124 MICROBIOLOGY 



drops of the staining solution on the film side of the fixed 

 preparation, which is either held horizontally with the fine 

 forceps or left resting on the tray or piece of filter paper. 

 Allow the stain to act for 2 or 3 minutes with old stains, which 

 act more quickly than those freshly prepared; then carefully 

 rinse it off in water, holding the cover firmly by one edge 

 with the forceps. After thoroughly rinsing, place the prepa- 

 ration, film downward, on a clean slide and dry the upper 

 surface with a piece of filter paper. It is now ready for the 

 microscopic examination. Use first the dry lens (1-8 in. obj.) 

 and then the oil-immersion objective. If the specimen is a 

 good one and it is desirable to preserve it, wipe off the drop 

 of oil with a piece of lens paper and run a drop of distilled 

 water under the cover-glass, which will float it, when it can 

 be easily removed with the forceps. Place it on the tray, 

 film upward, and when dry, mount it in a neutral or slightly 

 alkaline Canada balsam.* 



(6) With carbol fuchsin. Moisten the film side of the 

 cover- glass with water (using the pipette) ; then cover it 

 with the stain and allow it to act for from 10 to 30 seconds. 

 Then rinse it thoroughly in water, after which cover it with 

 0.1% solution of acetic acid or strong (95%) alcohol. Al- 

 low this to act for from 2 to 5 seconds, and again thoroughly 

 rinse in water and examine as above. 



Upon examination the preparation should be free from 

 deposits or stained background. The bacteria should, as a 

 rule, be isolated and distinct; unless they are, the prepara- 

 tions are not satisfactory. 



Staining bacteria in tissues. Occasionally it is important 

 to examine the bacteria in the tissues themselves. In such 

 cases the tissues should be taken soon after death, to prevent 

 as much as possible post mortem changes. Selected pieces 

 can be cut on the freezing microtome but the best results are 



* To neutralize balsam, add some pure sodium carbonate to it 

 and allow it to stand for about a month in a warm place, shaking 

 it from time to time. Then allow the sodium carbonate to settle. 

 The clear supernatant balsam will be found to be slightly alkaline. 



