40 BACTERIOLOGY FOR NURSES 



diaphragm should be widely opened and the flat side of the mirror 

 used. 



By means of the coarse adjustment H the tube of the microscope 

 can be raised or lowered ; it is used to bring the object to be studied 

 roughly into focus. The fine adjustment I raises or lowers the 

 tube much more slowly and evenly and is used with high-power 

 objectives in order to obtain a clear sharp definition of the object 

 after it has been brought into focus by the coarse adjustment. 

 The fine adjustment has a limited range and should never be forced ; 

 when it ceases to operate the tube should be raised by means of 

 the coarse adjustment and the screw should be turned back mid- 

 way within its range. 



To focus, lower the tube by means of the coarse adjustment 

 until the objective nearly, but not quite, touches the slide to be 

 examined. Then with the eye looking through the ocular raise 

 the tube until the object can be plainly seen ; when it is well in 

 focus use the fine adjustment to bring out more clearly the part 

 of the field to be studied. In focusing with the coarse adjustment 

 care should be taken never to lower the tube while the eye is look- 

 ing through the ocular ; if the light is too intense or the preparation 

 transparent the focal point may be passed without being perceived 

 and the result may be a broken slide and a damaged lens. 



Light. The best light for microscopic work is that obtained 

 from white clouds or blue sky with a northern exposure; direct 

 sunlight should be avoided. Satisfactory artificial light may be 

 obtained by means of a Welsbach burner and a whitened incan- 

 descent bulb. 



Dark Ground Illumination. An apparatus to be used in con- 

 junction with the microscope has been devised whereby minute 

 particles are made visible, particles which could not be seen other- 

 wise even with the highest magnification obtainable. The general 

 principle involved is to arrange for light to be thrown obliquely 

 on the object to be examined and to stop the rays passing directly 

 towards the tube of the microscope. An electric arc lamp is used 

 as a source of light ; the organisms appear as brightly illumined 

 objects while the fluid which surrounds them forms a dark back- 



