STAINING OF BACTERIA. 149 



The cover-glass is now inverted, with the moist speci- 

 men side downward, on a clean glass-slide. Or, the latter 

 may be brought down over the cover-glass till it touches. 

 The specimen should be examined to see if the upper sur- 

 face of the cover-glass is perfectly dry. Furthermore, just 

 sufficient water should be present between the cover-glass 

 and slide to keep the former in position. The .specimen 

 should never be examined in the dry condition. If there 

 is insufficient water, a drop should be placed near the 

 edge of the cover-glass. Too much water must be avoided, 

 since it would float the cover-glass, in which case the 

 examination with the oil immersion objective is quite im- 

 possible. Excess of water can be drained off by touching 

 the edge of the cover-glass with a piece of filter-paper. 

 The water under the cover-glass should be colorless, which 

 will be the case if the specimen was properly washed. 



The slide, thus prepared, is placed on the stage of the 

 microscope and examined first with the No. 7 objective and 

 subsequently with the iVth homogeneous oil immersion 

 objective, in the same way as was done when examining 

 hanging-drops. 



A good specimen should show the bacteria deeply 

 stained, not in masses, but separated from each other, and 

 evenly distributed over the entire cover-glass. If the 

 stained bacteria are seen to move about, it is due to insuf- 

 ficient fixing* in the flame. 



Failure to take the stain properly may be due to over- 

 heating while fixing the specimen, or to a weak dye, or to a 

 too short exposure to the dye. By repeated trials, the ex- 

 act conditions necessary can be ascertained, and then fol- 

 lowed without any difficulty. 



In the method as described above, the dye is not added 

 direct to the cover-glass, but to a drop of water which is 

 first placed upon it. This little deviation from the process, 

 as ordinarily described, serves to prevent over-staining and 

 deposition of coloring matter. 



