42 BACTERIOLOGY FOR NURSES 



is more easily distinguished than the middle it should be found 

 first with the low-power lens and then so arranged that the edge 

 of the drop crosses the center of the field. The high-power objec- 

 tive is then turned into position, the tube lowered until it almost 

 touches the coverslip, and then with the eye at the ocular it should 

 be carefully raised into focus. 



Hanging Block. The manner and rate of cell division can 

 perhaps be best studied by means of the hanging block. The 

 technique is as follows : A thin layer of melted agar is poured into 

 a Petri dish and allowed to harden, after which a small square 

 is removed, seeded with the organisms to be studied and carefully 

 placed on a coverslip, the bacteria being between the agar and the 

 glass. The preparation is placed over a hollow slide and examined 

 under the microscope in the manner described for the hanging drop. 



Film Preparation. This method is perhaps the most frequently 

 employed of all for bacterial examination; by it advantage may 

 be taken of the elective properties of certain bacteria for particular 

 stains and a diagnosis be thus more easily established. The 

 steps in the preparation are as follows : A thin even film of the 

 material to be examined is spread over the surface of a perfectly 

 clean slide ; if the material is fluid it is transferred by means of 

 a platinum loop,. if taken from solid culture medium a loopful of 

 sterile water is first placed upon the slide and a small particle 

 of a growth thoroughly mixed with it before spreading. The 

 film is allowed to dry in the air. The slide is then held by means 

 of a pair of forceps and passed slowly through the flame of a Bun- 

 sen burner three or four times, film side uppermost, in order to 

 fix the preparation on to the slide. Fixing may also be accom- 

 plished after the film is dry by immersing in chemicals such as 

 alcohol, formalin, glacial acetic acid, etc., in which case the slide 

 must be immersed in water to remove the excess chemical before the 

 next step. A few drops of the desired stain are then placed on 

 the slide and left there from a few seconds to several minutes 

 according to the dye used. The slide is then washed carefully 

 but thoroughly in water, after which it is dried by gently pressing 

 between layers of filter paper. 



