162 A MANUAL OF BACTERIOLOGY 



denser can then be approximately centred. The dia- 

 phragm is then closed to a point and further centration 

 adjusted in the same manner. The J-in. objective may 

 now be substituted for the |-in and the same procedure 

 repeated, so as to obtain centration more exactly. 



These adjustments assume that the diaphragm is 

 itself accurately centred, which is by no means always 

 the case. This can be detected after centring has been 

 performed, as described above, by the illumination on 

 one side of the diaphragm hole being brighter than on 

 the other side. If it be impossible to obtain equal illu- 

 mination all round the diaphragm hole, the diaphragm 

 is not centred and must be returned to the makers for 

 adjustment. 



Dark -ground Illumination 



By this method the objects appear illuminated upon a 

 dark background. This may be accomplished by intro- 

 ducing a " stop," such as a disc of metal or other opaque 

 material, at the central area of the lower surface of the 

 condenser, as in the old " stop lens." By this means all 

 central light is cut off and the field appears dark or 

 black. If small objects be then viewed with this arrange- 

 ment, a certain amount of light diffuses in laterally, and, 

 if it strike the objects, illuminates them, so that they 

 appear visible on the dark background. This is briefly 

 the principle of dark -ground illumination, but it is obvious 

 that by this simple arrangement only a small amount of 

 light will reach the objects, and in quite a haphazard way. 



In modern dark -ground illumination special " dark- 

 ground " condensers are employed. A similar central 

 " stopping out " of the light is obtained, as in the above- 

 described method, either by the use of an opaque disc 

 on the bottom of the condenser, as in the Zeiss paraboloid, 

 or by reflecting the central rays by a special construction 



