FIL M PREPARA TTONS, 95 



hanging-drop is made up with a 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, (a) Dry Method. 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 ob- 

 taining such. The test of 

 this being accomplished is 

 that, when the drop of 

 fluid containing the bac- 



teria is placed upon the Ite ' ^'~ C ^^^ f r h lding 

 glass, it can be uniformly 



spread with the platinum needle all over the surface 

 without showing any tendency to retract into drop- 

 lets. The best method 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 stored in absolute alcohol. 

 For use, a cover-glass is either dried by wiping with a clean 

 duster or is simply allowed to dry. This method will 

 amply repay the trouble, and really saves time in the end. 

 A clean cover having been obtained, the film preparation 

 can now be made. If a fluid is to be examined a loopful 

 may be placed on the cover-glass, and either spread out 

 over the surface with the needle, or another clean cover may 

 be placed on the top of the first, the drop thus spread out 

 between them and the two then drawn apart. When a 

 culture on a solid medium is to be examined a loopful of 



