MICROSCOPICAL TECHNIQUE 39 



adapt it to bacteriological work, are for the purpose of concen- 

 trating the light, and thus enable higher power objectives to 

 be used. 



Abbe condenser. The purpose of the condenser is to collect 

 a large number of rays of light and bring them to a focus in 

 the plane of the object to be examined. The plane mirror 

 should always be used with natural light ; the concave mirror 

 with artificial light. The condenser may be focused by means 

 of the screw beneath the stage of the microscope. 



Diaphragm. The purpose of the diaphragm is to regulate 

 the amount of light reflected by the mirror through the con- 

 denser. For stained preparations of bacteria the iris dia- 

 phragm must be open. In the examination of unstained 

 and living forms the light should be diminished by partial 

 closure of the diaphragm, so as to emphasize the slight vari- 

 ations in density of the protoplasm. 



Light. The mirror should be so adjusted as to illuminate 

 the field evenly when viewed through the tube of the micro- 

 scope with the ocular removed. North light is to be preferred, 

 and direct sunlight should never be used. The light coming 

 from a white cloud or the clear sky is best. 



Focusing. It should be a rule to work with as low a power 

 as possible, so as to cover as large a field as is permissible. 

 Always begin the examination of preparations of bacteria 

 with a 4-mm. objective and a 2-inch ocular, using the 2-mm. 

 oil-immersion objective for a more detailed examination. 



The proper objective should be turned into place, and with 

 the eye held at the side of the stage, the tube should be 

 lowered carefully by the use of the coarse adjustment until 

 the lens is nearly in contact with the cover glass of the prepa- 

 ration. With the eye at the ocular, then focus slowly upward 

 with the fine adjustment, meanwhile moving the preparation 



