588 LABORATORY EXERCISES 



2. Flame a cover-slip and place it on the filter paper on which rests the hanging- 

 drop slide. 



3. Place a drop of water on the center of the cover-glass by means of the platinum 

 loop. 



4. Remove some of the material in the culture flasks by means of a platinum loop 

 and mix it with the drop of water on the cover-slip. 



5. Raise the cover-glass with the points of a forceps and rapidly invert it on to 

 the ring cell of the hanging-drop slide, so that the drop of fluid occupies the center 

 of the ring. (In exact investigation, carefully avoid contact between the drops of 

 fluid and either the ring cell or the ring of vaseline. Should this happen, the in- 

 fected hanging-drop slide and its cover-slip must be dropped into lysol solution and 

 a new preparation made.) 



6. Press the cover-slip firmly down into the vaseline on to the top of the ring cell. 

 This spreads out the vaseline into a thin layer, and besides ensures the adhesion 

 of the cover-slip seals the cell and almost prevents evaporation. 



7. Examine microscopically (vide infra). 



Microscopic Examination of the Unstained Material. — i. Place the tube of the 

 microscope in a vertical position. 



2. .Arrange the hanging-drop slide on the microscope stage so that the drop of 

 fluid is in the optical axis of the instrument, and secure it in the position by means 

 of the spring clips. 



3. Use one-sixth inch objective, rack down the body tube until the front lens of 

 the objective is almost in contact with the cover-slip. 



4. Apply the eye to the eyepiece and adjust the plane mirror to the position 

 which secures the best illumination. 



5. Rack the condenser down slightly and cut down the aperture of the iris 

 diaphragm so that the light, although even, is dim. 



6. Rack up the body tube by means of the coarse adjustment until the organisms 

 come into view; then focus exactly by means of the fine adjustment. 



Some diflficulty is experienced at first in finding the hanging-drop, and if the first 

 attempt is unsuccessful, the student must not on any account, while still applying 

 his eye to the eyepiece, rack the body tube down, for by doing so there is every chance 

 of breaking the cover-glass and contaminating the objective. 



The examination of fresh material in a hanging-drop is directed to the 

 determination of: 



1. The nature of the bacteria and other organisms present. 



2. The purity of the culture. 



3. The presence or absence of motility. 



"When the examination is completed and the specimen finished the slide with 

 cover slip should in the study of contagious material be dropped into the lysol pot. 



Cf. KissKALT, K.: Prakticum der Bakteriologie und Protozoologie, Zweite 

 Auflage, Erster Teil (Bakteriologie), pp. 10-12 (1909). 



Mounting and Staining. — The mounting and staining of bacteria, protozoa and 

 other microorganisms may be accomplished as follows: 



I. Take the square, or round cover-slip, which has been previously cleaned out 

 of the alcohol pot, dry it between filter paper. 



