134 BACTERIOLOGY. 



broad pencil of light, corresponding to the diameter of the 

 front lens of the condenser, is brought to a focus at about 

 2 mm. above the upper spherical lens. An object, there- 

 fore, placed in this focus will receive all the light that 

 enters the condenser. In order to obtain the best results, 

 the condenser should always be focussed on the object. 

 When parallel light is employed, the reflected light from a 

 white cloud, the focus will lie nearest to the plane face 

 of the upper lens. When artificial light, as that of a 

 lamp, is employed, the rays that are brought to the con- 

 denser are divergent, and hence, the focus will be farther 

 away from the upper face of the condenser. The correct 

 adjustment of the Abbe condenser should, therefore, re- 

 ceive the same attention as the proper focussing of an 

 objective. ( The plane mirror should always be used with 

 the condenser, since the concave mirror, if employed, would 

 give off converging rays. These would be brought to a 

 focus so close to the upper face of the condenser, that, 

 owing to the thickness of the glass-slide, it would not be 

 possible to bring ihe object into this focus) 



The Abbe condenser is ordinarily employed with its 

 optical axis in line with that of the microscope. It thus 

 gives central illumination. The iris diaphragm on the 

 more expensive instruments can be placed in an excentric 

 position by means of a thumb-screw, and can be rotated in 

 this position about the optical axis. Hence, oblique light 

 can thus be admitted from any desired direction. 



The condenser is always accompanied by an iris dia- 

 phragm. This enables one to regulate the amount of light 

 which enters the condenser. If, for instance, an unstained 

 section of a liver is placed under the microscope, and care- 

 fully focussed, it will be seen perfectly, provided the dia- 

 phragm is partly closed. Now, if while looking at the section 

 through the microscope, the diaphragm be thrown wide open 

 the object will at once become invisible. The excessive 

 amount of light coming from the unrestricted condenser 



