

EXAMINATION OF HANGING- DROP CULTURES. 83 



which will dissolve the material by which the lens is fixed 

 in its metal carrier. 



Microscopic Examination of Bacteria, i. Hanging- 

 drop Preparations. Micro-organisms may be examined : 

 (i) alive or dead in fluids; (2) in film preparations; 

 (3) in sections of tissues. In the two last cases advan- 

 tage is always taken of the affinity of bacteria for certain 

 stains. When they are to be examined in fluids a drop 

 of the liquid may be placed on a slide and covered with 

 a cover- glass. 1 It is more usual, however, to employ 

 hanging -drop preparations. The technique of making 

 these has already been described. In examining them 

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

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

 power objective, 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 form such as anthrax. In fluid preparations the 

 natural appearance of bacteria may be studied, 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 



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

 thickness (i.e. , . 14 mm. thick) should be used, as those of greater thickness 

 are not suitable for T \ in. lens. 



