88 MICROSCOPIC METHODS. 



and covered with a cover-glass.^ It is more usual, however, to 

 employ hanging-drop preparations. The technique of making 

 these has already been described (p. 68). In examining them 

 microscopically, it is necessary to use a very small diaphragm- 

 It is best to focus the edge of the drop with a low-power objec- 

 tive, and, arranging the slide so that part of the edge crosses 

 the centre of the field, to clamp the preparation in this position. 

 A high-power lens is then turned into position and lowered by 

 the coarse adjustment to a short distance above its focal distance ; 

 it is now carefully screwed down by the fine adjustment, the eye 

 being kept at the tube meanwhile. The shadow of the edge will 

 be first recognised, and then the bacteria must be carefully 

 looked for. Often a dry lens is sufficient, but for some purposes 

 the oil immersion is required. If the bacteria are small and 

 motile a beginner may have great difficulty in seeing them, and 

 it is well to practise at first on some large non-motile form such 

 as anthrax. In fluid preparations the natural appearance of 

 bacteria may be studied, and their rate of growth determined. 

 The great use of such preparations, however, is to find whether 

 or not the bacteria are motile, and for determining this point 

 it is advisable to use either broth or agar cultures not more than 

 twenty-four hours old. In the latter case a small fragment of 

 growth is broken down in broth or in sterile water. Sometimes 

 it is an advantage to colour the solution in which the hanging- 

 drop is made up with a minute quantity of an aniline dye, say 

 a small crystal of gentian-violet to lOO 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 them. 



2. Film Preparations, {a) Dry Method. — This is the most 

 extensively applicable method of microscopically examining bac- 

 teria. 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 obtaining such. The test 



1 In bacteriological work it is essential that cover-glasses of No. i thickness 

 {i.e. .14 mm. thijk) should be used, as those of greater thickness are not suitable for 

 a jJj in. lens. 



