF APPLIED MICROSCOPY. 
phragm is to cut off adventitious light so that all the rays 
finally reaching the microscope shall come from the 
immediate vicinity of the object. A little experimentation 
will show that reducing the diaphragm opening often 
gives a sharper picture as well as one much less fying 
to the eyes; in general an opening about the size of the! 
front lens of the objective will yield good results. 
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Fic. 19.—ABBE CONDENSER. (After Hager-Mez.) 
Certain opaque objects are best examined by reflected 
light, and in many cases this method may well be used to 
supplement the ordinary one, the mirror being turned 
aside so that no light passes through the stage and the 
object being illuminated either with the light which nat- 
urally falls on it or with rays concentrated from above 
by. a lens or mirror. 
3. Focusing.—After the field is well lighted, the next 
step is to focus on the object, or to so adjust the relation 
of the lens systems that a clear image may be formed. 
