94 MICROSCOPIC METHODS. 



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 advantage 

 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 (p. 74). In examining them microscopic- 

 ally, it is necessary to use a very small diaphragm. It is 

 best 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 

 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 



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 a T V in. lens. 



