USE OF THE MICROSCOPE ol 
ing projecting beyond its surface the working distance is lessened 
thereby. 
Oculars. These have a simpler structure than the objectives. 
They serve as collective lenses, making all of the light constituting 
the image from the objective enter the eye of the observer. 
Ilumination. Practically all microscopic studies are carried out 
by transmitted light, and some special attachment is necessary to direct 
this light along the optical axis of the microscope. For this purpose 
substage or Abbe condensers are used. This condenser allows the 
utilization of all light which enters the object space. Light is sent 
through the object at an angle great enough to fill the objective. 
The condenser is constructed to bring the rays of light to a focus 
above the upper surface of the uppermost lens. When the concave 
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Fic. 26.—Showing Incidence of Light Rays in Condenser. (After Bausch.) 
mirror is used the rays of light are brought to a focus within the con- 
denser. The concave mirror acts as a lens and together with the con- 
verging effect of the lens in the condenser causes the light to converge 
more rapidly than that for which the microscope was standardized. 
Bausch has depicted this in Fig. 26. 
Bausch makes the following statement with regard to the use of 
the condenser: ‘In the use of the condenser with oil immersion 
objectives the custom prevails of using the condenser dry. It is well 
to point out, however, that both the condenser and the objective lose 
in their efficiency when the former is used dry, and for critical work 
the condenser should be in immersion contact with the slide. 
To make immersion contact between condenser and slide place 
a drop of oil on the top of condenser, drop the slide upon the stage, 
first turning the slips to one side. 
With immersion objectives the proper focusing of the condenser 
becomes a matter of nice distinction to obtain best results and can 
