84 MICROSCOPIC METHODS. 



minute quantity of any aniline dye, say a small crystal of 

 gentian violet to 100 c.c. of bouillon. Such a degree of 

 dilution will not have any effect on the vitality of the 

 bacteria. Ordinarily, living bacteria will not take up a 

 stain, but even though they do not, the contrast between 

 the unstained bacteria and the tinted fluid will enable the 

 observer more easily to recognise the former. 



2. Film Preparations. This is the most extensively 

 applicable method of microscopically examining bacteria. 

 Fluids containing bacteria, such as blood, pus, scrapings of 

 organs, can be thus investigated, as also cultures in fluid 

 and solid media. The first requisite is a perfectly clean 

 cover-glass. Many methods are recommended for obtain- 

 ing such. The result can be in the great majority of 

 cases obtained by the following procedure. The hands 

 being washed clean with soap and water, a cover-glass is 

 taken and a few drops of absolute alcohol or dilute acetic 

 acid placed on it. The wet glass is then firmly rubbed 

 between the right forefinger and thumb and then carefully 

 dried with a perfectly clean duster. It is then placed in a 

 pair of forceps (Cornet's pattern is very good, Fig. 34), and 



passed several times rapidly 

 through a Bun sen flame, 

 first the one side and then 

 the other being downmost. 

 By this means the last 

 FIG. 34. Cornet's forceps for holding traces of dirt are burned 



off. The great difficulty in 



cleaning covers is to remove the last trace of grease. The 

 test of this being accomplished is that, when the drop of fluid 

 containing the bacteria is placed upon the glass, it can be 

 uniformly spread with the platinum needle all over the surface 

 without showing any tendency to retract into droplets. The 

 best method, however, is that recommended by Van 

 Ermengem. The cover-glasses are placed for some time 

 in a mixture of concentrated sulphuric acid 6 parts, 

 potassium bichromate 6 parts, water 100 parts, then 

 washed thoroughly in water and kept in absolute alcohol. 



